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As you consider the reputation service and the needs of customers or individual consumers, as well as, perhaps, large organizations that are security conscious like our fictitious enterprise, Digital Diskus, what will be the expectations and requirements of the customers? Will consumers’ needs be different from those of enterprises? Who owns the data that is being served from the reputation service? In addition, what kinds of protections might a customer expect from other customers when accessing reputations?

Applied Security
Architecture and
Threat Models

Applied Security
Architecture and
Threat Models
Brook S.E. Schoenfield
Forewords by John N. Stewart and James F. Ransome

CRC Press
Taylor & Francis Group
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To the many teachers who’ve pointed me down the path; the managers who have sup-
ported my explorations; the many architects and delivery teams who’ve helped to refine
the work; to my first design mentors—John Caron, Roddy Erickson, and Dr. Andrew
Kerne—without whom I would still have no clue; and, lastly, to Hans Kolbe, who once
upon a time was our human fuzzer.
Each of you deserves credit for whatever value may lie herein.
The errors are all mine.

Dedication v
Contents vii
Foreword by John N. Stewart xiii
Foreword by Dr. James F. Ransome xv
Preface xix
Acknowledgments xxv
About the Author xxvii
Part I
Introduction 3
The Lay of Information Security Land 3
The Structure of the Book 7
References 8
Chapter 1: Introduction 9
1.1 Breach! Fix It! 11
1.2 Information Security, as Applied to Systems 14
1.3 Applying Security to Any System 21
References 25
Chapter 2: The Art of Security Assessment 27
2.1 Why Art and Not Engineering? 28
2.2 Introducing “The Process” 29

viii Securing Systems
2.3 Necessary Ingredients 33
2.4 The Threat Landscape 35
2.4.1 Who Are These Attackers? Why Do They Want
to Attack My System? 36
2.5 How Much Risk to Tolerate? 44
2.6 Getting Started 51
References 52
Chapter 3: Security Architecture of Systems 53
3.1 Why Is Enterprise Architecture Important? 54
3.2 The “Security” in “Architecture” 57
3.3 Diagramming For Security Analysis 59
3.4 Seeing and Applying Patterns 70
3.5 System Architecture Diagrams and Protocol Interchange
Flows (Data Flow Diagrams) 73
3.5.1 Security Touches All Domains 77
3.5.2 Component Views 78
3.6 What’s Important? 79
3.6.1 What Is “Architecturally Interesting”? 79
3.7 Understanding the Architecture of a System 81
3.7.1 Size Really Does Matter 81
3.8 Applying Principles and Patterns to Specific Designs 84
3.8.1 Principles, But Not Solely Principles 96
Summary 98
References 98
Chapter 4: Information Security Risk 101
4.1 Rating with Incomplete Information 101
4.2 Gut Feeling and Mental Arithmetic 102
4.3 Real-World Calculation 105
4.4 Personal Security Posture 106
4.5 Just Because It Might Be Bad, Is It? 107
4.6 The Components of Risk 108
4.6.1 Threat 110
4.6.2 Exposure 112
4.6.3 Vulnerability 117
4.6.4 Impact 121
4.7 Business Impact 122
4.7.1 Data Sensitivity Scales 125

Contents ix
4.8 Risk Audiences 126
4.8.1 The Risk Owner 127
4.8.2 Desired Security Posture 129
4.9 Summary 129
References 130
Chapter 5: Prepare for Assessment 133
5.1 Process Review 133
5.1.1 Credible Attack Vectors 134
5.1.2 Applying ATASM 135
5.2 Architecture and Artifacts 137
5.2.1 Understand the Logical and Component Architecture
of the System 138
5.2.2 Understand Every Communication Flow and Any
Valuable Data Wherever Stored 140
5.3 Threat Enumeration 145
5.3.1 List All the Possible Threat Agents for This Type
of System 146
5.3.2 List the Typical Attack Methods of the Threat Agents 150
5.3.3 List the System-Level Objectives of Threat Agents
Using Their Attack Methods 151
5.4 Attack Surfaces 153
5.4.1 Decompose (factor) the Architecture to a Level That
Exposes Every Possible Attack Surface 154
5.4.2 Filter Out Threat Agents Who Have No Attack
Surfaces Exposed to Their Typical Methods 159
5.4.3 List All Existing Security Controls for Each Attack
Surface 160
5.4.4 Filter Out All Attack Surfaces for Which There Is
Sufficient Existing Protection 161
5.5 Data Sensitivity 163
5.6 A Few Additional Thoughts on Risk 164
5.7 Possible Controls 165
5.7.1 Apply New Security Controls to the Set of Attack
Services for Which There Isn’t Sufficient Mitigation 166
5.7.2 Build a Defense-in-Depth 168
5.8 Summary 170
References 171
Part I
Summary 173

x Securing Systems
Part II
Introduction 179
Practicing with Sample Assessments 179
Start with Architecture 180
A Few Comments about Playing Well with Others 181
Understand the Big Picture and the Context 183
Getting Back to Basics 185
References 189
Chapter 6: eCommerce Website 191
6.1 Decompose the System 191
6.1.1 The Right Level of Decomposition 193
6.2 Finding Attack Surfaces to Build the Threat Model 194
6.3 Requirements 209
Chapter 7: Enterprise Architecture 213
7.1 Enterprise Architecture Pre-work: Digital Diskus 217
7.2 Digital Diskus’ Threat Landscape 218
7.3 Conceptual Security Architecture 221
7.4 Enterprise Security Architecture Imperatives
and Requirements 222
7.5 Digital Diskus’ Component Architecture 227
7.6 Enterprise Architecture Requirements 232
References 233
Chapter 8: Business Analytics 235
8.1 Architecture 235
8.2 Threats 239
8.3 Attack Surfaces 242
8.3.1 Attack Surface Enumeration 254
8.4 Mitigations 254
8.5 Administrative Controls 260
8.5.1 Enterprise Identity Systems (Authentication
and Authorization) 261
8.6 Requirements 262
References 266

Contents xi
Chapter 9: Endpoint Anti-malware 267
9.1 A Deployment Model Lens 268
9.2 Analysis 269
9.3 More on Deployment Model 277
9.4 Endpoint AV Software Security Requirements 282
References 283
Chapter 10: Mobile Security Software with Cloud Management 285
10.1 Basic Mobile Security Architecture 285
10.2 Mobility Often Implies Client/Cloud 286
10.3 Introducing Clouds 290
10.3.1 Authentication Is Not a Panacea 292
10.3.2 The Entire Message Stack Is Important 294
10.4 Just Good Enough Security 295
10.5 Additional Security Requirements for a Mobile and
Cloud Architecture 298
Chapter 11: Cloud Software as a Service (SaaS) 301
11.1 What’s So Special about Clouds? 301
11.2 Analysis: Peel the Onion 302
11.2.1 Freemium Demographics 306
11.2.2 Protecting Cloud Secrets 308
11.2.3 The Application Is a Defense 309
11.2.4 “Globality” 311
11.3 Additional Requirements for the SaaS Reputation Service 319
References 320
Part II
Summary 321
Part III
Introduction 327
Chapter 12: Patterns and Governance Deliver Economies of Scale 329
12.1 Expressing Security Requirements 337
12.1.1 Expressing Security Requirements to Enable 338
12.1.2 Who Consumes Requirements? 339

xii Securing Systems
12.1.3 Getting Security Requirements Implemented 344
12.1.4 Why Do Good Requirements Go Bad? 347
12.2 Some Thoughts on Governance 348
Summary 351
References 351
Chapter 13: Building an Assessment Program 353
13.1 Building a Program 356
13.1.1 Senior Management’s Job 356
13.1.2 Bottom Up? 357
13.1.3 Use Peer Networks 359
13.2 Building a Team 364
13.2.1 Training 366
13.3 Documentation and Artifacts 369
13.4 Peer Review 372
13.5 Workload 373
13.6 Mistakes and Missteps 374
13.6.1 Not Everyone Should Become an Architect 374
13.6.2 Standards Can’t Be Applied Rigidly 375
13.6.3 One Size Does Not Fit All, Redux 376
13.6.4 Don’t Issue Edicts Unless Certain of Compliance 377
13.7 Measuring Success 377
13.7.1 Invitations Are Good! 378
13.7.2 Establish Baselines 378
13.8 Summary 380
References 382
Part III
Summary and Afterword 383
Summary 383
Afterword 385
Index 387

As you read this, it is important to note that despite hundreds to thousands of people-
years spent to date, we are still struggling mightily to take the complex, de-compose
into the simple, and create the elegant when it comes to information systems. Our
world is hurtling towards an always on, pervasive, interconnected mode in which soft-
ware and life quality are co-dependent, productivity enhancements each year require
systems, devices and systems grow to 50 billion connected, and the quantifiable and
definable risks all of this creates are difficult to gauge, yet intuitively unsettling, and are
slowly emerging before our eyes.
“Arkhitekton”—a Greek word preceding what we speak to as architecture today, is
an underserved idea for information systems, and not unsurprisingly, security architec-
ture is even further underserved. The very notion that through process and product,
systems filling entire data centers, information by the pedabyte, transaction volumes
at sub-millisecond speed, and compute systems doubling capability every few years, is
likely seen as impossible—even if needed. I imagine the Golden Gate bridge seemed
impossible at one point, a space station also, and buildings such as the Burj Khalifa, and
yet here we are admiring each as a wonder unto themselves. None of this would be pos-
sible without formal learning, training architects in methods that work, updating our
training as we learn, and continuing to require a demonstration for proficiency. Each
element plays that key role.
The same is true for the current, and future, safety in information systems.
Architecture may well be the savior that normalizes our current inconsistencies, engen-
ders a provable model that demonstrates efficacy that is quantifiably improved, and
tames the temperamental beast known as risk. It is a sobering thought that when sys-
tems are connected for the first time, they are better understood than at any other time.
From that moment on, changes made—documented and undocumented—alter our
understanding, and without understanding comes risk. Information systems must be
understood for both operational and risk-based reasons, which means tight definitions
must be at the core—and that is what architecture is all about.
For security teams, both design and protect, it is our time to build the tallest, and
safest, “building.” Effective standards, structural definition, deep understanding with

xiv Securing Systems
validation, a job classification that has formal methods training, and every improving
and learning system that takes knowledge from today to strengthen systems installed
yesterday, assessments and inspection that look for weaknesses (which happen over
time), all surrounded by a well-built security program that encourages if not demands
security architecture, is the only path to success. If breaches, so oftentimes seen as
avoidable ex post facto, don’t convince you of this, then the risks should.
We are struggling as a security industry now, and the need to be successful is higher
than it has ever been in my twenty-five years in it. It is not good enough just to build
something and try and secure it, it must be architected from the bottom up with secu-
rity in it, by professionally trained and skilled security architects, checked and validated
by regular assessments for weakness, and through a learning system that learns from
today to inform tomorrow. We must succeed.
– John N. Stewart
SVP, Chief Security & Trust Officer
Cisco Systems, Inc.
About John N. Stewart:
John N. Stewart formed and leads Cisco’s Security and Trust Organization, underscor-
ing Cisco’s commitment to address two key issues in boardrooms and on the minds
of top leaders around the globe. Under John’s leadership, the team’s core missions are
to protect Cisco’s public and private customers, enable and ensure the Cisco Secure
Development Lifecycle and Trustworthy Systems efforts across Cisco’s entire mature
and emerging solution portfolio, and to protect Cisco itself from the never-ending, and
always evolving, cyber threats.
Throughout his 25-year career, Stewart has led or participated in security initiatives
ranging from elementary school IT design to national security programs. In addition to
his role at Cisco, he sits on technical advisory boards for Area 1 Security, BlackStratus,
Inc., RedSeal Networks, and Nok Nok Labs. He is a member of the Board of Directors
for Shape Security, Shadow Networks, Inc., and the National Cyber-Forensics Training
Alliance (NCFTA). Additionally, Stewart serves on the Cybersecurity Think Tank at
University of Maryland University College, and on the Cyber Security Review to Prime
Minister & Cabinet for Australia. Prior, Stewart served on the CSIS Commission on
Cybersecurity for the 44th Presidency of the United States, the Council of Experts for
the Global Cyber Security Center, and on advisory boards for successful companies
such as Akonix, Cloudshield, Finjan, Fixmo, Ingrian Networks, Koolspan, Riverhead,
and TripWire. John is a highly sought public and closed-door speaker and most recently
was awarded the global Golden Bridge Award and CSO 40 Silver Award for the 2014
Chief Security Officer of the Year.
Stewart holds a Master of Science degree in computer and information science from
Syracuse University, Syracuse, New York.

Cyberspace has become the 21st century’s greatest engine of change. And it’s every-
where. Virtually every aspect of global civilization now depends on interconnected
cyber systems to operate. A good portion of the money that was spent on offensive and
defensive capabilities during the Cold War is now being spent on cyber offense and
defense. Unlike the Cold War, where only governments were involved, this cyber chal-
lenge requires defensive measures for commercial enterprises, small businesses, NGOs,
and individuals. As we move into the Internet of Things, cybersecurity and the issues
associated with it will affect everyone on the planet in some way, whether it is cyber-
war, cyber-crime, or cyber-fraud.
Although there is much publicity regarding network security, the real cyber Achilles’
heel is insecure software and the architecture that structures it. Millions of software
vulnerabilities create a cyber house of cards in which we conduct our digital lives.
In response, security people build ever more elaborate cyber fortresses to protect this
vulnerable software. Despite their efforts, cyber fortifications consistently fail to pro-
tect our digital treasures. Why? The security industry has failed to engage fully with
the creative, innovative people who write software and secure the systems these solu-
tions are connected to. The challenges to keep an eye on all potential weaknesses are
skyrocketing. Many companies and vendors are trying to stay ahead of the game by
developing methods and products to detect threats and vulnerabilities, as well as highly
efficient approaches to analysis, mitigation, and remediation. A comprehensive approach
has become necessary to counter a growing number of attacks against networks, servers,
and endpoints in every organization.
Threats would not be harmful if there were no vulnerabilities that could be exploited.
The security industry continues to approach this issue in a backwards fashion by trying
to fix the symptoms rather than to address the source of the problem itself. As discussed
in our book Core Software Security: Security at the Source,* the stark reality is that the
* Ransome, J. and Misra, A. (2014). Core Software Security: Security at the Source. Boca Raton
(FL): CRC Press.

xvi Securing Systems
vulnerabilities that we were seeing 15 years or so ago in the OWASP and SANS Top Ten
and CVE Top 20 are almost the same today as they were then; only the pole positions
have changed. We cannot afford to ignore the threat of insecure software any longer
because software has become the infrastructure and lifeblood of the modern world.
Increasingly, the liabilities of ignoring or failing to secure software and provide the
proper privacy controls are coming back to the companies that develop it. This is and
will be in the form of lawsuits, regulatory fines, loss of business, or all of the above.
First and foremost, you must build security into the software development process. It is
clear from the statistics used in industry that there are substantial cost savings to fixing
security flaws early in the development process rather than fixing them after software is
fielded. The cost associated with addressing software problems increases as the lifecycle
of a project matures. For vendors, the cost is magnified by the expense of developing
and patching vulnerable software after release, which is a costly way of securing appli-
cations. The bottom line is that it costs little to avoid potential security defects early in
development, especially compared to costing 10, 20, 50, or even 100 times that amount
much later in development. Of course, this doesn’t include the potential costs of regula-
tory fines, lawsuits, and or loss of business due to security and privacy protection flaws
discovered in your software after release.
Having filled seven Chief Security Officer (CSO) and Chief Information Security
Officer (CISO) roles, and having had both software security and security architecture
reporting to me in many of these positions, it is clear to me that the approach for both
areas needs to be rethought. In my last book, Brook helped delineate our approach to
solving the software security problem while also addressing how to build in security
within new agile development methodologies such as Scrum. In the same book, Brook
noted that the software security problem is bigger than just addressing the code but also
the systems it is connected to.
As long as software and architecture is developed by humans, it requires the human
element to fix it. There have been a lot of bright people coming up with various techni-
cal solutions and models to fix this, but we are still failing to do so as an industry.
We have consistently focused on the wrong things: vulnerability and command and
control. But producing software and designing architecture is a creative and innovative
process. In permaculture, it is said that “the problem is the solution.” Indeed, it is that
very creativity that must be enhanced and empowered in order to generate security as
an attribute of a creative process. A solution to this problem requires the application of
a holistic, cost-effective, and collaborative approach to securing systems. This book is
a perfect follow-on to the message developed in Core Software Security: Security at the
Source* in that it addresses a second critical challenge in developing software: security
architecture methods and the mindset that form a frame for evaluating the security
of digital systems that can be used to prescribe security treatments for those systems.
Specifically, it addresses an applied approach to security architecture and threat models.
* Ibid.

Foreword xvii
It should be noted that systems security, for the most part, is still an art not a science.
A skilled security architect must bring a wealth of knowledge and understanding—
global and local, technical, human, organizational, and even geopolitical—to an assess-
ment. In this sense, Brook is a master of his craft, and that is why I am very excited
about the opportunity to provide a Foreword to this book. He and I have worked
together on a daily basis for over five years and I know of no one better with regard
to his experience, technical aptitude, industry knowledge, ability to think out of the
box, organizational collaboration skills, thoroughness, and holistic approach to systems
architecture—specifically, security as it relates to both software and systems design and
architecture. I highly recommend this book to security architects and all architects who
interact with security or to those that manage them. If you have a reasonable feel for
what the security architect is doing, you will be able to accommodate the results from
the process within your architectures, something that he and I have been able to do
successfully for a number of years now. Brook’s approach to securing systems addresses
the entire enterprise, not only its digital systems, as well as the processes and people
who will interact, design, and build the systems. This book fills a significant gap in the
literature and is appropriate for use as a resource for both aspiring and seasoned security
architects alike.
– Dr. James F. Ransome, CISSP, CISM
About Dr. James F. Ransome:
Dr. James Ransome, CISSP, CISM, is the Senior Director of Product Security at
McAfee—part of Intel Security—and is responsible for all aspects of McAfee’s Product
Security Program, a corporate-wide initiative that supports the delivery of secure soft-
ware products to customers. His career is marked by leadership positions in private and
public industries, having served in three chief information officer (CISO) and four
chief security officer (CSO) roles. Prior to the corporate world, Ransome had 23 years
of government service in various roles supporting the United States intelligence com-
munity, federal law enforcement, and the Department of Defense. He holds a Ph.D.
specializing in Information Security from a NSA/DHS Center of Academic Excellence
in Information Assurance Education program. Ransome is a member of Upsilon Pi
Epsilon, the International Honor Society for Computing and Information Disciplines
and a Ponemon Institute Distinguished Fellow. He recently completed his 10th infor-
mation security book Core Software Security: Security at the Source.*
* Ibid.

This book replies to a question that I once posed to myself. I know from my conversations
with many of my brother and sister practitioners that, early in your security careers, you have
also posed that very same question. When handed a diagram containing three rectangles and
two double-headed arrows connecting each box to one of the others, each of us has wondered,
“How do I respond to this?”
This is a book about security architecture. The focus of the book is upon how secu-
rity architecture methods and mindset form a frame for evaluating the security of digi-
tal systems in order to prescribe security treatments for those systems. The treatments
are meant to bring the system to a particular and verifiable risk posture.
“System” should be taken to encompass a gamut running from individual com-
puters, to networks of computers, to collections of applications (however that may
be defined) and including complex system integrations of all the above, and more.
“System” is a generic term meant to encompass rather than exclude. Presumably, a
glance through the examples in Part II of this book should indicate the breadth of reach
that has been attempted?
I will endeavor along the way, to provide situationally appropriate definitions for
“security architecture,” “risk,” “architecture risk assessment,” “threat model,” and
“applied.” These definitions should be taken as working definitions, fit only for the pur-
pose of “applied security architecture” and not as proposals for general models in any of
these fields. I have purposely kept a tight rein on scope in the hope that the book retains
enough focus to be useful. In my very humble experience, applied security architecture

xx Securing Systems
will make use of whatever skills—technical, interpersonal, creative, adaptive, and so
forth—that you have or can learn. This one area, applied security architecture, seems
big enough.
Who May Benefi t from This Book?
Any organization that places into service computer systems that have some chance of
being exposed to digital attack will encounter at least some of the problems addressed
within Securing Systems. Digital systems can be quite complex, involving various and
sometimes divergent stakeholders, and they are delivered through the collaboration of
multidisciplinary teams. The range of roles performed by those individuals who will
benefit from familiarity with applied security architecture, therefore, turns out to be
quite broad. The following list comprises nearly everyone who is involved in the specifi-
cation, implementation, delivery, and decision making for and about computer systems.
• Security architects, assessors, analysts, and engineers
• System, solution, infrastructure, and enterprise architects
• Developers, infrastructure engineers, system integrators, and implementation
• Managers, technical leaders, program and project managers, middle management,
and executives
Security architecture is and will remain, for some time, an experience-based prac-
tice. The security architect encounters far too many situations where the “right” answer
will be “it depends.” Those dependencies are, in part, what this book is about.
Certainly, engineering practice will be brought to bear on secure systems. Exploit
techniques tend to be particular. A firm grasp of the engineering aspects of soft-
ware, networks, operating systems, and the like is essential. Applied cryptography is
not really an art. Cryptographic techniques do a thing, a particular thing, exactly.
Cryptography is not magic, though application is subtle and algorithms are often
mathematically and algorithmically complex. Security architecture cannot be per-
formed without a firm grounding in many aspects of computer science. And, at a
grosser granularity, there are consistent patterns whose solutions tend to be amenable
to clear-cut engineering resolution.
Still, in order to recognize the patterns, one must often apply deep and broad
experience. This book aims to seed precisely that kind of experience for practitioners.
Hopefully, alongside the (fictitious but commonly occurring) examples, I will have
explained the reasoning and described the experience behind my analysis and the deci-
sions depicted herein such that even experts may gain new insight from reading these
and considering my approaches. My conclusions aren’t necessarily “right.” (Being a risk-
driven practice, there often is no “right” answer.)

Preface xxi
Beyond security architects, all architects who interact with security can benefit from
this work. If you have a reasonable feel for what the security architect is doing, you will
be able to accommodate the results from the process within your architectures. Over
the years, many partner architects and I have grown so attuned, that we could finish
each other’s sentences, speak for each other’s perspectives, and even include each other’s
likely requirements within our analysis of an architecture. When you have achieved
this level of understanding and collaboration, security is far more easily incorporated
from the very inception of a new idea. Security becomes yet another emerging attribute
of the architecture and design, just like performance or usability. That, in my humble
opinion, is an ideal to strive for.
Developers and, particularly, development and technical leaders will have to translate
the threat model and requirements into things that can be built and coded. That’s not an
easy transformation. I believe that this translation from requirement through to func-
tional test is significantly eased through a clear understanding of the threat model. In
fact, at my current position, I have offered many participatory coaching sessions in the
ATASM process described in this book to entire engineering teams. These sessions have
had a profound effect, causing everyone involved—from architect to quality engineer—
to have a much clearer understanding of why the threat model is key and how to work
with security requirements. I hope that reading this book will provide a similar ground-
ing for delivery teams that must include security architecture in their work.
I hope that all of those who must build and then sustain a security architecture prac-
tice will find useful tidbits that foster high-functioning technical delivery teams that
must include security people and security architecture—namely, project and program
managers, line managers, middle management, or senior and executive management.
Beyond the chapter specifically devoted to building a program, I’ve also included a con-
siderable explanation of the business and organizational context in which architecture
and risk assessment programs exist. The nontechnical factors must comprise the basis
from which security architecture gets applied. Without the required business acumen
and understanding, security architecture can easily devolve to ivory tower, isolated, and
unrealistic pronouncements. Nobody actually reads those detailed, 250-page architec-
ture documents that are gathering dust on the shelf. My sincere desire is that this body
of work remains demonstratively grounded in real-world situations.
All readers of this book may gain some understanding of how the risk of system
compromise and its impacts can be generated. Although risk remains a touted corner-
stone of computer security, it is poorly understood. Even the term, “risk,” is thrown
about with little precision, and with multiple and highly overloaded meanings. Readers
will be provided with a risk definition and some specificity about its use, as well as given
a proven methodology, which itself is based upon an open standard. We can all benefit
from just a tad more precision when discussing this emotionally loaded topic, “risk.”
The approach explained in Chapter 4 underlies the analysis in the six example (though
fictitious) architectures. If you need to rank risks in your job, this book will hopefully
provide some insight and approaches.

xxii Securing Systems
Background and Origins
I was thrown into the practice of securing systems largely because none of the other
security architects wanted to attend the Architecture Technical Review (ATR) meet-
ings. During those meetings, every IT project would have 10 minutes to explain what
they were intending to accomplish. The goal of the review was to uncover the IT ser-
vices required for project success. Security was one of those IT services.
Security had no more than 5 minutes of that precious time slot to decide whether
the project needed to be reviewed more thoroughly. That was a hard task! Mistakes and
misses occurred from time to time, but especially as I began to assess the architectures
of the projects.
When I first attended ATR meetings, I felt entirely unqualified to make the engage-
ment decisions; in fact, I felt pretty incompetent to be assessing IT projects, at all. I
had been hired to provide long-term vision and research for future intrusion detec-
tion systems and what are now called “security incident event management systems.”
Management then asked me to become “Infosec’s” first application security architect. I
was the newest hire and was just trying to survive a staff reduction. It seemed a precari-
ous time to refuse job duties.
A result that I didn’t expect from attending the ATR meetings was how the wide
exposure would dramatically increase my ability to spot architecture patterns. I saw
hundreds of different architectures in those couple of years. I absorbed IT standards
and learned, importantly, to quickly cull exceptional and unique situations. Later, when
new architects took ATR duty, I was forced to figure out how to explain what I was
doing to them. And interacting with all those projects fostered relationships with teams
across IT development. When inevitable conflicts arose, those relationships helped us
to cooperate across our differences.
Because my ATR role was pivotal to the workload for all the security architects
performing reviews, I became a connecting point for the team. After all, I saw almost
all the projects first. And that connecting role afforded me a view of how each of these
smart, highly skilled individuals approached the problems that they encountered as
they went through their process of securing IT’s systems and infrastructures.
Security architecture was very much a formative practice in those days. Systems
architecture was maturing; enterprise architecture was coalescing into a distinct body
of knowledge and practice. The people performing system architecture weren’t sure that
the title “architect” could be applied to security people. We were held somewhat at arm’s
length, not treated entirely as peers, not really allowed into the architects’ “club,” if you
will? Still, it turns out that it’s really difficult to secure a system if the person trying does
not have architectural skills and does not examine the system holistically, including
having the broader context for which the system is intended. A powerful lesson.
At that time, there were few people with a software design background who also
knew anything about computer security. That circumstance made someone like me
a bit of a rarity. When I got started, I had very little security knowledge, just enough
knowledge to barely get by. But, I had a rich software design background from which

Preface xxiii
to draw. I could “do” architecture. I just didn’t know much about security beyond hav-
ing written simple network access control lists and having responded to network attack
logs. (Well, maybe a little more than that?)
Consequently, people like Steve Acheson, who was already a security guru and had,
in those early days, a great feel for design, were willing to forgive me for my inex-
perience. I suspect that Steve tolerated my naiveté because there simply weren’t that
many people who had enough design background with whom he could kick around
the larger issues encountered in building a rigorous practice of security architecture. At
any rate, my conversations with Steve and, slightly later, Catherine Blackader Nelson,
Laura Lindsey, Gavin Reid, and somewhat later, Michele Guel, comprise the seeds out
of which this book was born. Essentially, perhaps literally, we were trying to define the
very nature of security architecture and to establish a body of craft for architecture risk
assessment and threat models.
A formative enterprise identity research team was instigated by Michele Guel in
early 2001. Along with Michele, Steve Acheson and I, (then) IT architect Steve Wright,
and (now) enterprise architect, Sergei Roussakov, probed and prodded, from diverse
angles, the problems of identity as a security service, as an infrastructure, and as an
enterprise necessity. That experience profoundly affects not only the way that I practice
security architecture but also my understanding of how security fits into an enterprise
architecture. Furthermore, as a team encompassing a fairly wide range of different per-
spectives and personalities, we proved that diverse individuals can come together to
produce seminal work, and relatively easily, at that. Many of the lessons culled from
that experience are included in this volume.
For not quite 15 years, I have continued to explore, investigate, and refine these
early experiments in security architecture and system assessment in concert with those
named above, as well as many other practitioners. The ideas and approaches set out
herein are this moment’s summation of not only of my experience but also that of many
of the architects with whom I’ve worked and interacted. Still, it’s useful to remember
that a book is merely a point in time, a reflection of what is understood at that moment.
No doubt my ideas will change, as will the practice of security architecture.
My sincere desire is that I’m offering both an approach and a practicum that will
make the art of securing systems a little more accessible. Indeed, ultimately, I’d like
this book to unpack, at least a little bit, the craft of applied security architecture for the
many people who are tasked with providing security oversight and due diligence for
their digital systems.
Brook S.E. Schoenfield
Camp Connell, California, USA, December 2014

There are so many people who have contributed to the content of this book—from
early technical mentors on through my current collaborators and those people who were
willing to wade through my tortured drivel as it has come off of the keyboard. I direct
the reader to my blog site,, if you’re curious about my technical
history and the many who’ve contributed mightily to whatever skills I’ve gained. Let it
suffice to say, “Far too many to be named here.” I’ll, therefore, try to name those who
contributed directly to the development of this body of work.
Special thanks are due to Laura Lindsey, who coached my very first security review
and, afterwards, reminded me that, “We’re not the cops, Brook.” Hopefully, I continue
to pass on your wisdom?
Michelle Koblas and John Stewart not only “got” my early ideas but, more impor-
tantly, encouraged me, supporting me through the innumerable and inevitable mis-
takes and missteps. Special thanks are offered to you, John, for always treating me as a
respected partner in the work, and to both of you for offering me your ongoing personal
friendship. Nasrin Rezai, I continue to carry your charge to “teach junior people,” so
that security architecture actually has a future.
A debt of gratitude is owed to every past member of Cisco’s “WebArch” team during
the period when I was involved. Special thanks go to Steve Acheson for his early faith
in me (and friendship).
Everyone who was involved with WebArch let me prove that techniques gleaned
from consensus, facilitation, mediation, and emotional intelligence really do provide
a basis for high-functioning technical teams. We collectively proved it again with the
“PAT” security architecture virtual team, under the astute program management of
Ferris Jabri, of “We’re just going to do it, Brook,” fame. Ferris helped to manifest some
of the formative ideas that eventually became the chapter I wrote (Chapter 9) in Core
Software Security: Security at the Source,* by James Ransome and Anmol Misra, as well.
* Schoenfi eld, B. (2014). “Applying the SDL Framework to the Real World” (Ch. 9). In Core
Software Security: Security at the Source, pp. 255–324. Boca Raton (FL): CRC Press.

xxvi Securing Systems
A special note is reserved for Ove Hansen who, as an architect on the WebArch team,
challenged my opinions on a regular basis and in the best way. Without that counter-
vail, Ove, that first collaborative team experiment would never have fully succeeded.
The industry continues to need your depth and breadth.
Aaron Sierra, we proved the whole concept yet again at WebEx under the direction
and support of Dr. James Ransome. Then, we got it to work with most of Cisco’s bur-
geoning SaaS products. A hearty thanks for your willingness to take that journey with
me and, of course, for your friendship.
Vinay Bansal and Michele Guel remain great partners in the shaping of a security
architecture practice. I’m indebted to Vinay and to Ferris for helping me to generate
a first outline for a book on security architecture. This isn’t that book, which remains
Thank you to Alan Paller for opportunities to put my ideas in front of wider audi-
ences, which, of course, has provided an invaluable feedback loop.
Many thanks to the readers of the book as it progressed: Dr. James Ransome, Jack
Jones, Eoin Carroll, Izar Tarandach, and Per-Olof Perrson. Please know that your com-
ments and suggestions have improved this work immeasurably. You also validated that
this has been a worthy pursuit.
Catherine Blackader Nelson and Dr. James Ransome continue to help me refine this
work, always challenging me to think deeper and more thoroughly. I treasure not only
your professional support but also the friendship that each of you offers to me.
Thanks to Dr. Neal Daswani for pointing out that XSS may also be mitigated
through output validation (almost an “oops” on my part).
This book simply would not exist without the tireless logistical support of Theron
Shreve and the copyediting and typesetting skills of Marje Pollack at DerryField
Publishing Services. Thanks also go to John Wyzalek for his confidence that this body
of work could have an audience and a place within the CRC Press catalog. And many
thanks to Webb Mealy for help with graphics and for building the Index.
Finally, but certainly not the least, thanks are owed to my daughter, Allison, who
unfailingly encourages me in whatever creative efforts I pursue. I hope that I return that
spirit of support to you. And to my sweetheart, Cynthia Mealy, you have my heartfelt
gratitude. It is you who must put up with me when I’m in one of my creative binges,
which tend to render me, I’m sure, absolutely impossible to deal with. Frankly, I have
no idea how you manage.
Brook S.E. Schoenfield
Camp Connell, California, USA, October 2014

About the Author
Brook S.E. Schoenfield is a Master Principal Product Security Architect at a global
technology enterprise. He is the senior technical leader for software security across a
division’s broad product portfolio. He has held leadership security architecture posi-
tions at high-tech enterprises for many years.
Brook has presented at conferences such as RSA, BSIMM, and SANS What Works
Summits on subjects within security architecture, including SaaS security, information
security risk, architecture risk assessment and threat models, and Agile security. He has
been published by CRC Press, SANS, Cisco, and the IEEE.
Brook lives in the Sierra Mountains of California. When he’s not thinking about,
writing about, and speaking on, as well as practicing, security architecture, he can be
found telemark skiing, hiking, and fly fishing in his beloved mountains, or playing
various genres of guitar—from jazz to percussive fingerstyle.

Part I

Part I
The Lay of Information Security Land
[S]ecurity requirements should be developed at the same time system planners define
the requirements of the system. These requirements can be expressed as technical features
(e.g., access controls), assurances (e.g., background checks for system developers), or
operational practices (e.g., awareness and training).1
How have we come to this pass? What series of events have led to the necessity for per-
vasive security in systems big and small, on corporate networks, on home networks, and
in cafes and trains in order for computers to safely and securely provide their benefits?
How did we ever come to this? Isn’t “security” something that banks implement? Isn’t
security an attribute of government intelligence agencies? Not anymore.
In a world of pervasive and ubiquitous network interconnection, our very lives are
intertwined with the successful completion of millions of transactions initiated on our
behalf on a rather constant basis. At the risk of stating the obvious, global commerce
has become highly dependent upon the “Internet of Things.”2 Beyond commerce, so
has our ability to solve large, complex problems, such as feeding the hungry, under-
standing the changes occurring to the ecosystems on our planet, and finding and
exploiting resources while, at the same time, preserving our natural heritage for future
generations. Indeed, war, peace, and regime change are all dependent upon the global
commons that we call “The Public Internet.” Each of these problems, as well as all of us
connected humans, have come to rely upon near-instant connection and seamless data
exchange, just as each of us who use small, general-purpose computation devices—that
is, your “smart phone,”—expect snappy responses to our queries and interchanges. A
significant proportion of the world’s 7 billion humans* have become interconnected.
* As of this writing, the population of the world is just over 7 billion. About 3 billion of these
people are connected to the Internet.

4 Securing Systems
And we expect our data to arrive safely and our systems and software to provide a
modicum of safety. We’d like whatever wealth we may have to be held securely. That’s
not too much to expect, is it?
We require a modicum of security: the same protection that our ancestors expected
from the bank and solicitor. Or rather, going further back, these are the protections that
feudal villages expected from their Lord. Even further back, the village or clan warriors
supposedly provided safety from a dangerous “outside” or “other.”
Like other human experiments in sharing a commons,* the Internet seems to suffer
from the same forces that have plagued common areas throughout history: bandits,
pirates, and other groups taking advantage of the lack of barriers and control.
Early Internet pundits declared that the Internet would prove tremendously
As we approach the twenty-first century, America is turning into an electronic republic,
a democratic system that is vastly increasing the people’s day-to-day influence on the
decisions of state . . . transforming the nature of the political process . . .3
Somehow, I doubt that these pundits quite envisioned the “democracy” of the
modern Internet, where salacious rumors can become worldwide “facts” in hours, where
news about companies’ mistakes and misdeeds cannot be “spun” by corporate press
corps, and where products live or die through open comment and review by consumers.
Governments are not immune to the power of instant interconnectedness. Regimes
have been shaken, even toppled it would seem, by the power of the instant message.
Nation-state nuclear programs have been stymied through “cyber offensives.” Corporate
and national secrets have been stolen. Is nothing on the Internet safe?
Indeed, it is a truism in the Age of the Public Internet (if I may title it so?), “You
can’t believe anything on the Internet.” And yet, Wikipedia has widely replaced the
traditional, commercial encyclopedia as a reference source. Wikipedia articles, which
are written by its millions of participants—“crowd-sourced”—rather than being writ-
ten by a hand-selected collection of experts, have proven to be quite reliable, if not
always perfectly accurate. “Just Good Enough Reference”? Is this the power of Internet
Realizing the power of unfettered interconnection, some governments have gone to
great lengths to control connection and content access. For every censure, clever techni-
cians have devised methods of circumventing those governmental controls. Apparently,
people all over the world prefer to experience the content that they desire and to com-
municate with whom they please, even in the face of arrest, detention, or other sanction.
Alongside the growth of digital interconnection have grown those wishing to take
advantage of the open structure of our collective, global commons. Individuals seeking
* A commons is an asset held in common by a community—for example, pasture land that
every person with livestock might use to pasture personal animals. Th e Public Internet is a
network and a set of protocols held in common for everyone with access to it.

Part I-Introduction 5
advantage of just about every sort, criminal gangs large and small, pseudo- governmental
bodies, cyber armies, nation-states, and activists of every political persuasion have all
used and misused the openness built into the Internet.
Internet attack is pervasive. It can take anywhere from less than a minute to as
much as eight hours for an unprotected machine connected to the Internet to be com-
pletely compromised. The speed of attack entirely depends upon at what point in the
address space any of the hundreds of concurrent sweeps happen to be at the moment.
Compromise is certain; the risk of compromise is 100%. There is no doubt. An unpro-
tected machine that is directly reachable (i.e., has a routable and visible address) from
the Internet will be controlled by an attacker given a sufficient exposure period. The
exposure period has been consistently shortening, from weeks, to days, then to hours,
down to minutes, and finally, some percentage of systems have been compromised
within seconds of connection.
In 1998, I was asked to take over the security of the single Internet router at the
small software house for which I worked. Alongside my duties as Senior Designer and
Technical Lead, I was asked, “Would you please keep the Access Control Lists (ACL)
updated?”* Why was I chosen for these duties? I wrote the TCP/IP stack for our real-
time operating system. Since supposedly I knew something about computer network-
ing, we thought I could add few minor maintenance duties. I knew very little about
digital security at the time. I learned.
As I began to study the problem, I realized that I didn’t have a view into potential
attacks, so I set up the experimental, early Intrusion Detection System (IDS), Shadow,
and began monitoring traffic. After a few days of monitoring, I had a big shock. We, a
small, relatively unknown (outside our industry) software house with a single Internet
connection, were being actively attacked! Thus began my journey (some might call it
descent?) into cyber security.
Attack and the subsequent “compromise,” that is, complete control of a system on
the Internet, is utterly pervasive: constant and continual. And this has been true for
quite a long time. Many attackers are intelligent and adaptive. If defenses improve,
attackers will change their tactics to meet the new challenge. At the same time, once
complex and technically challenging attack methods are routinely “weaponized,”
turned into point-and-click tools that the relatively technically unsophisticated can
easily use. This development has exponentially expanded the number of attackers. The
result is a broad range of attackers, some highly ingenious alongside the many who can
and will exploit well-known vulnerabilities if left unpatched. It is a plain fact that as of
this writing, we are engaged in a cyber arms race of extraordinary size, composition,
complexity, and velocity.
Who’s on the defending side of this cyber arms race? The emerging and burgeoning
information security industry.
As the attacks and attackers have matured, so have the defenders. It is information
security’s job to do our best to prevent successful compromise of data, communications,
* Subsequently, the company’s Virtual Private Network (VPN) was added to my security duties.

6 Securing Systems
the misuse of the “Internet of Things.” “Infosec”* does this with technical tools that aid
human analysis. These tools are the popularly familiar firewalls, intrusion detection
systems (IDS), network (and other) ACLs, anti-virus and anti-malware protections,
Security Information and Event Managers (SIEM), the whole panoply of software tools
associated with information security. Alongside these are tools that find issues in soft-
ware, such as vulnerability scanners and “static” analysis tools. These scanners are used
as software is written.†
Parallel to the growth in security software, there has been an emerging trend to
codify the techniques and craft used by security professionals. These disciplines have
been called “security engineering,” “security analysis,” “security monitoring,” “security
response,” “security forensics,” and most importantly for this work, “security archi-
tecture.” It is security architecture with which we are primarily concerned. Security
architecture is the discipline charged with integrating into computer systems the
security features and controls that will provide the protection expected of the system
when it is deployed for use. Security architects typically achieve a sufficient breadth of
knowledge and depth of understanding to apply a gamut of security technologies and
processes to protect systems, system interconnections, and the data in use and storage:
Securing Systems.
In fact, nearly twenty years after the publication of NIST-14 (quoted above), organi-
zations large and small—governmental, commercial, and non-profit—prefer that some
sort of a “security review” be conducted upon proposed and/or preproduction systems.
Indeed, many organizations require a security review of systems. Review of systems to
assess and improve system security posture has become a mandate.
Standards such as the NIST 800-53 and ISO 27002, as well as measures of existing
practice, such as the BSIMM-V, all require or measure the maturity of an organiza-
tion’s “architecture risk assessment” (AR A). When taken together, it seems clear that
a security review of one sort or another has become a security “best practice.” That is,
organizations that maintain a cyber-security defense posture typically require some sort
of assessment or analysis of the systems to be used by the organization, whether those
systems are homegrown, purchased, or composite. Ergo, these organizations believe it is
in their best interest to have a security expert, typically called the “security architect.”‡
However “security review” often remains locally defined. Ask one practitioner and
she will tell you that her review consists of post-build vulnerability scanning. Another
answer might be, “We perform a comprehensive attack and penetration on systems
before deployment.” But neither of these responses captures the essence and timing
of, “[S]ecurity requirements should be developed at the same time system planners define
* “Infosec” is a common nickname for an information security department.
† Static analyzers are the security equivalent of the compiler and linker that turn software
source code written in programming languages into executable programs.
‡ Th ough these may be called a “security engineer,” or a “security analyst,” or any number of
similar local variations.

Part I-Introduction 7
the requirements of the system.”4 That is, the “review,” the discovery of “requirements”
is supposed to take place proactively, before a system is completely built! And, in my
experience, for many systems, it is best to gather security requirements at various points
during system development, and at increasing levels of specificity, as the architecture
and design are thought through. The security of a system is best considered just as
all the other attributes and qualities of the system are pulled together . It remains an
on going mistake to leave security to the end of the development cycle.
By the time a large and complex system is ready for deployment, the possibility of
structural change becomes exponentially smaller. If a vulnerability (hole) is found in
the systems logic or that its security controls are incomplete, there is little likelihood
that the issue can or will be repaired before the system begins its useful life. Too much
effort and resources have already been expended. The owners of the system are typically
stuck with what’s been implemented. They owners will most likely bear the residual
risk, at least until some subsequent development cycle, perhaps for the life of the system.
Beyond the lack of definition among practitioners, there is a dearth of skilled secu-
rity architects. The United States Department of Labor estimated in 2013 that there
would be zero unemployment of information security professionals for the foreseeable
future. Demand is high. But there are few programs devoted to the art and practice
of assessing systems. Even calculating the risk of any particular successful attack has
proven a difficult problem, as we shall explore. But risk calculation is only one part of
an assessment. A skilled security architect must bring a wealth of knowledge and under-
standing—global and local, technical, human, organizational, and even geo political—
to an assessment. How does a person get from here to there, from engineer to a security
architect who is capable of a skilled security assessment?
Addressing the skill deficit on performing security “reviews,” or more properly, secu-
rity assessment and analysis, is the object of this work. The analysis must occur while
there is still time to make any required changes. The analyst must have enough infor-
mation and skill to provide requirements and guidance sufficient to meet the security
goals of the owners of the system. That is the goal of this book and these methods, to
deliver the right security at the right time in the implementation lifecycle. In essence,
this book is about addressing pervasive attacks through securing systems.
The Structure of the Book
There are three parts to this book: Parts I, II, and III. Part I presents and then attempts
to explain the practices, knowledge domains, and methods that must be brought to
bear when performing assessments and threat models.
Part II is a series of linked assessments. The assessments are intended to build upon
each other; I have avoided repeating the same analysis and solution set over and over
again. In the real world, unique circumstances and individual treatments exist within
a universe of fairly well known and repeating architecture patterns. Alongside the need

8 Securing Systems
for a certain amount of brevity, I also hope that each assessment may be read by itself,
especially for experienced security architects who are already familiar with the typical,
repeating patterns of their practice. Each assessment adds at least one new architecture
and its corresponding security solutions.
Part III is an abbreviated exploration into building the larger practice encompass-
ing multiple security architects and engineers, multiple stakeholders and teams, and
the need for standards and repeating practices. This section is short; I’ve tried to avoid
repeating the many great books that already explain in great detail a security program.
These usually touch upon an assessment program within the context of a larger com-
puter security practice. Instead, I’ve tried to stay focused on those facets that apply
directly to an applied security architecture practice. There is no doubt that I have left
out many important areas in favor of keeping a tight focus.
I assume that many readers will use the book as a reference for their security archi-
tecture and system risk-assessment practice. I hope that by clearly separating tools and
preparation from analysis, and these from program, it will be easier for readers to find
what they need quickly, whether through the index or by browsing a particular part
or chapter.
In my (very humble) experience, when performing assessments, nothing is as neat
as the organization of any methodology or book. I have to jump from architecture to
attack surface, explain my risk reasoning, only to jump to some previously unexplored
technical detail. Real-world systems can get pretty messy, which is why we impose the
ordering that architecture and, specifically, security architecture provides.
1. Swanson, M. and Guttman B. (September 1996). “Generally Accepted Principles and
Practices for Securing Information Technology Systems.” National Institute of Standards
and Technology, Technology Administration, US Department of Commerce (NIST
800-14, p. 17).
2. Ashton, K. (22 June 2009).  “Th at ‘Internet of Th ings’ Th ing: In the real world things
matter more than ideas.” RFID Journal. Retrieved from http://www.rfi
3. Grossman,  L. K. (1995).  Electronic Republic: Reshaping American Democracy for the
Information Age (A Twentieth Century Fund Book), p. 3. Viking Adult.
4. Swanson, M. and Guttman B. (September 1996). “Generally Accepted Principles and
Practices for Securing Information Technology Systems.” National Institute of Standards
and Technology, Technology Administration, US Department of Commerce (NIST
800-14, p. 17).

Chapter 1
Often when the author is speaking at conferences about the practice of security archi-
tecture, participants repeatedly ask, “How do I get started?” At the present time, there
are few holistic works devoted to the art and the practice of system security assessment.*
Yet despite the paucity of materials, the practice of security assessment is growing
rapidly. The information security industry has gone through a transformation from
reactive approaches such as Intrusion Detection to proactive practices that are embed-
ded into the Secure Development Lifecycle (SDL). Among the practices that are typi-
cally required is a security architecture assessment. Most Fortune 500 companies are
performing some sort of an assessment, at least on critical and major systems.
To meet this demand, there are plenty of consultants who will gladly offer their
expensive services for assessments. But consultants are not typically teachers; they are
not engaged long enough to provide sufficient longitudinal mentorship. Organizations
attempting to build an assessment practice may be stymied if they are using a typi-
cal security consultant. Consultants are rarely geared to explaining what to do. They
usually don’t supply the kind of close relationship that supports long-term training.
Besides, this would be a conflict of interest—the stronger the internal team, the less
they need consultants!
Explaining security architecture assessment has been the province of a few mentors
who are scattered across the security landscape, including the author. Now, therefore,
seems a like a good time to offer a book describing, in detail, how to actually perform a
security assessment, from strategy to threat model, and on through producing security
requirements that can and will get implemented.
* Th ere are numerous works devoted to organizational “security assessment.” But few describe
in any detail the practice of analyzing a system to determine what, if any, security must be
added to it before it is used.

10 Securing Systems
Training to assess has typically been performed through the time-honored system
of mentoring. The prospective security architect follows an experienced practitioner for
some period, hoping to understand what is happening. The mentee observes the mentor
as he or she examines in depth systems’ architectures.
The goal of the analysis is to achieve the desired security posture. How does the
architect factor the architecture into components that are relevant for security analysis?
And, that “desired” posture? How does the assessor know what that posture is? At the
end of the analysis, through some as yet unexplained “magic”—really, the experience
and technical depth of the security architect—requirements are generated that, when
implemented, will bring the system up to the organization’s security requirements. The
author has often been asked by mentees, “How do you know what questions to ask?”
or, “How can you find the security holes so quickly?”
Securing Systems is meant to step into this breach, to fill the gap in training and men-
torship. This book is more than a step-by-step process for performing an analysis. For
instance, this book offers a set of prerequisite knowledge domains that is then brought
into a skilled analysis. What does an assessor need to understand before she or he can
perform an assessment?
Even before assembling the required global and local knowledge set, a security archi-
tect will have command of a number of domains, both within security and without.
Obviously, it’s imperative to have a grasp of typical security technologies and their
application to systems to build the defense. These are typically called “security con-
trols,” which are usually applied in sets intended to build a “defense-in-depth,” that
is, a multilayered set of security controls that, when put together, complement each
other as well as provide some protection against the failure of each particular control.
In addition, skilled security architects usually have at least some grounding in system
architecture—the practice of defining the structure of large-scale systems. How can
one decompose an architecture sufficiently to provide security wisdom if one cannot
understand the architecture itself? Implicit in the practice of security architecture is
a grasp of the process by which an architect arrives at an architecture, a firm grasp
on how system structures are designed. Typically, security architects have significant
experience in designing various types of computer systems.
And then there is the ongoing problem of calculating information security risk.
Despite recent advances in understanding, the industry remains largely dependent upon
expert opinion. Those opinions can be normalized so that they are comparable. Still, we,
the security industry, are a long way from hard, mathematically repeatable calculations.
How does the architect come to an understanding whereby her or his risk “calculation”
is more or less consistent and, most importantly, trustworthy by decision makers?
This book covers all of these knowledge domains and more. Included will be the
author’s tips and tricks. Some of these tips will, by the nature of the work, be technical.
Still, complex systems are built by teams of highly skilled professionals, usually cross-
ing numerous domain and organizational boundaries. In order to secure those systems,
the skilled security architect must not alienate those who have to perform the work or

Introduction 11
who may have a “no” vote on requirements. Accumulated through the “hard dint” of
experience, this book will offer tricks of the trade to cement relationships and to work
with inevitable resistance, the conflict that seems to predictably arise among teams with
different viewpoints and considerations who must come to definite agreements.
There is no promise that reading this book will turn the reader into a skilled security
architect. However, every technique explained here has been practiced by the author
and, at least in my hands, has a proven track record. Beyond that endorsement, I have
personally trained dozens of architects in these techniques. These architects have then
taught the same techniques and approaches down through several generations of archi-
tecture practice. And, indeed, these techniques have been used to assess the security of
literally thousands of individual projects, to build living threat models, and to provide
sets of security requirements that actually get implemented. A few of these systems have
resisted ongoing attack through many years of exposure; their architectures have been
canonized into industry standards.*
My promise to the reader is that there is enough information presented here to
get one started. Those who’ve been tasked for the first time with the security assess-
ment of systems will find hard answers about what to learn and what to do. For the
practitioner, there are specific techniques that you can apply in your practice. These
techniques are not solely theoretical, like, “programs should . . .” And they aren’t just
“ivory tower” pronouncements. Rather, these techniques consist of real approaches that
have delivered results on real systems. For assessment program managers, I’ve provided
hints along the way about successful programs in which I’ve been involved, including
a final chapter on building a program. And for the expert, perhaps I can, at the very
least, spark constructive discussion about what we do and how we do it? If something
that I’ve presented here can seed improvement to the practice of security architecture in
some significant way, such an advance would be a major gift.
1.1 Breach! Fix It!
Advances in information security have been repeatedly driven by spectacular attacks
and by the evolutionary advances of the attackers. In fact, many organizations don’t
really empower and support their security programs until there’s been an incident. It is
a truism among security practitioners to consider a compromise or breach as an “oppor-
tunity.” Suddenly, decision makers are paying attention. The wise practitioner makes
use of this momentary attention to address the weaker areas in the extant program.
For example, for years, the web application security team on which I worked, though
reasonably staffed, endured a climate in which mid-level management “accepted” risks,
that is, vulnerabilities in the software, rather than fix them. In fact, a portfolio of
* Most notably, the Cisco SAFE eCommerce architecture closely models Cisco’s external web
architecture, to which descendant architects and I contributed.

12 Securing Systems
thousands of applications had been largely untested for vulnerabilities. A vulnerability
scanning pilot revealed that every application tested had issues. The security “debt,”
that is, an unaddressed set of issues, grew to be much greater than the state of the art
could address. The period for detailed assessment grew to be estimated in multiple
years. The application portfolio became a tower of vulnerable cards, an incident waiting
to happen. The security team understood this full well.
This sad state of affairs came through a habit of accepting risk rather than treating
it. The team charged with the security of the portfolio was dispirited and demoralized.
They lost many negotiations about security requirements. It was difficult to achieve
security success against the juggernaut of manage ment unwilling to address the mount-
ing problem.
Then, a major public hack occurred.
The password file for millions of customers was stolen through the front end of a
web site pulling in 90% of a multi-billion dollar revenue stream. The attack was suc-
cessful through a vector that had been identified years before by the security team. The
risk had been accepted by corporate IT due to operational and legacy demands. IT
didn’t want to upset the management who owned the applications in the environments.
Immediately, that security team received more attention, first negative, then con-
structive. The improved program that is still running successfully 10 years later was
built out on top of all this senior management attention. So far as I know, that company
has not endured another issue of that magnitude through its web systems. The loss of
the password file turned into a powerful imperative for improvement.
Brad Arkin, CSO for Adobe Systems, has said, “Never waste a crisis.”1 Savvy secu-
rity folk leverage significant incidents for revolutionary changes. For this reason, it
seems that these sea changes are a direct result, even driven out of, successful attacks.
Basically, security leaders are told, “There’s been a breach. Fix it!” Once into a “fix it”
cycle, a program is much more likely to receive the resource expansions, programmatic
changes, and tool purchases that may be required.
In parallel, security technology makers are continually responding to new attack
methods. Antivirus, anti-malware, next-generation firewall, and similar vendors contin-
ually update the “signatures,” the identifying attributes, of malicious software, and usu-
ally very rapidly, as close to “real-time” as they are able. However, it is my understanding
that new variations run in the hundreds every single day; there are hundreds of millions
of unique, malicious software samples in existence as of this writing. Volumes of this
magnitude are a maintenance nightmare requiring significant investment in automation
in order to simply to keep track, much less build new defenses. Any system that handles
file movements is going to be handling malicious pieces of software at some point, per-
haps constantly exposed to malicious files, depending upon the purpose of the system.
Beyond sheer volume, attackers have become ever more sophisticated. It is not
unusual for an Advanced Persistent Attack (APT) to take months or even years to plan,
build, disseminate, and then to execute. One well-known attack described to the author
involved site visits six months before the actual attack, two diversionary probes in parallel

Introduction 13
to the actual data theft, the actual theft being carried out over a period of days and per-
haps involving an attack team staying in a hotel near the physical attack site. Clever
name-resolution schemes such as fast-flux switching allow attackers to efficiently hide
their identities without cost. It’s a dangerous cyber world out there on the Internet today.
The chance of an attempted attack of one kind or another is certain. The probability
of a web attack is 100%; systems are being attacked and will be attacked regularly and
continually. Most of those attacks will be “door rattling,” reconnaissance probes and well-
known, easily defended exploit methods. But out of the fifty million attacks each week
that most major web sites must endure, something like one or two within the mountain
of attack events will likely be highly sophisticated and tightly targeted at that particular
set of systems. And the probability of a targeted attack goes up exponentially when the
web systems employ well-known operating systems and execution environments.
Even though calculating an actual risk in dollars lost per year is fairly difficult, we
do know that Internet system designers can count on being attacked, period. And these
attacks may begin fairly rapidly upon deployment.
There’s an information security saying, “the defender must plug all the holes. The
attacker only needs to exploit a single vulnerability to be successful.” This is an over-
simplification, as most successful data thefts employ two or more vulnerabilities strung
together, often across multiple systems or components.
Indeed, system complexity leads to increasing the difficulty of defense and, inversely,
decreasing the difficulty of successful exploitation. The number of flows between sys-
tems can turn into what architects call, “spaghetti,” a seeming lack of order and regu-
larity in the design. Every component within the system calls every other component,
perhaps through multiple flows, in a disorderly matrix of calls. I have seen complex
systems from major vendors that do exactly this. In a system composed of only six
components, that gives 62=36 separate flows (or more!). Missing appropriate security
on just one of these flows might allow an attacker a significant possibility to gain a
foothold within the trust boundaries of the entire system. If each component blindly
trusts every other component, let’s say, because the system designers assumed that the
surrounding network would provide enough protection, then that foothold can easily
allow the attacker to own the entire system. And, trusted systems make excellent beach
heads from which to launch attacks at other systems on a complex enterprise network.
Game over. Defenders 0, attacker everything.
Hence, standard upon standard require organizations to meet the challenge through
building security into systems from the very start of the architecture and then on through
design. It is this practice that we will address.
• When should the architect begin the analysis?
• At what points can a security architect add the most value?
• What are the activities the architect must execute?
• How are these activities delivered?
• What is the set of knowledge domains applied to the analysis?

14 Securing Systems
• What are the outputs?
• What are the tips and tricks that make security architecture risk assessment easier?
If a breach or significant compromise and loss creates an opportunity, then that
opportunity quite often is to build a security architecture practice. A major part or focus
of that maturing security architecture practice will be the assessment of systems for the
purpose of assuring that when deployed, the assessed systems contain appropriate secu-
rity qualities and controls.
• Sensitive data will be protected in storage, transmission, and processing.
• Sensitive access will be controlled (need-to-know, authentication, and
• Defenses will be appropriately redundant and layered to account for failure.
• There will be no single point of failure in the controls.
• Systems are maintained in such a way that they remain available for use.
• Activity will be monitored for attack patterns and failures.
1.2 Information Security, as Applied to Systems
One definition of security architecture might be, “applied information security.” Or
perhaps, more to the point of this work, security architecture applies the principles of
security to system architectures. It should be noted that there are (at least) two uses of
the term, “security architecture.” One of these is, as defined above, to ensure that the
correct security features, controls, and properties are included into an organization’s
digital systems and to help implement these through the practice of system architecture.
The other branch, or common usage, of “security architecture” is the architecture of
the security systems of an organization. In the absence of the order provided through
architecture, organizations tend to implement various security technologies “helter-
skelter,” that is, ad hoc. Without security architecture, the intrusion system (IDS) might
be distinct and independent from the firewalls (perimeter). Firewalls and IDS would
then be unconnected and independent from anti-virus and anti-malware on the end-
point systems and entirely independent of server protections. The security architect first
uncovers the intentions and security needs of the organization: open and trusting or
tightly controlled, the data sensitivities, and so forth. Then, the desired security posture
(as it’s called) is applied through a collection of coordinated security technologies. This
can be accomplished very intentionally when the architect has sufficient time to strate-
gize before architecting, then to architect to feed a design, and to have a sound design
to support implementation and deployment.*
* Of course, most security architects inherit an existing set of technologies. If these have grown
up piecemeal over a signifi cant period of time, there will be considerable legacy that hasn’t
been architected with which to contend. Th is is the far more common case.

Introduction 15
[I]nformation security solutions are often designed, acquired and installed on a tactical
basis. . . . [T]here is no strategy that can be identifiably said to support the goals of
the business. An approach that avoids these piecemeal problems is the development
of an enterprise security architecture which is business-driven and which describes a
structured inter-relationship between the technical and procedural solutions to support
the long-term needs of the business.2
Going a step further, the security architect who is primarily concerned with deploy-
ing security technologies will look for synergies between technologies such that the sum
of the controls is greater than any single control or technology. And, there are products
whose purpose is to enhance synergies. The purpose of the security information and
event management (SIEM) products is precisely this kind of synergy between the event
and alert flows of disparate security products. Depending upon needs, this is exactly the
sort of synergistic view of security activity that a security architect will try to enhance
through a security architecture (this second branch of the practice). The basic question
the security architect implementing security systems asks is, “How can I achieve the
security posture desired by the organization through a security infrastructure, given
time, money, and technology restraints.”
Contrast the foregoing with the security architect whose task it is to build security
into systems whose function has nothing to do with information security. The security
architecture of any system depends upon and consumes whatever security systems have
been put into place by the organization. Oftentimes, the security architecture of non-
security systems assumes the capabilities of those security systems that have been put into
place. The systems that implement security systems are among the tools that the system
security architect will employ, the “palette” from which she or he draws, as systems are
analyzed and security requirements are uncovered through the analysis. You may think
of the security architect concerned with security systems, the designer of security systems,
as responsible for the coherence of the security infrastructure. The architect concerned
with non-security systems will be utilizing the security infrastructure in order to add
security into or underneath the other systems that will get deployed by the organization.
In smaller organizations, there may be no actual distinction between these two roles:
the security architect will design security systems and will analyze the organization’s
other systems in light of the security infrastructure. The two, systems and security sys-
tems, are intimately linked and, typically, tightly coupled. Indeed, as stated previously,
at least a portion of the security infrastructure will usually provide security services
such as authentication and event monitoring for the other systems. And, firewalls and
the like will provide protections that surround the non-security systems.
Ultimately, the available security infrastructure gives rise to an organization’s tech-
nical standards. Although an organization might attempt to create standards and then
build an infrastructure to those standards, the dictates of resources, technology, skill,
and other constraints will limit “ivory tower” standards; very probably, the ensuing
infrastructure will diverge significantly from standards that presume a perfect world
and unlimited resources.

16 Securing Systems
When standards do not match what can actually be achieved, the standards become
empty ideals. In such a case, engineers’ confidence will be shaken; system project teams
are quite likely to ignore standards, or make up their own. Security personnel will lose
considerable influence. Therefore, as we shall see, it’s important that standards match
capabilities closely, even when the capabilities are limited. In this way, all participants
in the system security process will have more confidence in analysis and requirements.
Delivering ivory tower, unrealistic requirements is a serious error that must be avoided.
Decision makers need to understand precisely what protections can be put into place
and have a good understanding of any residual, unprotected risks that remain.
From the foregoing, it should be obvious that the two concentrations within security
architecture work closely together when these are not the same person. When the roles
are separate disciplines, the architect concerned with the infrastructure must under-
stand what other systems will require, the desired security posture, perimeter protec-
tions, and security services. The architect who assesses the non-security systems must
have a very deep and thorough understanding of the security infrastructure such that
these services can be applied appropriately. I don’t want to over specify. If an infrastruc-
ture provides strong perimeter controls (firewalls), there is no need to duplicate those
controls locally. However, the firewalls may have to be updated for new system bound-
aries and inter-trust zone communications.
In other words, these two branches of security architecture work very closely together
and may even be fulfilled by the same individual.
No matter how the roles are divided or consolidated, the art of security analysis of a
system architecture is the art of applying the principles of information security to that
system architecture. A set of background knowledge domains is applied to an architec-
ture for the purpose of discovery. The idea is to uncover points of likely attack: “attack
surfaces.” The attack surfaces are analyzed with respect to active threats that have the
capabilities to exercise the attack surfaces. Further, these threats must have access in
order to apply their capabilities to the attack surfaces. And the attack surfaces must
present a weakness that can be exploited by the attacker, which is known as a “vulner-
ability.” This weakness will have some kind of impact, either to the organization or to
the system. The impact may be anywhere from high to low.
We will delve into each of these components later in the book. When all the requisite
components of an attack come together, a “credible attack vector” has been discovered.
It is possible in the architecture that there are security controls that protect against the
exercise of a credible attack vector. The combination of attack vector and mitigation
indicates the risk of exploitation of the attack vector. Each attack vector is paired to
existing (or proposed) security controls. If the risk is low enough after application of the
mitigation, then that credible attack vector will receive a low risk. Those attack vectors
with a significant impact are then prioritized.
The enumeration of the credible attack vectors, their impacts, and their mitigations
can be said to be a “threat model,” which is simply the set of credible attack vectors and
their prioritized risk rating.

Introduction 17
Since there is no such thing as perfect security, nor are there typically unlimited
resources for security, the risk rating of credible attack vectors allows the security archi-
tect to focus on meaningful and significant risks.
Securing systems is the art and craft of applying information security principles,
design imperatives, and available controls in order to achieve a particular security
posture. The analyst must have a firm grasp of basic computer security objectives for
confidentiality, integrity, and availability, commonly referred to as “CIA.” Computer
security has been described in terms of CIA. These are the attributes that will result
from appropriate security “controls.” “Controls” are those functions that help to provide
some assurance that data will only be seen or handled by those allowed access, that data
will remain or arrive intact as saved or sent, and that a particular system will continue
to deliver its functionality. Some examples of security controls would be authentication,
authorization, and network restrictions. A system-monitoring function may provide
some security functionality, allowing the monitoring staff to react to apparent attacks.
Even validation of user inputs into a program may be one of the key controls in a sys-
tem, preventing misuse of data handling procedures for the attacker’s purposes.
The first necessity for secure software is specifications that define secure behavior
exhibiting the security properties required. The specifications must define functionality
and be free of vulnerabilities that can be exploited by intruders. The second necessity
for secure software is correct implementation meeting specifications. Software is correct
if it exhibits only the behavior defined by its specification – not, as today is often the
case, exploitable behavior not specified, or even known to its developers and testers.3
The process that we are describing is the first “necessity” quoted above, from the
work of Redwine and Davis* (2004)3: “specifications that define secure behavior exhibit-
ing the security properties required.” Architecture risk assessment (AR A) and threat
modeling is intended to deliver these specifications such that the system architecture
and design includes properties that describe the system’s security. We will explore the
architectural component of this in Chapter 3.
The assurance that the implementation is correct—that the security properties have
been built as specified and actually protect the system and that vulnerabilities have
not been introduced—is a function of many factors. That is, this is the second “neces-
sity” given above by Redwine and David (2004).3 These factors must be embedded
into processes, into behaviors of the system implementers, and for which the system
is tested. Indeed, a fair description of my current thinking on a secure development
lifecycle (SDL) can be found in Core Software Security: Security at the Source, Chapter 9
(of which I’m the contributing author), and is greatly expanded within the entire book,
written by Dr. James Ransome and Anmol Misra.4 Architecture analysis for security fits
within a mature SDL. Security assessment will be far less effective standing alone, with-
* With whom I’ve had the privilege to work.

18 Securing Systems
out all the other activities of a mature and holistic SDL or secure project development
lifecycle. However, a broad discussion of the practices that lead to assurance of imple-
mentation is not within the scope of this work. Together, we will limit our explora tion
to AR A and threat modeling, solely, rather than attempting cover an entire SDL.
A suite of controls implemented for a system becomes that system’s defense. If well
designed, these become a “defense-in-depth,” a set of overlapping and somewhat redun-
dant controls. Because, of course, things fail. One security “principle” is that no single
control can be counted upon to be inviolable. Everything may fail. Single points of
failure are potentially vulnerable.
I drafted the following security principles for the enterprise architecture practice of
Cisco Systems, Inc. We architected our systems to these guidelines.
1. Risk Management: We strive to manage our risk to acceptable business levels.
2. Defense-in-Depth: No one solution alone will provide sufficient risk mitigation.
Always assume that every security control will fail.
3. No Safe Environment: We do not assume that the internal network or that any
environment is “secure” or “safe.” Wherever risk is too great, security must be
4. CIA: Security controls work to provide some acceptable amount of Confidential-
ity, Integrity, and/or Availability of data (CIA).
5. Ease Security Burden: Security controls should be designed so that doing the
secure thing is the path of least resistance. Make it easy to be secure, make it easy
to do the right thing.
6. Industry Standard: Whenever possible, follow industry standard security
7. Secure the Infrastructure: Provide security controls for developers not by them.
As much as possible, put security controls into the infrastructure. Developers
should develop business logic, not security, wherever possible.
The foregoing principles were used* as intentions and directions for architecting
and design. As we examined systems falling within Cisco’s IT development process, we
applied specific security requirements in order to achieve the goals outlined through
these principles. Requirements were not only technical; gaps in technology might
be filled through processes, and staffing might be required in order to carry out the
processes and build the needed technology. We drove toward our security principles
through the application of “people, process, and technology.” It is difficult to architect
without knowing what goals, even ideals, one is attempting to achieve. Principles help
* Th ese principles are still in use by Enterprise Architecture at Cisco Systems, Inc., though they
have gone through several revisions. National Cyber Security Award winner Michele Guel
and Security Architect Steve Acheson are coauthors of these principles.

Introduction 19
to consider goals as one analyzes a system for its security: The principles are the proper-
ties that the security is supposed to deliver.
These principles (or any similar very high level guidance) may seem like they are
too general to help? But experience taught me that once we had these principles firmly
communicated and agreed upon by most, if not all, of the architecture community,
discussions about security requirements were much more fruitful. The other archi-
tects had a firmer grasp on precisely why security architects had placed particular
requirements on a system. And, the principles helped security architects remember to
analyze more holistically, more thoroughly, for all the intentions encapsulated within
the principles.
ARAs are a security, “rubber meets the road” activity. The following is a generic state-
ment about what the practice of information security is about, a definition, if you will.
Information assurance is achieved when information and information systems are
protected against attacks through the application of security services such as availability,
integrity, authentication, confidentiality, and nonrepudiation. The application of these
services should be based on the protect, detect, and react paradigm. This means that in
addition to incorporating protection mechanisms, organizations need to expect attacks
and include attack detection tools and procedures that allow them to react to and
recover from these unexpected attacks.5
This book is not a primer in information security. It is assumed that the reader
has at least a glancing familiarity with CIA and the paradigm, “protect, detect, react,”
as described in the quote above. If not, then perhaps it might be of some use to take
a look at an introduction to computer security before proceeding? It is precisely this
paradigm whereby:
• Security controls are in-built to protect a system.
• Monitoring systems are created to detect attacks.
• Teams are empowered to react to attacks.
The Open Web Application Security Project (OWASP) provides a distillation of
several of the most well known sets of computer security principles:
ο Apply defense-in-depth (complete mediation).
ο Use a positive security model (fail-safe defaults, minimize attack surface).
ο Fail securely.
ο Run with least privilege.
ο Avoid security by obscurity (open design).
ο Keep security simple (verifiable, economy of mechanism).
ο Detect intrusions (compromise recording).
ο Don’t trust infrastructure.

20 Securing Systems
ο Don’t trust services.
ο Establish secure defaults6
Some of these principles imply a set of controls (e.g., access controls and privilege
sets). Many of these controls, such as “Avoid security by obscurity” and “Keep secu-
rity simple,” are guides to be applied during design, approaches rather than specific
demands to be applied to a system. When assessing a system, the assessor examines for
attack surfaces, then applies specific controls (technologies, processes, etc.) to realize
these principles.
These principles (and those like the ones quoted) are the tools of computer security
architecture. Principles comprise the palette of techniques that will be applied to sys-
tems in order to achieve the desired security posture. The prescribed requirements fill
in the three steps enumerated above:
• Protect a system through purpose-built security controls.
• Attempt to detect attacks with security-specific monitors.
• React to any attacks that are detected.
In other words, securing systems is the application of the processes, technologies,
and people that “protect, detect, and react” to systems. Securing systems is essentially
applied information security. Combining computer security with information security
risk comprises the core of the work.
The output of this “application of security to a system” is typically security “require-
ments.” There may also be “nice-to-have” guidance statements that may or may not
be implemented. However, there is a strong reason to use the word “requirement.”
Failure to implement appropriate security measures may very well put the survival of
the organi zation at risk.
Typically, security professionals are assigned a “due diligence” responsibility to
prevent disastrous events. There’s a “buck stops here” part of the practice: Untreated
risk must never be ignored. That doesn’t mean that security’s solution will be adopted.
What it does mean is that the security architect must either mitigate information
security risks to an acceptable, known level or make the appropriate decision maker
aware that there is residual risk that either cannot be mitigated or has not been miti-
gated sufficiently.
Just as a responsible doctor must follow a protocol that examines the whole health
of the patient, rather than only treating the presenting problem, so too must the secu-
rity architect thoroughly examine the “patient,” any system under analysis, for “vital
signs”—that is, security health.
The requirements output from the analysis are the collection of additions to the sys-
tem that will keep the system healthy as it endures whatever level of attack is predicted
for its deployment and use. Requirements must be implemented or there is residual risk.
Residual risk must be recognized because of due diligence responsibility. Hence, if the

Introduction 21
analysis uncovers untreated risk, the output of that analysis is the necessity to bring the
security posture up and risk down to acceptable levels. Thus, risk practice and architec-
ture analysis must go hand-in-hand.
So, hopefully, it is clear that a system is risk analyzed in order to determine how
to apply security to the system appropriately. We then can define Architecture Risk
Analysis (AR A) as the process of uncovering system security risks and applying infor-
mation security techniques to the system to mitigate the risks that have been discovered.
1.3 Applying Security to Any System
This book describes a process whereby a security architect analyzes a system for its
security needs, a process that is designed to uncover the security needs for the system.
Some of those security needs will be provided by an existing security infrastructure.
Some of the features that have been specified through the analysis will be services
consumed from the security infrastructure. And there may be features that need to be
built solely for the system at hand. There may be controls that are specific to the system
that has been analyzed. These will have to be built into the system itself or added to
the security architecture, depending upon whether these features, controls, or services
will be used only by this system, or whether future systems will also make use of these.
A typical progression of security maturity is to start by building one-off security
features into systems during system implementation. During the early periods, there
may be only one critical system that has any security requirements! It will be easier
and cheaper to simply build the required security services as a part of the system as it’s
being implemented. As time goes on, perhaps as business expands into new territories
or different products, there will be a need for common architectures, if for no other
reason than maintainability and shared cost. It is typically at this point that a security
infrastructure comes into being that supports at least some of the common security
needs for many systems to consume. It is characteristically a virtue to keep complexity
to a minimum and to reap scales of economy.
Besides, it’s easier to build and run a single security service than to maintain many
different ones whose function is more or less the same. Consider storage of credentials
(passwords and similar).
Maintaining multiple disparate stores of credentials requires each of these to be held
at stringent levels of security control. Local variations of one of the stores may lower
the overall security posture protecting all credentials, perhaps enabling a loss of these
sensitive tokens through attack, whereas maintaining a single repository at a very high
level, through a select set of highly trained and skilled administrators (with carefully
controlled boundaries and flows) will be far easier and cheaper. Security can be held at
a consistently high level that can be monitored more easily; the security events will be
consistent, allowing automation rules to be implemented for raising any alarms. And
so forth.

22 Securing Systems
An additional value from a single authentication and credential storing service is
likely to be that users may be much happier in that they have only a single password to
remember! Of course, once all the passwords are kept in a single repository, there may
be a single point of failure. This will have to be carefully considered. Such considera-
tions are precisely what security architects are supposed to provide to the organization.
It is the application of security principles and capabilities that is the province and
domain of security architecture as applied to systems.
The first problem that must be overcome is one of discovery.
• What risks are the organization’s decision makers willing to undertake?
• What security capabilities exist?
• Who will attack these types of systems, why, and to attain what goals?
Without the answers to these formative questions, any analysis must either treat
every possible attack as equally dangerous, or miss accounting for something impor-
tant. In a world of unlimited resources, perhaps locking everything down completely
may be possible. But I haven’t yet worked at that organization; I don’t practice in that
world. Ultimately, the goal of a security analysis isn’t perfection. The goal is to imple-
ment just enough security to achieve the surety desired and to allow the organization to
take those risks for which the organization is prepared. It must always be remembered
that there is no usable perfect security.
A long-time joke among information security practitioners remains that all that’s
required to secure a system is to disconnect the system, turn the system off, lock it into
a closet, and throw away the key. But of course, this approach disallows all purposeful
use of the system. A connected, running system in purposeful use is already exposed
to a certain amount of risk. One cannot dodge taking risks, especially in the realm of
computer security. The point is to take those risks that can be borne and avoid those
which cannot. This is why the first task is to find out how much security is “enough.”
Only with this information in hand can any assessment and prescription take place.
Erring on the side of too much security may seem safer, more reasonable. But, secu-
rity is expensive. Taken among the many things to which any organi zation must attend,
security is important but typically must compete with a host of other organizational
priorities. Of course, some organizations will choose to give their computer security
primacy. That is what this investigation is intended to uncover.
Beyond the security posture that will further organizational goals, an inventory of
what security has been implemented, what weaknesses and limitations exist, and what
security costs must be borne by each system is critical.
Years ago, when I was just learning system assessment, I was told that every applica-
tion in the application server farm creating a Secure Sockets Layer (SSL)* tunnel was
required to implement bi directional, SSL certificate authentication. Such a connection
* Th is was before the standard became Transport Layer Security (TLS).

Introduction 23
presumes that at the point at which the SSL is terminated on the answering (server)
end, the SSL “stack,” implementing software, will be tightly coupled, usually even con-
trolled by the application that is providing functionality over the SSL tunnel. In the
SSL authentication exchange, first, the server (listener) certificate is authenticated by
the client (caller). Then, the client must respond with its certificate to be authenticated
by the server. Where many different and disparate, logically separated applications
coexist on the same servers, each application would then have to be listening for its own
SSL connections. You typically shouldn’t share a single authenticator across all of the
applications. Each application must have its own certificate. In this way, each authen-
tication will be tied to the relevant application. Coupling authenticator to application
then provides robust, multi-tenant application authentication.
I dutifully provided a requirement to the first three applications that I analyzed
to use bidirectional, SSL authentication. I was told to require this. I simply passed
the requirement to project teams when encountering a need for SSL. Case closed?
Unfortunately not.
I didn’t bother to investigate how SSL was terminated for our application server farms.
SSL was not terminated at the application, at the application server software, or even
at the operating system upon which each server was running. SSL was terminated on
a huge, specialized SSL adjunct to the bank of network switches that routed network
traffic to the server farm. The receiving switch passed all SSL to the adjunct, which
terminated the connection and then passed the normal (not encrypted SSL) connection
request onwards to the application servers.
The key here is that this architecture separated the network details from the applica-
tion details. And further and most importantly, SSL termination was quite a distance
(in an application sense) from any notion of application. There was no coupling whatso-
ever between application and SSL termination. That is, SSL termination was entirely
independent from the server-side entities (applications), which must offer the connect-
ing client an authentication certificate. The point being that the infrastructure had
designed “out” and had not accounted for a need for application entities to have indivi-
dual SSL certificate authenticators. The three applications couldn’t “get there from
here”; there was no capability to implement bidirectional SSL authentication. I had
given each of these project teams a requirement that couldn’t be accomplished without
an entire redesign of a multi-million dollar infrastructure. Oops!
Before rushing full steam ahead into the analysis of any system, the security architect
must be sure of what can be implemented and what cannot, what has been designed
into the security infrastructure, and what has been designed out of it. There are usually
at least a few different ways to “skin” a security problem, a few different approaches
that can be applied. Some of the approaches will be possible and some difficult or
even impossible, just as my directive to implement bidirectional SSL authentication
was impossible given the existing infrastructure for those particular server farms and
networks. No matter how good a security idea may seem on the face of it, it is illusory if
it cannot be made real, given the limits of what exists or accounting for what can be put

24 Securing Systems
into place. I prefer never to assume; time spent understanding existing security infra-
structure is always time well spent. This will save a lot of time for everyone involved.
Some security problems cannot be solved without a thorough understanding of the
existing infrastructure.
Almost every type and size of a system will have some security needs. Although it
may be argued that a throw-away utility, written to solve a singular problem, might not
have any security needs, if that utility finds a useful place beyond its original problem
scope, the utility is likely to develop security needs at some point. Think about how
many of the UNIX command line programs gather a password from the user. Perhaps
many of these utilities were written without the need to prompt for the user’s creden-
tials and subsequently to perform an authentication on the user’s behalf? Still, many of
these utilities do so today. And authentication is just one security aspect out of many
that UNIX system utilities perform. In other words, over time, many applications will
eventually grapple with one or more security issues.
Complex business systems typically have security requirements up front. In addi-
tion, either the implementing organization or the users of the system or both will have
security expectations of the system. But complexity is not the determiner of security.
Consider a small program whose sole purpose is to catch central processing unit (CPU)
memory faults. If this software is used for debugging, it will probably have to, at the
very least, build in access controls, especially if the software allows more than one user
at a time (multiuser). Alternatively, if the software catches the memory faults as a part
of a security system preventing misuse of the system through promulgation of memory
faults, preventing say, a privilege escalation through an executing program via a mem-
ory fault, then this small program will have to be self-protective such that attackers
cannot turn it off, remove it, or subvert its function. Such a security program must not,
under any circumstances, open a new vector of attack. Such a program will be targeted
by sophisticated attackers if the program achieves any kind of broad distribution.
Thus, the answer as to whether a system requires an AR A and threat model is tied
to the answers to a number of key questions:
• What is the expected deployment model?
• What will be the distribution?
• What language and execution environment will run the code?
• On what operating system(s) will the executables run?
These questions are placed against probable attackers, attack methods, network
exposures, and so on. And, of course, as stated above, the security needs of the organi-
zation and users must be factored against these.
The answer to whether a system will benefit from an AR A/Threat model is a func-
tion of the dimensions outlined above, and perhaps others, depending upon consider-
ation of those domains on which analysis is dependent. The assessment preprocess or
triage will be outlined in a subsequent chapter. The simple answer to “which systems?”

Introduction 25
is any size, shape, complexity, but certainly not all systems. A part of the art of the secu-
rity architecture assessment is deciding which systems must be analyzed, which will
benefit, and which may pass. That is, unless in your practice you have unlimited time
and resources. I’ve never had this luxury. Most importantly, even the smallest applica-
tion may open a vulnerability, an attack vector, into a shared environment.
Unless every application and its side effects are safely isolated from every other appli-
cation, each set of code can have effects upon the security posture of the whole. This is
particularly true in shared environments. Even an application destined for an endpoint
(a Microsoft Windows™ application, for instance) can contain a buffer overflow that
allows an attacker an opportunity, perhaps, to execute code of the attacker’s choosing.
In other words, an application doesn’t have to be destined for a large, shared server farm
in order to affect the security of its environment. Hence, a significant step that we will
explore is the security triage assessment of the need for analysis.
Size, business criticality, expenses, and complexity, among others, are dimensions
that may have a bearing, but are not solely deterministic. I have seen many Enterprise
IT efforts fail, simply because there was an attempt to reduce this early decision to a
two-dimensional space, yes/no questions. These simplifications invariably attempted to
achieve efficiencies at scale. Unfortunately, in practice today, the decision to analyze the
architecture of a system for security is a complex, multivariate problem. That is why this
decision will have its own section in this book. It takes experience (and usually more
than a few mistakes) to ask appropriate determining questions that are relevant to the
system under discussion.
The answer to “Systems? Which systems?” cannot be overly simplified. Depending
upon use cases and intentions, analyzing almost any system may produce significant
security return on time invested. And, concomitantly, in a world of limited resources,
some systems and, certainly, certain types of system changes may be passed without
review. The organization may be willing to accept a certain amount of unknown risk as
a result of not conducting a review.
1. Arkin, B. (2012). “Never Waste a Crisis – Necessity Drives Software Security.” RSA Conference
2012, San Francisco, CA, February 29, 2012. Retrieved from http://www.rsaconference.
2. Sherwood, J., Clark, A., and Lynas, D. “Enterprise Security Architecture.” SABSA White
Paper, SABSA Limited, 1995–2009. Retrieved from
members/sites/default/inline-fi les/SABSA_White_Paper .
3. Redwine, S. T., Jr., and Davis, N., eds. (2004). “Processes to Produce Secure Software:
Towards more Secure Software.” Software Process Subgroup, Task Force on Security across
the Software Development Lifecycle, National Cyber Security Summit, March 2004.
4. Ransome, J. and Misra, A. (2014). Core Software Security: Security at the Source. Boca
Raton (FL): CRC Press.

26 Securing Systems
5. NSA. “Defense in Depth: A practical strategy for achieving Information Assurance
in today’s highly networked environments.” National Security Agency, Information
Assurance Solutions Group – STE 6737. Available from: les/
support/defenseindepth .
6. Open Web Application Security Project (OWASP) (2013). Some Proven Application
Security Principles. Retrieved from

Chapter 2
The Art of Security
Despite the fact that general computer engineering is taught as a “science,” there is a gap
between what can be engineered in computer security and what remains, as of this writ-
ing, as “art.” Certainly, it can be argued that configuring Access Control Lists (ACL) is
an engineering activity. Cold hard logic is employed to generate linear steps that must
flow precisely and correctly to form a network router’s ACL. Each ACL rule must lie in
precisely the correct place so as not to disturb the functioning of the other rules. There
is a definite and repeatable order in the rule set. What is known as the “default deny”
rule must be at the very end of the list of rules. For some of the rules’ ordering, there is
very little slippage room, and sometimes absolutely no wiggle room as to where the rule
must be placed within the set. Certain rules must absolutely follow other rules in order
for the entire list to function as designed.
Definition of “engineering”:
The branch of science and technology concerned with the design, building, and use of
engines, machines, and structures.1
Like an ACL list, the configuration of alerts in a security monitoring system, the
use of a cryptographic function to protect credentials, and the handling of the crypto-
graphic keying material are all engineering tasks. There are specific demands that must
be met in design and implementation. This is engineering. Certainly, a great deal in
computer security can be described as engineering.
There is no doubt that the study of engineering requires a significant investment in
time and effort. I do not mean to suggest otherwise. In order to construct an effective

28 Securing Systems
ACL, a security engineer must understand network routing, TCP/IP, the assignment
and use of network ports for application functions, and perhaps even some aspects and
details of the network protocols that will be allowed or blocked. Alongside this general
knowledge of networking, a strong understanding of basic network security is essential.
And, a thorough knowledge of the configuration language that controls options for the
router or firewall on which the rule set will be applied is also essential. This is a con-
siderable and specific knowledge set. In large and/or security-conscious organizations,
typically only experts in all of these domains are allowed to set up and maintain the
ACL lists on the organization’s networking equipment.
Each of these domains follows very specific rules. These rules are deterministic;
most if not all of the behaviors can be described with Boolean logic. Commands must
be entered precisely; command-line interpreters are notoriously unforgiving. Hence,
hopefully, few will disagree that writing ACLs is an engineering function.
2.1 Why Art and Not Engineering?
In contrast, a security architect must use her or his understanding of the currently active
threat agents in order to apply these appropriately to a particular system. Whether a
particular threat agent will aim at a particular system is as much a matter of under-
standing, knowledge, and experience as it is cold hard fact.* Applying threat agents and
their capabilities to any particular system is an essential activity within the art of threat
modeling. Hence, a security assessment of an architecture is an act of craft.
Craftsmen know the ways of the substances they use. They watch. Perception and
systematic thinking combine to formulate understanding.2
Generally, effective security architects have a strong computer engineering back-
ground. Without the knowledge of how systems are configured and deployed, and
without a broad understanding of attack methods—maybe even a vast array of attack
methods and their application to particular scenarios—the threat model will be incom-
plete. Or the modeler will not be able to prioritize attacks. All attacks will, therefore,
have to be considered as equally probable. In security assessment, art meets science;
craft meets engineering; and experience meets standard, policy, and rule. Hence, the
methodology presented here is a combination of art and science, craft and engineering.
It would be prohibitively expensive and impractical to defend every possible
* Th ough we do know with absolute certainty that any system directly addressable on the
Public Internet will be attacked, and that the attacks will be constant and unremitting.

The Art of Security Assessment 29
Perhaps someday, security architecture risk assessment (AR A) and threat model-
ing will become a rigorous and repeatable engineering activity? As of the writing of
this book, however, this is far from the case. Good assessors bring a number of key
knowledge domains to each assessment. It is with these domains that we will start. Just
as an assessment begins before the system is examined, so in this chapter we will explore
the knowledge and understanding that feeds into and underpins an analysis of a system
for security purposes.
You may care to think of these pre-assessment knowledge domains as the homework
or pre-work of an assessment. When the analyst does not have this information, she or
he will normally research appropriately before entering into the system assessment. Of
course, if during an assessment you find that you’ve missed something, you can always
stop the analysis and do the necessary research. While I do set this out in a linear fash-
ion, the linearity is a matter of convenience and pedagogy. There have been many times
when I have had to stop an assessment in order to research a technology or a threat
agent capability about which I was unsure.
It is key to understand that jumping over or missing any of the prerequisite knowledge
sets is likely to cause the analysis to be incomplete, important facets to be missed. The
idea here is to help you to be holistic and thorough. Some of the biggest mistakes I’ve
made have been because I did not look at the system as a whole but rather focused on a
particular problem to the detriment of the resulting analysis. Or I didn’t do thorough
research. I assumed that what I knew was complete when it wasn’t. My assessment mis-
takes could likely fill an entire volume by themselves. Wherever relevant, I will try to
highlight explanations with both my successes and my failures.
Because we are dealing with experience supporting well-educated estimates, the
underpinning knowledge sets are part of the assessor’s craft. It is in the application of
controls for risk mitigation that we will step into areas of hard engineering, once again.
2.2 Introducing “The Process”
It certainly may appear that an experienced security architect can do a system assess-
ment, even the assessment of something fairly complex, without seeming to have any
structure to the process at all. Most practitioners whom I’ve met most certainly do have a
system and an approach. Because we security architects have methodologies, or I should
say, I have a map in my mind while I assess, I can allow myself to run down threads into
details without losing the whole of both the architecture and the methodology. But,
unfortunately, that’s very hard to teach. Without structure, the whole assessment may
appear aimless and unordered? I’ve had many people follow me around through many,
many reviews. Those who are good at following and learning through osmosis “get it.”
But many people require a bit more structure in order to fit the various elements that
must be covered into a whole and a set of steps.

30 Securing Systems
Because most experienced architects actually have a structure that they’re following,
that structure gives the architect the opportunity to allow discussion to flow where
it needs to rather than imposing a strict agenda. This approach is useful, of course,
in helping everyone involved feel like they’re part of a dialogue rather than an inter-
rogation. Still, anyone who doesn’t understand the map may believe that there is no
structure at all. In fact, there is a very particular process that proceeds from threat
and attack methods, through attack surfaces, and ultimately resulting in requirements.
Practitioners will express these steps in different ways, and there are certainly many dif-
ferent means to express the process, all of them valid. The process that will be explained
in this book is simply one expression and certainly not absolute in any sense of the word.
Further, there is certain information, such as threat analysis, that most practitioners
bring to the investigation. But the architect may not take the time to describe this pre-
assessment information to other participants. It was only when I started to teach the
process to others that I realized I had to find a way to explain what I was doing and
what I knew to be essential to the analysis.
Because this book explains how to perform an assessment, I will try to make plain
all that is necessary. Please remember when you’re watching an expert that she or he will
apply existing knowledge to an analysis but may not explain all the pre-work that she
or he has already expended. The security architect will have already thought through
the appropriate list of threat agents for the type of system under consideration. If this
type of system is analyzed every day, architects live and breathe the appropriate infor-
mation. Hence, they may not even realize the amount of background that they bring
to the analysis.
I’m going to outline with broad strokes a series of steps that can take one from pre-
requisite know ledge through a system assessment. This series of steps assumes that the
analyst has sufficient understanding of system architecture and security architecture
going into the analysis. It also assumes that the analyst is comfortable uncovering risk,
rating that risk, and expressing it appropriately for different audiences. Since each of
these, architecture and risk, are significant bodies of knowledge, before proceeding into
the chapters on analysis, we will take time exploring each domain in a separate section.
As you read the following list, please remember that there are significant prerequisite
understandings and knowledge domains that contribute to a successful AR A.
○ Enumerate inputs and connections
○ Enumerate threats for this type of system and its intended deployment
– Consider threats’ usual attack methods
– Consider threats’ usual goals
○ Intersect threat’s attack methods against the inputs and connections. These are the
set of attack surfaces
○ Collect the set of credible attack surfaces
○ Factor in each existing security control (mitigations)
○ Risk assess each attack surface. Risk rating will help to prioritize attack surfaces
and remediations

The Art of Security Assessment 31
Each of the foregoing steps hides a number of intermediate steps through which an
assessment must iterate. The above list is obviously a simplification. A more complete
list follows. However, these intermediate steps are perceived as a consequence of the
investigation. At this point, it may be more useful to understand that relevant threats
are applied to the attack surfaces of a system to understand how much additional secu-
rity needs to be added.
The analysis is attempting to enumerate the set of “credible attack surfaces.” I use
the word “credi ble” in order to underline the fact that every attack method is not appli-
cable to every input. In fact, not every threat agent is interested in every system. As we
consider different threat agents, their typical methods, and most importantly, the goals
of their attacks, I hope that you’ll see that some attacks are irrelevant against some
systems: These attacks are simply not worth consideration. The idea is to filter out the
noise such that the truly relevant, the importantly dangerous, get more attention than
anything else.
Credible attack vector: A credible threat exercising an exploit on an exposed
I have defined the term “credible attack vector.” This is the term that I use to indi-
cate a composite of factors that all must be true before an attack can proceed. I use
the term “true” in the Boolean sense: there is an implicit “if ” statement (for the pro-
gramming language minded) in the term “credible”: if the threat can exercise one of
the threat’s exploit techniques (attack method) upon a vulnerability that is sufficiently
exposed such that the exploit may proceed successfully.
There are a number of factors that must each be true before a particular attack sur-
face becomes relevant. There has to be a known threat agent who has the capability to
attack that attack surface. The threat agent has to have a reason for attacking. And most
importantly, the attack surface needs to be exposed in some way such that the threat
agent can exploit it. Without each of these factors being true, that is, if any one of them
is false, then the attack cannot be promulgated. As such, that particular attack is not
worth considering. A lack of exposure might be due to an existing set of controls. Or,
there might be architectural reasons why the attack surface is not exposed. Either way,
the discussion will be entirely theoretical without exposure.
Consider the following pseudo code:
Credible attack vector = (active threat agent & exploit & exposure & vulnerability)
The term “credible attack vector” may only be true if each of the dependent
conditions is true. Hence, an attack vector is only interesting if its component
terms all return a “true” value. The operator combining each terms is Boolean And.
Understanding the combinatory quality of these terms is key in order to filter out
hypothetical attacks in favor of attacks that have some chance of succeeding if these
attacks are not well defended.

32 Securing Systems
Also important: If the attacker cannot meet his or her goals by exploiting a par-
ticular attack surface, the discussion is also moot. As an example, consider an overflow
condition that can only be exploited with elevated, super-user privileges. At the point
at which attackers have gained superuser privileges, they can run any code they want
on most operating systems. There is no advantage to exploiting an additional overflow.
It has no attack value. Therefore, any vulnerability such as the one outlined here is
theoretical. In a world of limited resources, concentrating on such an overflow wastes
energy that is better spent elsewhere.
In this same vein, a credible attack vector has little value if there’s no reward for the
attacker. Risk, then, must include a further term: the impact or loss. We’ll take a deeper
dive into risk, subsequently.
An analysis must first uncover all the credible attack vectors of the system. This
simple statement hides significant detail. At this point in this work, it may be suffi-
cient to outline the following mnemonic, “ATASM.” Figure 2.1 graphically shows an
ATASM flow:
Figure 2.1 Architecture, threats, attack surfaces, and mitigations.
Threats are applied to the attack surfaces that are uncovered through decomposing
an architecture. The architecture is “factored” into its logical components—the inputs
to the logical components and communication flows between components. Existing
mitigations are applied to the credible attack surfaces. New (unimplemented) mitiga-
tions become the “security requirements” for the system. These four steps are sketched
in the list given above. If we break these down into their constituent parts, we might
have a list something like the following, more detailed list:
• Diagram (and understand) the logical architecture of the system.
• List all the possible threat agents for this type of system.
• List the goals of each of these threat agents.
• List the typical attack methods of the threat agents.
• List the technical objectives of threat agents applying their attack methods.
• Decompose (factor) the architecture to a level that exposes every possible attack
• Apply attack methods for expected goals to the attack surfaces.
• Filter out threat agents who have no attack surfaces exposed to their typical

The Art of Security Assessment 33
• Deprioritize attack surfaces that do not provide access to threat agent goals.
• List all existing security controls for each attack surface.
• Filter out all attack surfaces for which there is sufficient existing protection.
• Apply new security controls to the set of attack services for which there isn’t
sufficient mitigation. Remember to build a defense-in-depth.
• The security controls that are not yet implemented become the set of security
requirements for the system.
Even this seemingly comprehensive set of steps hides significant detail. The details
that are not specified in the list given above comprise the simplistic purpose of this book.
Essentially, this work explains a complex process that is usually treated atomically, as
though the entire art of security architecture assessment can be reduced to a few easily
repeated steps. However, if the process of AR A and threat modeling really were this
simple, then there might be no reason for a lengthy explication. There would be no
need for the six months to three years of training, coaching, and mentoring that is typi-
cally undertaken. In my experience, the process cannot be so reduced. Analyzing the
security of complex systems is itself a complex process.
2.3 Necessary Ingredients
Just as a good cook pulls out all the ingredients from the cupboards and arranges them
for ready access, so the experienced assessor has at her fingertips information that must
feed into the assessment. In Figure 2.2, you will see the set of knowledge domains that
Figure 2.2 Knowledge sets that feed a security analysis.

34 Securing Systems
feed into an architecture analysis. Underlying the analysis set are two other domains
that are discussed, separately, in subsequent chapters: system architecture and specifi-
cally security architecture, and information security risk. Each of these requires its own
explanation and examples. Hence, we take these up below.
The first two domains from the left in Figure 2.2 are strategic: threats and risk pos-
ture (or tolerance). These not only feed the analysis, they help to set the direction and
high-level requirements very early in the development lifecycle. For a fuller discussion
on early engagement, please see my chapter, “The SDL in the Real World,” in Core
Software Security.4 The next two domains, moving clockwise—possible controls and
existing limitations—refer to any existing security infrastructure and its capabilities:
what is possible and what is difficult or excluded. The last three domains—data sensi-
tivity, runtime/execution environment, and expected deployment model—refer to the
system under discussion. These will be discussed in a later chapter.
Figure 2.3 places each contributing knowledge domain within the area for which it
is most useful. If it helps you to remember, these are the “3 S’s.” Strategy, infrastructure
and security structures, and specifications about the system help determine what is
important: “Strategy, Structures, Specification.” Indeed, very early in the lifecycle, per-
haps as early as possible, the strategic understandings are critically important in order
to deliver high-level requirements. Once the analysis begins, accuracy, relevance, and
deliverability of the security requirements may be hampered if one does not know what
security is possible, what exists, and what the limitations are. As I did in my first couple
of reviews, it is easy to specify what cannot actually be accomplished. As an architecture
begins to coalesce and become more solid, details such as data sensitivity, the runtime
and/or execution environment, and under what deployment models the system will run
become clearer. Each of these strongly influences what is necessary, which threats and
attack methods become relevant, and which can be filtered out from consideration.
Figure 2.3 Strategy knowledge, structure information, and system specifi cs.

The Art of Security Assessment 35
It should be noted that the process is not nearly as linear as I’m presenting it. The
deployment model, for instance, may be known very early, even though it’s a fairly
specific piece of knowledge. The deployment model can highly influence whether secu-
rity is inherited or must be placed into the hands of those who will deploy the system.
As soon as this is known, the deployment model will engender some design imperatives
and perhaps a set of specific controls. Without these specifics, the analyst is more or less
shooting in the dark.
2.4 The Threat Landscape
Differing groups target and attack different types of systems in different ways for dif-
ferent reasons. Each unique type of attacker is called a “threat agent.” The threat agent
is simply an individual, organi zation, or group that is capable and motivated to pro-
mulgate an attack of one sort or another. Threat agents are not created equal. They
have different goals. They have different methods. They have different capabilities and
access. They have different risk profiles and will go to quite different lengths to be suc-
cessful. One type of attacker may move quickly from one system to another searching
for an easy target, whereas another type of attacker or threat agent may expend con-
siderable time and resources to carefully target a single system and goal. This is why
it is important to understand who your attackers are and why they might attack you.
Indeed, it helps when calculating the probability of attack to know if there are large
numbers or very few of each sort of attackers. How active is each threat agent? How
might a successful attack serve a particular threat agent’s goals?
You may note that I use the word “threat” to denote a human actor who promul-
gates attacks against computer systems. There are also inanimate threats. Natural
disasters, such as earthquakes and tornadoes, are most certainly threats to computer
systems. Preparing for these types of events may fall onto the security architect. On the
other hand, in many organizations, responding to natural disasters is the responsibility
of the business continuity function rather than the security function. Responding to
natural disaster events and noncomputer human events, such as riots, social disruption,
or military conflict, do require forethought and planning. But, it is availability that is
mostly affected by this class of events. And for this reason generally, the business con-
tinuity function takes the lead rather than security. We acknowledge the seriousness
of disastrous events, but for the study of architecture analysis for security, we focus on
human attackers.
It should be noted that there are research laboratories who specialize in understand-
ing threat agents and attack methods. Some of these, even commercial research, are
regularly published for the benefit of all. A security architect can consume these public
reports rather than trying to become an expert in threat research. What is important
is to stay abreast of current trends and emerging patterns. Part of the art of security

36 Securing Systems
assessment is planning for the future. As of this writing, two very useful reports are
produced by Verizon and by McAfee Labs.*
Although a complete examination of every known computer attacker is far beyond
the scope of this work, we can take a look at a few examples to outline the kind of
knowledge about threats that is necessary to bring to an assessment.
There are three key attributes of human attackers, as follows:
• Intelligence
• Adaptivity
• Creativity
This means that whatever security is put into place can and will be probed, tested,
and reverse engineered. I always assume that the attacker is as skilled as I am, if not
more so. Furthermore, there is a truism in computer security: “The defender must
close every hole. The attacker only needs one hole in order to be successful.” Thus, the
onus is on the defender to understand his adversaries as well as possible. And, as has
been noted several times previously, the analysis has to be thorough and holistic. The
attackers are clever; they only need one opportunity for success. One weak link will
break the chain of defense. A vulnerability that is unprotected and exposed can lead to
a successful attack.
2.4.1 Who Are These Attackers? Why Do They Want to Attack
My System?
Let’s explore a couple of typical threat agents in order to understand what it is we need
to know about threats in order to proceed with an analysis.† Much media attention has
been given to cyber criminals and organized cyber crime. We will contrast cyber crimi-
nals with industrial espionage threats (who may or may not be related to nation-state
espionage). Then we’ll take a look at how cyber activists work, since their goals and
methods differ pretty markedly from cyber crime. These three threat agents might be
the only relevant ones to a particular system. But these are certainly not the only threat
agents who are active as of this writing. It behooves you, the reader, to take advantage
of public research in order to know your attackers, to understand your adversaries.
* Full disclosure: At the time of this writing, the author works for McAfee Inc. However,
citing these two reports from among several currently being published is not intended as
an endorsement of either company or their products. Verizon and McAfee Labs are given as
example reports. Th ere are others.
† Th e threat analysis presented in this work is similar in intention and spirit to Intel’s
Th reat Agent Risk Assessment (TAR A). However, my analysis technique was developed
independently, without knowledge of TAR A. Any resemblance is purely coincidental.

The Art of Security Assessment 37
Currently, organized cyber criminals are pulling in billions and sometimes tens of
billions of dollars each year. Email spam vastly outweighs in volume the amount of
legitimate email being exchanged on any given day. Scams abound; confidence games
are ubiquitous. Users identities are stolen every day; credit card numbers are a dime a
dozen on the thriving black market. Who are these criminals and what do they want?
The simple answer is money. There is money to be made in cyber crime. There are
thriving black markets in compromised computers. People discover (or automate exist-
ing) and then sell attack exploits; the exploit methods are then used to attack systems.
Fake drugs are sold. New computer viruses get written. Some people still do, appar-
ently, really believe that a Nigerian Prince is going to give them a large sum of money if
they only supply a bank account number to which the money will supposedly be wired.
Each of these activities generates revenue for someone. That is why people do these
things, for income. In some instances, lots of income. The goal of all of this activity is
really pretty simple, as I understand it. The goal of cyber criminals can be summed up
with financial reward. It’s all about the money.
But, interestingly, cyber criminals are not interested in computer problems, per se.
These are a means to an end. Little hard exploit research actually occurs in the cyber
crime community. Instead, these actors tend to prefer to make use of the work of others,
if possible. Since the goal is income, like any business, there’s more profit when cost of
goods, that is, when the cost of research can be minimized.
This is not to imply that cyber criminals are never sophisticated. One only has to
investigate fast flux DNS switching to realize the level of technical skill that can be
brought to bear. Still, the goal is not to be clever, but to generate revenue.
Cyber crime can be an organized criminal’s “dream come true.” Attacks can be
largely anonymous. Plenty of attack scenarios are invisible to the target until after suc-
cess: Bank accounts can be drained in seconds. There’s typically no need for heavy
handed thuggery, no guns, no physical interaction whatsoever. These activities can be
conducted with far less risk than physical violence. “Clean crime?”
Hence, cyber criminals have a rather low risk tolerance, in general. Attacks tend
to be poorly targeted. Send out millions of spams; one of them will hit somewhere to
someone. If you wonder why you get so many spams, it’s because these continue to hit
pay dirt; people actually do click those links, they do order those fake drugs, and they
do believe that they can make $5000 per week working from home. These email scams
are successful or they would stop. The point here is that if I don’t order a fake drug, that
doesn’t matter; the criminal moves on to someone who will.
If a machine can’t easily be compromised, no matter. Cyber criminals simply move
on to one that can fall to some well-known vulnerability. If one web site doesn’t offer
any cross-site scripting (XSS) opportunities from which to attack users, a hundred thou-
sand other web sites do offer this vulnerability. Cyber criminals are after the gullible,
the poorly defended, the poorly coded. They don’t exhibit a lot of patience. “There’s a
sucker born every day,” as T.E. Barnum famously noted.

38 Securing Systems
From the foregoing, you may also notice that cyber criminals prefer to put in as little
work as possible. I call this a low “work factor.” The pattern then is low risk, low work
factor. The cyber criminal preference is for existing exploits against existing vulnerabili-
ties. Cyber criminals aren’t likely to carefully target a system or a particular individual,
as a generalization. (Of course, there may be exceptions to any broad characterization.)
There are documented cases of criminals carefully targeting a particular organi-
zation. But even in this case, the attacks have gone after the weak links of the system,
such as poorly constructed user passwords and unpatched systems with well-known
vulnerabilities, rather than highly sophisticated attack scenarios making use of
unknown vulnerabilities.
Further, there’s little incentive to carefully map out a particular person’s digital life.
That’s too much trouble when there are so many (unfortunately) who don’t patch their
systems and who use the same, easily guessed password for many systems. It’s a simple
matter of time and effort. When not successful, move on to the next mark.
This Report [2012 Attorney General Breach Report*], and other studies, have repeatedly
shown that cybercrime is largely opportunistic.† In other words, the organizations and
individuals who engage in hacking, malware, and data breach crimes are mostly
looking for “low-hanging fruit” — today’s equivalent of someone who forgets to lock
her car door.5
If you’ve been following along, I hope that you have a fair grasp of the methods, goals,
and profile of the cyber criminal? Low work factor, easy targets, as little risk as possible.
Let’s contrast cyber crime to some of the well-known industrial espionage cases.
Advanced persistent threats (APTs) are well named because these attack efforts can be
multi-year, multidimensional, and are often highly targeted. The goals are informa-
tion and disruption. The actors may be professionals (inter-company espionage), quasi-
state sponsored (or, at least, state tolerated), and nation-states themselves. Many of the
threat agents have significant numbers of people with which to work as well as being
well funded. Hence, unlike organized cyber criminals, no challenge is too difficult.
Attackers will spend the time and resources necessary to accomplish the job.
I am convinced that every company in every conceivable industry with significant size
and valuable intellectual property and trade secrets has been compromised (or will be
shortly) . . . In fact, I divide the entire set of Fortune Global 2,000 firms into two
categories: those that know they’ve been compromised and those that don’t yet know.6
* Harris, K. D. (2013). 2012 Attorney General Breach Report. Retrieved from http://oag.
25-million> (as of Jan. 8, 2014).

The Art of Security Assessment 39
We have collected logs that reveal the full extent of the victim population since mid-
2006 when the log collection began.7
That is, Operation “Shady R AT” likely began in 2006, whereas the McAfee research
was published in 2011. That is an operation of at least five years. There were at least 70
organizations that were targeted. In fact, as the author suggests, all of the Fortune 2000
companies were likely successfully breached. These are astounding numbers.
More astounding then the sheer breadth of Shady R AT is the length, sophistication,
and persistence of this single set of attacks, perhaps promulgated by a single group or
under a single command structure (even if multiple groups). APT attacks are multi-
month, often multi-year efforts. Sometimes a single set of data is targeted, and some-
times the attacks seem to be after whatever may be available. Multiple diversionary
attacks may be exercised to hide the data theft. Note the level of sophistication here:
• Carefully planned and coordinated
• Highly secretive
• Combination of techniques (sometimes highly sophisticated)
The direct goal is rarely money (though commercial success or a nation-state advan-
tage may ultimately be the goal). The direct goal of the attack is usually data, informa-
tion, or disruption. Like cyber criminals, APT is a risk averse strategy, attempting to
hide the intrusion and any compromise. Persistence is an attribute. This is very unlike
the pattern of cyber criminals, who prefer to find an easier or more exposed target.
For industrial spies, breaking through a defense-in-depth is an important part of the
approach. Spies will take the time necessary to study and then to target indivi duals.
New software attacks are built. Nation-states may even use “zero day” (previously
unknown) vulnerabilities and exploits. The United States’ STUXNET attack utilized
an exploit never before seen.
Although both cyber criminals and industrial spies are fairly risk averse, their
methods differ somewhat—that is, both threats make use of anonymizing services, but
spies will attempt to cover their tracks completely. They don’t want the breach to be
discovered, ever, if possible. In contrast, criminals tend to focus on hiding only their
identity. Once the theft has occurred, they don’t want to be caught and punished; their
goal is to hang on to their illegitimate gains. The fact that a crime has occurred will
eventually be obvious to the victim.
These two approaches cause different technical details to emerge through the
attacks. And, defenses need to be different.
Since the cyber criminal will move on in the event of resistance, an industry stan-
dard defense is generally sufficient. As long as the attack work-factor is kept fairly high,
the attackers will go somewhere else that offers easier pickings. The house with the dog
and burglar alarm remains safe. Next door, the house with poor locks that is regularly
unoccupied is burglarized repeatedly.

40 Securing Systems
The industrial spy spends weeks, months, years researching the target organization’s
technology and defenses. The interests and social relations of potentially targetable
users are carefully studied. In one famous attack, the attacker knew that on a particu-
lar day, a certain file was distributed to a given set of individuals with an expected file
name. By spoofing the document and the sender, several of the recipients were fooled
into opening the document, which contained the attack.
It is difficult to resist a targeted “spear phishing” attack: An email or URL that
appears to be sent such that the email masquerades as something expected, of particular
interest, from someone trusted. To resist an APT effort, defenses must be thorough and
in depth. No single defense can be a single point of failure. Each defense is assumed
to fail. As the principles previously outlined state, each defense must “fail securely.”
The entire defense cannot count on any single security control surviving; controls are
layered, with spheres of control overlapping significantly. The concept being that one
has built sufficient barriers for the attackers to surmount such that an attack will be
identified before it can fully succeed.* It is assumed that some protections will fail to
the technical excellence of the attackers. But the attacks will be slower than the reacti on
to them.
Figure 2.4 attempts to provide a visual mapping of the relationships between various
attributes that we might associate with threat agents. This figure includes inanimate
threats, with which we are not concerned here. Attributes include capabilities, activity
level, risk tolerance, strength of the motivation, and reward goals.
If we superimpose attributes from Table 2.1’s cyber-crime attributes onto Figure 2.4,
we can render Figure 2.5. Figure 2.5 gives us a visual representation of cyber criminal
threat agent attributes and their relationships in a mind map format.
[I]f malicious actors are interested in a company in the aerospace sector, they may try to
compromise the website of one of the company’s vendors or the website of an aerospace
industry-related conference. That website can become a vector to exploit and infect
employees who visit it in order to gain a foothold in the intended target company.8
We will not cover every active threat here. Table 2.1 summarizes the attributes that
characterize each of the threat agents that we’re examining. In order to illustrate the
differences in methods, goals, effort, and risk tolerance of differing threat agents, let’s
now briefly examine the well-known “hacktivist” group, Anonymous.
Unlike either cyber criminals or spies, activists typically want the world to know about
a breach. In the case of the HP Gary Federal hack (2011), the email, user credentials,
and other compromised data were posted publicly after the successful breach. Before
the advent of severe penalties for computer breaches, computer activists sometimes did
* Astute readers may note that I did not say, “attack prevented.” Th e level of focus, eff ort, and
sophistication that nation-state cyber spies can muster implies that most protections can be
breached, if the attackers are suffi ciently motivated.

The Art of Security Assessment 41
not hide their attack at all.* As of this writing, activists do try to hide their identities
because current US law provides serious penalties for any breach, whether politically
motivated or not: All breaches are treated as criminal acts. Still, hacktivists go to no
great pains to hide the compromise. Quite the opposite. The goal is to uncover wrong-
doing, perhaps even illegal actions. The goal is an open flow of information and more
transparency. So there is no point in hiding an attack. This is completely opposite to
how spies operate.
Figure 2.4 Threat agent attribute relationships.
Table 2.1 Summarized Threat Attributes
Threat Agent Goals Risk Tolerance Work Factor Methods
Cyber criminals Financial Low Low to medium Known proven
Industrial spies Information and
Low High to extreme Sophisticated and
Hacktivists Information,
disruption, and
media attention
Medium to high Low to medium System administration
errors and social
* Under the current US laws, an activist (Aaron Schwartz) who merely used a publicly available
system (MIT library) faced terrorism charges for downloading readily available scientifi c
papers without explicit permission from the library and each author. Th is shift in US law has
proven incredibly chilling to transparent cyber activism.

42 Securing Systems
The technical methods that were used by Anonymous were not particularly sophisti-
cated.* At HP Gary Federal, a very poorly constructed and obvious password was used
for high-privilege capabilities on a key system. The password was easily guessed or oth-
erwise forced. From then on, the attackers employed social engineering, not technical
acumen. Certainly, the attackers were familiar with the use of email systems and the
manipulation of servers and their operating systems. Any typical system administrator
would have the skills necessary. This attack did not require sophisticated reverse engi-
neering skills, understanding of operating system kernels, system drivers, or wire-level
network communications. Anonymous didn’t have to break any industrial-strength
cryptography in order to breach HB Gary Federal.
Computer activists are volunteers. They do not get paid (despite any propaganda you
may have read). If they do have paying jobs, their hacktivism has to be performed during
Figure 2.5 Cyber criminal attributes.
* I drew these conclusions after reading a technically detailed account of the HB Gary
attack in Unmasked, by Peter Bright, Nate Anderson, and Jacqui Cheng (Amazon Kindle,
2011).9 Th e conclusions that I’ve drawn about Anonymous were further bolstered by
an in-depth analysis appearing in Rolling Stone Magazine, “Th e Rise and Fall of Jeremy
Hammond: Enemy of the State,” by  Janet Reitman, appearing in the December  7,
2012, issue.10 It can be retrieved from:

The Art of Security Assessment 43
their non-job hours. Although there is some evidence that Anonymous did coordinate
between the various actors, group affiliation is loose. There are no leaders who give the
orders and coordinate the work of the many to a single goal. This is quite unlike the
organization of cyber criminals or cyber spies.
In our short and incomplete survey, I hope you now have a feel for the differences
between at least some of the currently active threat agents.
• Cyber crimes: The goal is financial. Risk tolerance is low. Effort tends to be low
to medium; cyber criminals are after the low hanging fruit. Their methods tend
to be proven.
• Industrial espionage: The goal is information and disruption. Risk tolerance is
low. Effort can be quite high, perhaps even extreme. Difficult targets are not a
barrier. Methods are very sophisticated.
• Computer activists: The goal is information, disruption, and media attention.
Risk tolerance is medium to high (they are willing to go to jail for their beliefs).
Their methods are computer savvy but not necessarily sophisticated. They are
willing to put in the time necessary to achieve their goal.
These differences are summarized in Table 2.1, above.
Each of these threat agents operates in a different way, for different motivations, and
with different methods. Although many of the controls that would be put into place to
protect against any of them are the same, a defense-in-depth has to be far more rigor-
ous and deep against industrial espionage or nation-state spying versus cyber criminals
or activists.
If a system does not need to resist industrial espionage, it may rely on a less rigorous
defense. Instead, shoring up significant barriers to attack at the entrances to systems
should be the focus. On the other hand, preparing to resist a nation-state attack will
likely also discourage cyber criminals. Attending to basics appropriately should deter
many external activists.*
Hopefully, at this point you can see that knowing who your attackers are and some-
thing about them influences the way you build your defenses. An organization will
need to decide which of the various threat agents pose the most likely attack scenarios
and which, if any, can be ignored. Depending upon the use of the system, its exposure,
the data it handles, and the organizations that will deploy and use the system, certain
threat agents are likely to be far more important and dangerous to the mission of the
organization than others. An organization without much controversy may very well
not have to worry about computer activism. An organization that offers little financial
reward may not have to worry about cyber crime (other than the pervasive cyber crime
* Edward Snowden, the NSA whistleblower, was given almost free rein to access systems as a
trusted insider. In his case, he required no technical acumen in order to retrieve much of the
information that he has made public. He was given access rights.

44 Securing Systems
that’s aimed at every individual who uses a computer). And likewise, an organization
that handles a lot of liquid funds may choose to focus on cyber crime.
I do not mean to suggest that there’s only one threat that any particular system must
resist. Rather, the intersection of organization, organizational mission, and systems can
help focus on those threats that are of concern while, at the same time, allowing some
threat agents and their attack methods to be de-prioritized.
2.5 How Much Risk to Tolerate?
As we have seen, different threat agents have different risk tolerances. Some attempt
near perfect secrecy, some need anonymity, and some require immediate attention for
success. In the same way, different organizations have different organizational risk pos-
tures. Some businesses are inherently risky; the rewards need to be commensurate with
the risk. Some organizations need to minimize risk as much as possible. And, some
organizations have sophisticated risk management processes. One only needs to con-
sider an insurance business or any loan-making enterprise. Each of these makes a profit
through the sophisticated calculation of risk. An insurance company’s management of
its risk will, necessarily, be a key activity for a successful business. On the other hand,
an entrepreneurial start-up run by previously successful businesspeople may be able to
tolerate a great deal of risk. That, in fact, may be a joy for the entrepreneur.
Since there is no perfect security, and there are no guarantees that a successful attack
will always be prevented, especially in computer security, risk is always inherent in
the application of security to a system. And, since there are no guarantees, how much
security is enough? This is ultimately the question that must be answered before the
appropriate set of security controls can be applied to any system.
I remind the reader of a definition from the Introduction:
Securing systems is the art and craft of applying information security principles, design
imperatives, and available controls in order to achieve a particular security posture.
I have emphasized “a particular security posture.” Some security postures will be
too little to resist the attacks that are most likely to come. On the other hand, deep,
rigorous, pervasive information security is expensive and time consuming. The classic
example is the situation where the security controls cost more than the expected return
on investment for the system. It should be obvious that such an expensive security
posture would then be too much? Security is typically only one of many attributes that
contribute to the success of a particular system, which then contributes to the success of
the organi zation. When resources are limited (and aren’t they always?), difficult choices
need to be made.
In my experience, it’s a great deal easier to make these difficult choices when one has
a firm grasp on what is needed. A system that I had to assess was subject to a number of
the organization’s standards. The system was to be run by a third party, which brought

The Art of Security Assessment 45
it under the “Application Service Provider Policy.” That policy and standard was very
clear: All third parties handling the organization’s data were required to go through an
extensive assessment of their security practices. Since the proposed system was to be
exposed to the Internet, it also fell under standards and policies related to protection
of applications and equipment exposed to the Public Internet. Typically, application
service provider reviews took two or three months to complete, sometimes considerably
longer. If the third party didn’t see the value in participating or was resistive for any
other reason, the review would languish waiting for their responses. And, oftentimes
the responses would be incomplete or indicate a misunderstanding of one or more of the
review questions. Though unusual, a review could take as long as a year to complete.
The Web standards called for the use of network restrictions and firewalls between
the various components, as they change function from Web to application to data
(multi-tier protections). This is common in web architectures. Further, since the organi-
zation putting forth the standards deployed huge, revenue-producing server farms, its
standards were geared to large implementations, extensive staff, and very mature pro-
cesses. These standards would be overwhelming for a small, nimble, poorly capitalized
company to implement.
When the project manager driving the project was told about all the requirements
that would be necessary and the likely time delays that meeting the requirements would
entail, she was shocked. She worked in a division that had little contact with the web
security team and, thus, had not encountered these policies and standards previously.
She then explained that the company was willing to lose all the money to be expended
on this project: The effort was an experiment in a new business model. That’s why they
were using a third party. They wanted to be able to cut loose from the effort and the
application on a moment’s notice. The company’s brand name was not going to be asso-
ciated with this effort. So there was little danger of a brand impact should the system be
successfully breached. Further, there was no sensitive data: All the data was eminently
discardable. This application was to be a tentative experiment. The goal was simply to
see if there was interest for this type of application. In today’s lexicon, the company for
which I worked was searching for the “right product,” rather than trying to build the
product “right.”
Any system connected to the Internet, of course, must have some self-protection
against the omnipresent level of attack it must face. But the kind of protections that
we would normally have put on a web system were simply too much for this particular
project. The required risk posture was quite low. In this case, we granted exceptions to
the policies so that the project could go forward quickly and easily. The controls that we
actually implemented were just sufficient to stave off typical, omnipresent web attack.
It was a business decision to forgo a more protective security posture.
The primary business requirements for information security are business-specific. They
will usually be expressed in terms of protecting the availability, integrity, authenticity
and confidentiality of business information, and providing accountability and
auditability in information systems.11

46 Securing Systems
There are two risk tolerances that need to be understood before going into a system
security assessment.
• What is the general risk tolerance of the owners of the system?
• What is the risk tolerance for this particular system?
Systems critical to the functioning of an organization will necessarily have far less
risk tolerance and a far higher security posture than systems that are peripheral. If a
business can continue despite the loss of a system or its data, then that system is not
nearly as important as a system whose functioning is key. It should be noted that in a
shared environment, even the least critical application within the shared environment
may open a hole that degrades the posture of the entire environment. If the environ-
ment is critical, then the security of each component, no matter how peripheral, must
meet the standards of the entire environment. In the example above, the system under
assessment was both peripheral and entirely separate. Therefore, that system’s loss could
not have significant impact on the whole. On the other hand, an application on that
organization’s shared web infrastructure with a vulnerability that breached the tiered
protections could open a disastrous hole, even if completely insignificant. (I did prevent
an application from doing exactly that in another, unrelated, review.)
It should be apparent that organizations willing to take a great deal of risk as a
general part of their approach will necessarily be willing to lose systems. A security
architect providing security controls for systems being deployed by such an organiza-
tion needs to understand what risks the organization is willing to take. I offer as an
example a business model that typically interacts with its customers exactly one single
time. In such a model, the business may not care if customers are harmed through their
business systems. Cross-site scripting (XSS) is typically an attack through a web system
against the users of the system. In this business model, the owners of the system may
not care that some percentage of their customers get attacked, since the organization
won’t interact with these customers again; they have no need for customer loyalty.*
On the other hand, if the business model requires the retention, loyalty, and good-
will of as many customers as possible, then having one’s customers get attacked because
of flaws in one’s commerce systems is probably not a risk worth taking. I use these two
polar examples to illustrate how the organization’s operational model influences its risk
stance. And, the risk tolerance of the organization significantly influences how much
security is required to protect its systems.
How does one uncover the risk tolerance of an organization? The obvious answer is
to simply ask. In organizations that have sophisticated and/or mature risk management
* I do not mean to suggest that ignoring your customers’ safety is a particularly moral stance.
My own code entreats me to “do no harm.” However, I can readily imagine types of businesses
that don’t require the continuing goodwill of their customers.

The Art of Security Assessment 47
practices, it may be a matter of simply asking the right team or group. However, for any
organization that doesn’t have this information readily available, some investigation is
required. As in the case with the project manager whose project was purely experimen-
tal and easily lost, simply asking, “What is the net effect of losing the data in the sys-
tem?” may be sufficient. But in situations where the development team hasn’t thought
about this issue, the most likely people to understand the question in the broader orga-
nizational sense will be those who are responsible and accountable. In a commercial
organization, this may be senior management, for instance, a general manager for a
division, and others in similar positions. In organizations with less hierarchy, this may
be a discussion among all the leaders—technical, management, whoever’s responsible,
or whoever takes responsibility for the success of the organization.
Although organizational risk assessment is beyond the scope of this book, one can
get a good feel simply by asking pointed questions:
• How much are we willing to lose?
• What loss would mean the end of the organization?
• What losses can this organization sustain? And for how long?
• What data and systems are key to delivering the organizational mission?
• Could we make up for the loss of key systems through alternate means? For how
long can we exist using alternate means?
These and similar questions are likely to seed informative conversations that will
give the analyst a better sense of just how much risk and of what sort the organization
is willing to tolerate.
As an example, for a long time, an organization at which I worked was willing to
tolerate accumulating risk through its thousands of web applications. For most of these
applications, loss of any particular one of them would not degrade the overall enterprise
significantly. While the aggregate risk continued to increase, each risk owner, usually
a director or vice president, was willing to tolerate this isolated risk for their particular
function. No one in senior management was willing to think about the aggregate risk
that was being accumulated. Then, a nasty compromise and breach occurred. This
highlighted the pile of unmitigated risk that had accumulated. At this point, executive
management decided that the accumulated risk pile needed to be addressed; we were
carrying too much technology debt above and beyond the risk tolerance of the organi-
zation. Sometimes, it takes a crisis in order to fully understand the implications for the
organization. As quoted earlier, in Chapter 1, “Never waste a crisis.”12 The short of it
is, it’s hard to build the right security if you don’t know what “secure enough” is. Time
spent fact finding can be very enlightening.
With security posture and risk tolerance of the overall organization in hand, spe-
cific questions about specific systems can be placed within that overall tolerance. The
questions are more or less the same as listed above. One can simply change the word
“organization” to “system under discussion.”

48 Securing Systems
There is one additional question that should be added to our list: “What is the high-
est sensitivity of the data handled by the system?” Most organizations with any security
maturity at all will have developed a data-sensitivity classification policy and scale. These
usually run from public (available to the world) to secret (need-to-know basis only).
There are many variations on these policies and systems, from only two classifications
to as many as six or seven. An important element for protecting the organi zation’s data
is to understand how restricted the access to particular data within a particular system
needs to be. It is useful to ask for the highest sensitivity of data since controls will have
to be fit for that, irrespective of other, lower classification data that is processed or stored.
Different systems require different levels of security. A “one-size-fits-all” approach is
likely to lead to over specifying some systems. Or it may lead to under specifying most
systems, especially key, critical systems. Understanding the system risk tolerance and
the sensitivity of the data being held are key to building the correct security.
For large information technology (IT) organizations, economies of scale are typi-
cally achieved by treating as many systems as possible in the same way, with the same
processes, with the same infrastructure, with as few barriers between information flow
as possible. In the “good old days” of information security, when network restrictions
ruled all, this approach may have made some sense. Many of the attacks of the time
were at the network and the endpoint. Sophisticated application attacks, combination
attacks, persistent attacks, and the like were extremely rare. The castle walls and the
perimeter controls were strong enough. Security could be served by enclosing and iso-
lating the entire network. Information within the “castle” could flow freely. There were
only a few tightly controlled ingress and egress points.
Those days are long gone. Most organizations are so highly cross-connected that we
live in an age of information ecosystems rather than isolated castles and digital city-
states. I don’t mean to suggest that perimeter controls are useless or passé. They are
one part of a defense-in-depth. But in large organizations, certainly, there are likely to
be several, if not many, connections to third parties, some of whom maintain radically
different security postures. And, on any particular day, there are quite likely to be any
number of people whose interests are not the same as the organization’s but who’ve been
given internal access of one kind or another.
Added to highly cross-connected organizations, many people own many connecting
devices. The “consumerization” of IT has opened the trusted network to devices that are
owned and not at all controlled by the IT security department. Hence, we don’t know
what applications are running on what devices that may be connecting (through open
exchanges like HTTP/HTML) to what applications. We can authen ticate and autho-
rize the user. But from how safe a device is the user connecting? Generally, today, it is
safer to assume that some number of the devices accessing the organization’s network
and resources are already compromised. That is a very different picture from the highly
restricted networks of the past.
National Cyber Security Award winner Michelle Guel has been touting “islands
of security” for years now. Place the security around that which needs it rather than

The Art of Security Assessment 49
trusting the entire castle. As I wrote above, it’s pretty simple: Different systems require
different security postures. Remember, always, that one system’s security posture affects
all the other systems’ security posture in any shared environment.
What is a security posture?
Security posture is the overall capability of the security organization to assess its unique
risk areas and to implement security measures that would protect against exploitation.13
If we replace “organization” with “system,” we are close to a definition of a system’s
security posture. According to Michael Fey’s definition, quoted above, an architecture
analysis for security is a part of the security posture of the system (replacing “organi-
zation” with “system”). But is the analysis to determine system posture a part of that
posture? I would argue, “No.” At least within the context of this book, the analysis is
outside the posture. If the analysis is to be taken as a part of the posture, then sim-
ply performing the analysis will change the posture of the system. And our working
approach is that the point of the analysis is to determine the current posture of the
system and then to bring the system’s posture to a desired, intended state. If we then
rework the definition, we have something like the following:
System security posture: The unique risk areas of a system against which to implement
security measures that will protect against exploitation of the system.
Notice that our working definition includes both risk areas and security measures.
It is the sum total of these that constitute a “security posture.” A posture includes both
risk and protection. Once again, “no risk” doesn’t exist. Neither does “no protection,”
as most modern operating environments have some protections in-built. Thus, posture
must include the risks, the risk mitigations, and any residual risk that remains unpro-
tected. The point of an AR A—the point of securing systems—is to bring a system to
an intended security posture, the security posture that matches the risk tolerance of
the organization and protects against those threats that are relevant to that system and
its data.
Hence, one must ascertain what’s needed for the system that’s under analysis. The
answers that you will collect to the risk questions posed above point in the right direc-
tion. An analysis aims to discover the existing security posture of a system and to cal-
culate through some risk-based method, the likely threats and attack scenarios. It then
requires those controls that will bring the system to the intended security posture.
The business model (or similar mission of system owners) is deeply tied into the
desired risk posture. Let’s explore some more real-life examples. We’ve already examined
a system that was meant to be temporary and experimental. Let’s find a polar opposite,
a system that handles financial data for a business that must retain customer loyalty.
In the world of banking, there are many offerings, and competition for customers is
fierce. With the growth of online banking services, customers need significant reasons

50 Securing Systems
to bank with the local institution, even if there is only a single bank in town. A friend of
mine is a bank manager in a small town of four thousand people, in central California.
Even in that town, there are several brick and mortar banks. She vies for the loyalty of
her customers with personal services and through paying close attention to individual
needs and the town’s overall economic concerns.
Obviously, a front-end banking system available to the Internet may not be able to
offer the human touch that my friend can tender to her customers. Hopefully, you still
agree that loyalty is won, not guaranteed? Part of that loyalty will be the demonstration,
over time, that deposits are safely held, that each customer’s information is secure.
Beyond the customer-retention imperative, in most countries, banks are subject to a
host of regulations, some of which require and specify security. The regulatory picture
will influence the business’ risk posture, alongside its business imperatives. Any system
deployed by the bank for its customers will have to have a security posture sufficient for
customer confidence and that meets jurisdictional regulations, as well.*
As we have noted, any system connected to the Public Internet is guaranteed to be
attacked, to be severely tested continuously. Financial institutions, as we have already
examined, will be targeted by cyber criminals. This gives us our first posture clue: The
system will have to have sufficient defense to resist this constant level of attack, some of
which will be targeted and perhaps sophisticated.
But we also know that our customers are targets and their deposits are targeted.
These are two separate goals: to gain, through our system, the customers’ equipment
and data (on their endpoint). And, at the same time, some attackers will be targeting the
funds held in trust. Hence, this system must do all that it can to prevent its use to attack
our customers. And, we must protect the customers’ funds and data; an ideal would be
to protect “like a safety deposit box.”
Security requirements for an online bank might include demilitarized zone (DMZ)
hardening, administration restrictions, protective firewall tiers between HTTP termi-
nations, application code and the databases to support the application, robust authen-
tication and authorization systems (which mustn’t be exposed to the Internet, but only
to the systems that need to authenticate), input validation (to prevent input validation
errors), stored procedures (to prevent SQL injection errors), and so forth. As you can see,
the list is quite extensive. And I have not listed everything that I would expect for this
system, only the most obvious.
If the bank chose to outsource the system and its operations, then the chosen vendor
would have to demonstrate all of the above and more, not just once, but repeatedly
through time.
Given these different types of systems, perhaps you are beginning to comprehend
why the analysis can only move forward successfully with both the organization posture
* I don’t mean to reduce banking to two imperatives. I’m not a banking security expert. And,
online banking is beyond our scope. I’ve reduced the complexity, as an example.

The Art of Security Assessment 51
and the system posture understood? The bank’s internal company portal through which
employees get the current company news and access various employee services, would,
however, have a different security posture. The human resources (HR) system may
have significant security needs, but the press release feed may have signi ficantly less.
Certainly, the company will prefer not to have fake news posted. Fake company news
postings may have a much less significant impact on the bank than losing the account
holdings of 30% of the banks customers?
Before analysis, one needs to have a good understanding of the shared services that
are available, and how a security posture may be shared across systems in any particular
environment. With the required system risk posture and risk tolerance in hand, one
may proceed with the next steps of the syste m analysis.
2.6 Getting Started
Before I can begin to effectively analyze systems for an organization, I read the security
policy and standards. This gives me a reasonable feel for how the organization approaches
security. Then, I speak with leaders about the risks they are willing to take, and those
that they cannot—business risks that seem to have nothing to do with computers may
still be quite enlightening. I further query technical leaders about the security that they
think systems have and that systems require.
I then spend time learning the infrastructure—how it’s implemented, who admin-
isters it, the processes in place to grant access, the organization’s approach to security
layers, monitoring, and event analysis. Who performs these tasks, with what technol-
ogy help, and under what response timing (“SLA”). In other words, what security is
already in place and how does a system inherit that security?
My investigations help me understand the difference between past organization
expectations and current ones. These help me to separate my sense of appropriate secu-
rity from that of the organization. Although I may be paid to be an expert, I’m also paid
to execute the organization’s mission, not my own. As we shall see, a big part of risk is
separating my risk tolerance from the desired risk tolerance.
Once I have a feel for the background knowledge sets listed in this introduction,
then I’m ready to start looking at systems. I try to remember that I’ll learn more as I
analyze. Many assessments are like peeling an onion: I test my understandings with
the stakeholders. If I’m off base or I’ve missed something substantive, the stakeholders
will correct me. I may check each “fact” as I believe that I’ve come to understand
something about the system. There are a lot of questions. I need to be absolutely cer-
tain of every relevant thing that can be known at the time of the assessment. I reach
for absolute technical certainty. Through the process, my understanding will mature
about each system under consideration and about the surrounding and supporting
environment. As always, I will make mistakes; for these, I prepare myself and I prepare
the organization.

52 Securing Systems
1. Oxford Dictionary of English. (2010). 3rd ed. UK: Oxford University Press.
2. Buschmann, F., Henney, K., and Schmidt, D. C. (2007). “Foreword.” In Pattern-Oriented
Software Architecture: On Patterns and Pattern Languages. Vol. 5. John Wiley & Sons.
3. Rosenquist, M. (2009). “Prioritizing Information Security Risks with Th reat Agent
Risk Assessment.” IT@Intel White Paper, Intel Information Technology. Retrieved from
Prioritizing_Info_Security_Risks_with_TAR A .
4. Schoenfi eld, B. (2014). “Applying the SDL Framework to the Real World” (Ch. 9). In
Core Software Security: Security at the Source, pp. 255–324. Boca Raton (FL): CRC Press.
5. Harris, K. D. (2014). “Cybersecurity in the Golden State.” California Department of
6. Alperovitch, D. (2011-08-02). “Revealed: Operation Shady R AT.” McAfee, Inc. White
7. Ibid.
8. Global Th reat Report 2013 YEAR IN REVIEW, Crowdstrike, 2013. Available at:
9. Bright, P., Anderson, N., and Cheng, J. (2011). Unmasked. Amazon Kindle. Retrieved
10. Reitman, J. (Dec. 7, 2012). “Th e Rise and Fall of Jeremy Hammond: Enemy of the State.”
Rolling Stone Magazine. Retrieved from
11. Sherwood, J., Clark, A., and Lynas, D. “Enterprise Security Architecture.” SABSA White
Paper, SABSA Limited, 1995–2009. Retrieved from
members/sites/default/inline-fi les/SABSA_White_Paper .
12. Arkin, B. (2012). “Never Waste a Crisis – Necessity Drives Software Security.” RSA Conference
2012, San Francisco, CA, February 29, 2012. Retrieved from http://www.rsaconference.
13. Fey, M., Kenyon, B., Reardon, K. T., Rogers, B., and Ross, C. (2012). “Assessing Mission
Readiness” (Ch. 2). In Security Battleground: An Executive Field Manual. Intel Press.

Chapter 3
Security Architecture
of Systems
A survey of 7,000 years of history of human kind would conclude that the only known
strategy for accommodating extreme complexity and high rates of change is architecture.
If you can’t describe something, you can’t create it, whether it is an airplane, a hundred
storey building, a computer, an automobile . . . or an enterprise. Once you get a
complex product created and you want to change it, the basis for change is its descriptive
If the only viable strategy for handling complex things is the art of architecture, then
surely the practice of architecture is key to the practice of security for computers. This is
John Zachman’s position in the quote introducing this chapter. The implication found
in this quote is that the art of representing a complex system via an abstraction helps us
cope with the complexity because it allows us to understand the structure of a thing—
for our purposes, computer systems.
Along with a coping strategy for complexity, the practice of architecture gives us a
tool for experimenting with change before we actually build the system. This is a pro-
found concept that bears some thinking. By creating an abstraction that represents a
structure, we can then play with that structure, abstractly. In this way, when encounter-
ing change, we can try before we build, in a representative sense.
For a fairly common but perhaps trivial example, what happens when we place
the authentication system in our demilitarized zone (DMZ)—that is, in the layer
closest to the Internet? What do we have to do to protect the authentication system?
Does this placement facilitate authentication in some way? How about if we move the
authentica tion system to a tier behind the DMZ, thus, a more trusted zone? What are

54 Securing Systems
the implications of doing so for authentication performance? For security? I’ve had pre-
cisely these discussions, more than once, when architecting a web platform. These are
discussions about structures; these are architecture discussions.
Computer security is a multivariate, multidimensional field. Hence, by its very
nature, computer security meets a test for complexity. Architecture then becomes a tool
to apply to that complexity.
Computer security is dynamic; the attackers are adaptive and unpredictable. This
dynamism guarantees change alongside the inherent complexity. The complexity of
the problem space is mirrored within the complexity of the systems under discussion
and the security mechanisms that must be built in order to protect the systems. And as
John Zachman suggests in the quote introducing this chapter, complex systems that are
going to change require some kind of descriptive map so as to manage the change in an
orderly fashion: “the basis for change is its descriptive representations.”2
3.1 Why Is Enterprise Architecture Important?
The field of enterprise architecture supplies a mapping to generate order for a modern,
cross-connected digital organization.* I think Pallab Saha sums up the discipline of
Enterprise architecture in the following quote. Let this be our working definition for
enterprise—that is, an enterprise of “systems”—architecture.
Enterprise architecture (EA) is the discipline of designing enterprises guided with
principles, frameworks, methodologies, requirements, tools, reference models, and
Enterprise architecture is focused on the entire enterprise, not only its digital sys-
tems, including the processes and people who will interact, design, and build the sys-
tems. An often-quoted adage, “people, process, and technology,” is used to include
human, non-digital technology, and digital domains in the enterprise architecture.
Enterprise architects are not just concerned with technology. Any process, manual or
digital, that contributes to the overall goals of the enterprise, of the entire system taken
as a whole, is then, necessarily, a part of the “enterprise architecture.” Thus, a manu-
ally executed process will, by definition, include the people who execute that process:
“People, process, and technology.”
I’ve thrown around the term “enterprise” since the very beginning of this book. But,
I haven’t yet defined it. I’ve found most definitions of “enterprise,” in the sense that it is
used here and in enterprise architecture, rather lacking. There’s often some demarcation
below which an organization doesn’t meet the test. Yet, the organizations who fail to
meet the criteria would still benefit from architecture, perhaps enterprise architecture,
certainly enterprise security architecture. Consider the following criteria:
* Large business organizations are often called “enterprises.”

Security Architecture of Systems 55
• Greater than 5000 employees (10,000? 50,000? 100,000?)
• Greater than $1 billion in sales ($2 billion? $5 billion? $10 billion?)
• Fortune 1000 company (Fortune 500? Fortune 100? Fortune 50?)
Each of these measures presumes a for-profit goal. That leaves out non- governmental
organizations (NGOs) and perhaps governments.
A dictionary definition also doesn’t seem sufficient to our purpose:
[A] unit of economic organization or activity; especially : a business organization4
For the purposes of this book, I will offer a working definition not meant for any
purposes but my own:
Enterprise: An organization whose breadth and depth of activities cannot easily be
held simultaneously in one’s conscious mind.
That is, for our purposes only, if a person (you? I?) can’t keep the relationships and
processes of an organization in mind, it’s probably complex enough to meet our, not
very stringent, requirement and, thus, can be called an “enterprise.”
The emphasis here is on complexity. At the risk of forming a tautology, if the orga-
nization needs an architecture practice in order to transcend ad hoc and disparate solu-
tions to create some semblance of order, then it’s big enough to benefit from enterprise
architecture. Our sole concern in this discussion concerns whether or not an organiza-
tion may benefit from enterprise architecture as a methodology to provide order and to
reap synergies between the organization’s activities. If benefit may be derived from an
architectural approach, then we can apply enterprise architecture to the organization,
and specifically, a security architecture.
If enterprise architecture is concerned with the structure of the enterprise as a func-
tioning system, then enterprise security architecture will be concerned with the secu-
rity of the enterprise architecture as a functioning system. We emphasize the subset of
enterprise security architecture that focuses on the security of digital systems that are to
be used within the enterprise architecture. Often, this more granular architecture prac-
tice is known as “solutions” architectu re although, as of this writing, I have not seen the
following term applied to security: “solutions security architecture.” The general term,
“security architecture,” will need to suffice (though, as has been previously noted, the
term “security architecture” is overloaded).
Generally, if there is an enterprise architecture practice in an organization, the enter-
prise architecture is a good place from which to start. Systems intended to function
within an enterprise architecture should be placed within that overall enterprise struc-
ture and will contribute to the working and the goals of the organization. The enterprise
architecture then is an abstract, and hopefully ordered, representation of those systems
and their interactions. Because the security architecture of the organization is one part
of the overarching architecture (or should be!), it is useful for the security architect to

56 Securing Systems
understand and become conversant in architectures at this gross, organizational level of
granularity. Hence, I introduce some enterprise architecture concepts in order to place
system security assessments within the larger framework in which they may exist.
Still, it’s important to note that most system assessments—that is, architecture risk
assessment (AR A) and threat modeling—will take place at the systems or solutions
level, not at the enterprise view. Although understanding the enterprise architecture
helps to find the correct security posture for systems, the system-oriented pieces of the
enterprise security architecture emerge from the individual systems that make up the
total enterprise architecture. The caveat to this statement is the security infrastructure
into which systems are placed and which those systems consume for security services.
The security infrastructure must be one key component of an enterprise architecture.
This is why enterprise security architects normally work closely with, and are peers of,
the enterprise architects in an organization. Nevertheless, security people charged with
the architectural assessment of systems will typically be working at the system or solu-
tion level, placing those systems within the enterprise architecture and, thus, within an
enterprise security architecture.
Being a successful security architect means thinking in business terms at all times, even
when you get down to the real detail and the nuts and bolts of the construction. You
always need to have in mind the questions: Why are you doing this? What are you
trying to achieve in business terms here?5
In this book, we will take a cursory tour through some enterprise architecture con-
cepts as a grounding and path into the practice of security architecture. In our security
architecture journey, we can borrow the ordering and semantics of enterprise architecture
concepts for our security purposes. Enterprise architecture as a practice has been develop-
ing somewhat longer than security architecture.* Its framework is reasonably mature.
An added benefit of adopting enterprise security architecture terminology will then
be that the security architect can gently and easily insert him or herself in an organi-
zation’s architecture practice without perturbing already in-flight projects and pro-
cesses. A security architect who is comfortable interacting within existing and accepted
architecture practices will likely be more successful in adding security requirements
to an architecture. By using typical enterprise architecture language, it is much easier
for non-security architects to accept what may seem like strange concepts—attack
vectors and misuse cases, threat analysis and information security risk rating, and so
forth. Security concepts can run counter to the goals of the other architects. The bridge
* Th e Open Group off ers a certifi cation for Enterprise Architects. In 2008, I asked several
principals of the Open Group about security architecture as a practice. Th ey replied that
they weren’t sure such an architecture practice actually existed. Since then, the Open
Group has initiated an enterprise security architect certifi cation. So, apparently we’ve now
been recognized.

Security Architecture of Systems 57
between security and solution is to understand enterprise and solutions architecture
first, and then to build the security picture from those practices.
I would suggest that architecture is the total set of descriptive representations relevant
for describing something, anything complex you want to create, which serves as the
baseline for change if you ever want to change the thing you have created.6
I think that Zachman’s architecture definition at the beginning of the chapter applies
very well to the needs of securing systems. In order to apply information security prin-
ciples to a system, that system needs to be describable through a representation—that
is, it needs to have an architecture. As Izar Taarandach told me, “if you can’t describe
it—it is not time to do security architecture yet.” A security assessment doesn’t have
to wait for a completely finished system architecture. Assessment can’t wait for perfec-
tion because high-level security requirements need to be discovered early enough to get
into the architecture. But Izar is right in that without a system architecture, how does
the security architect know what to do? Not to mention that introducing even more
change by attempting to build security before sufficient system architecture exists is
only going to add more complexity before the structure of the system is understood well
enough. Furthermore, given one or more descriptive representations of the system, the
person who assesses the system for security will have to understand the representation
as intended by the creators of the representation (i.e., the “architects” of the system).
3.2 The “Security” in “Architecture”
The assessor cannot stop at an architectural understanding of the system. This is where
security architecture and enterprise, solutions, or systems architects part company. In
order to assess for security, the representation must be viewed both as its functioning is
intended and, just as importantly, as it may be misused. The system designers are inter-
ested in “use cases.” Use cases must be understood by the security architect in the context
of the intentions of the system. And, the security architect must generate the “misuse
cases” for the system, how the system may be abused for purposes that were not intended
and may even run counter to the goals of the organization sponsoring the system.
An assessor (usually a security architect) must then be proficient in architecture
in order to understand and manipulate system architectures. In addition, the security
architect also brings substantial specialized knowledge to the practice of security assess-
ment. Hence, we start with solutions or systems architectures and their representations
and then apply security to them.
This set of descriptive representations thereby becomes the basis for describing the
security needs of the system. If the security needs are not yet built, they will cause a
“change” to the system, as explained in Zachman’s definition describing architecture as
providing a “baseline for change” (see above).7

58 Securing Systems
Let me suggest a working definition for our purposes that might be something simi-
lar to the following:
System architecture is the descriptive representation of the system’s component functions
and the communication* flows between those components.
My definition immediately raises some important questions.
• What are “components”?
• Which functions are relevant?
• What is a communication flow?
It is precisely these questions that the security architect must answer in order to
understand a system architecture well enough to enumerate the system’s attack sur-
faces. Ultimately, we are interested in attack surfaces and the risk treatments that will
protect them. However, the discovery of attack surfaces is not quite as straightforward
a problem as we might like. Deployment models, runtime environments, user expecta-
tions, and the like greatly influence the level of detail at which a system architecture
will need to be examined. Like computer security itself, the architectural representation
is the product of a multivariate, complex problem. We will examine this problem in
some detail.
Mario Godinez et al. (2010)8 categorize architectures into several different layers, as
• Conceptual Level—This level is closest to business definitions, business processes,
and enterprise standards.
• Logical Level—This level of the Reference Architecture translates conceptual
design into logical design.
• Physical Level—This level of the Reference Architecture translates the logical
design into physical structures and often products.
The Logical Level is broken down by Godinez et al. (2010) into two interlocking
and contributing sub-models:
ο Logical Architecture—The Logical Architecture shows the relationships of
the different data domains and functionalities required to manage each type of
* I use “communication fl ow” because, sometimes, people forget those communications
between systems that aren’t considered “data” connections. In order to communicate, digital
entities need to exchange data. So, essentially, all communication fl ows are data fl ows. In
this context we don’t want to constrain ourselves to common conceptions of data fl ows, but
rather, all exchange of bits between one function and another.

Security Architecture of Systems 59
ο Component Model—Technical capabilities and the architecture building blocks
that execute them are used to delineate the Component Model.9
For complex systems, and particularly at the enterprise architecture level, a single
repre sentation will never be sufficient. Any attempt at a complete representation is likely
to be far too “noisy” to be useful to any particular audience: There are too many possible
representations, too many details, and too many audiences. Each “audience”—that is,
each stakeholder group—has unique needs that must be reflected in a representation of
the system. Organizational leaders (senior management, typically) need to understand
how the organization’s goals will be carried out through the system. This view is very
different from what is required by network architects building a network infrastructure
to support the system. As we shall see, what the security architect needs is also different,
though hopefully not entirely unique. Due to these factors, the practice of enterprise
architecture creates different views representing the same architecture.
For the purposes of security evaluation, we are concerned primarily with the Logical
Level—both the logical architecture and component model. Often, the logical archi-
tecture, the different domains and functionalities, as well as the component model, are
superimposed upon the same system architecture diagram. For simplicity, we will call
this the “logical system architecture.” The most useful system architecture diagram will
contain sufficient logical separation to represent the workings of the system and the
differing domains. And the diagram should explain the component model sufficiently
such that the logical functions can be tied to technical components.
Security controls tend to be “point”—that is, they implement a single function that
will then be paired to one or more attack vectors. The mapping is not one-to-one, vec-
tor to control or control to attack method. The associations are much looser (we will
examine this in greater detail later). Due to the lack of absolute coherence between the
controls that can be implemented and the attack vectors, the technical components are
essential for understanding just precisely which controls can be implemented and which
will contribute towards the intended defense-in-depth.
Eventually, any security services that a system consumes or implements will, of
course, have to be designed at the physical level. Physical servers, routers, firewalls, and
monitoring systems will have to be built. But these are usually dealt with logically, first,
leaving the physical implementation until the logical and component architectures are
thoroughly worked out. The details of firewall physical implementation often aren’t
important during the logical security analysis of a system, so long as the logical controls
produce the tiers and restrictions, as required. Eventually, the details will have to be
decided upon, as well, of course.
3.3 Diagramming For Security Analysis
Circles and arrows leave one free to describe the interrelationships between things in a
way that tables, for example, do not.10

60 Securing Systems
It may be of help to step back from our problem (assessing systems for security) to
examine different ways in which computer systems are described visually. The archi-
tecture diagram is a critical prerequisite for most architects to conduct an assessment.
What does an architecture diagram look like?
In Figure 3.1, I have presented a diagram of an “architecture” that strongly resem-
bles a diagram that I once received from a team.* The diagram does show something
of the system: There is some sort of interaction between a user’s computer and a server.
The server interacts with another set of servers in some manner. So there are obviously
at least three different components involved. The brick wall is a standard representation
of a firewall. Apparently, there’s some kind of security control between the user and the
middle server. Because the arrows are double headed, we don’t know which component
calls the others. It is just as likely that the servers on the far right call the middle server
as the other way around. The diagram doesn’t show us enough specificity to begin to
think about trust boundaries. And, are the two servers on the right in the same trust
area? The same network? Or are they separated in some manner? We don’t know from
this diagram. How are these servers managed? Are they managed by a professional,
security-conscious team? Or are they under someone’s desk, a pilot project that has
gone live without any sort of administrative security practice? We don’t know if these
are web and database protocols or something else. We also do not know anything about
the firewall. Is it stateful? Deep packet inspection? A web application firewall (WAF)?
Or merely a router with an Access Control List (ACL) applied?
An astute architect might simply make queries about each of these facets (and more).
Or the architect might request more details in order to help the team create a diagram
with just a little bit more specificity.
I include Figure 3.2 because although this diagram may enhance the sales of a
product, it doesn’t tell us very much about those things with which we must deal. This
diagram is loosely based upon the “architecture” diagram that I received from a busi-
ness data processing product† that I was reviewing. What is being communicated by the
diagram, and what is needed for an assessment?
Figure 3.1 A simplistic Web architecture diagram.
* Figure 3.1 includes no references that might endanger or otherwise identify a running system
at any of my former or current employers.
† Although based upon similar concepts, this diagram is entirely original. Any resemblance to
an existing product is purely coincidental.

Security Architecture of Systems 61
From Figure 3.2, we know that, somehow, a “warehouse” (whatever that is) commu-
nicates with data sources. And presumably, the application foundation supports various
higher-level functions? This may be very interesting for someone buying the product.
However, this diagram does not give us sufficient information about any of the compo-
nents for us to begin to identify attack surfaces, which is the point of a security analysis.
The diagram is too high level, and the components displayed are not tied to things that
we can protect, such as applications, platforms, databases, applications, and so forth.
Even though we understand, by studying Figure 3.2, that there’s some sort of “appli-
cation platform”—an operating environment that might call various modules that
are being considered as “applications”—we do not know what that execution entails,
whether “application” in this diagram should be considered as atomic, with attack sur-
faces exposed, or whether this is simply a functional nomenclature to express func-
tionality about which customers will have some interest. Operating systems provide
application execution. But so do “application servers.” Each of these presents rather dif-
ferent attack possibilities. An analysis of this “architecture” could not proceed without
more specificity about program execution.
In this case, the real product’s platform was actually a Java web application server
(a well-known version), with proprietary code running within the application server’s
usual web application runtime. The actual applications were packaged as J2EE serve-
lets. That means that custom code was running within a well-defined and publicly
available specification. The diagram that the vendor had given to me did not give me
much useful information; one could not even tell how “sources” were accessed, for what
Figure 3.2 Marketing architecture for a business intelligence product.

62 Securing Systems
operations (Read only? Write? Execute?). And which side, warehouse or source, initiated
the connection? From the diagram, it was impossible to know. Do the source commu-
nications require credentials? How might credentials be stored and protected? We don’t
have a clue from the diagram that authentication by each source is even supported.
[T]he System Context Diagram . . . is a methodological approach to assist in the
detailing of the conceptual architecture all the way down to the Operational Model
step by step and phase by phase.11
As may be seen from the foregoing explanation, the diagram in Figure 3.2 was quite
insufficient for the purposes of a security assessment. In fact, neither of these diagrams
(Figures 3.1 or 3.2) meets Zachman’s definition, “the total set of descriptive representa-
tions relevant for describing something.”12 Nor would either of these diagrams suitably
describe “all the way down to the Operational Model step by step.”13 Each of these
diagrams describes some of the system in an incomplete way, not only for the purposes
of security assessment, but incomplete in a more general architectural sense, as well.
Figures 3.1 and 3.2 may very well be sufficient for other purposes beyond general sys-
tem architecture or security architecture. My point is that these representations were
Figure 3.3 Sample external web architecture.14 (Courtesy of the SANS Institute.)

Security Architecture of Systems 63
insufficient for the kind of analysis about which this book is written. Since systems vary
so tremendously, it is difficult to provide a template for a system architecture that is
relevant across the extant variety and complexity. Still, a couple of examples may help?
Figure 3.3 is reproduced from an ISA Smart Guide that I wrote to explain how
to securely allow HTTP traffic to be processed by internal resources that were not
originally designed to be exposed to the constant attack levels of the Internet. The
diagram was not intended for architecture analysis. However, unlike Figure 3.1, several
trust-level boundaries are clearly delineated. Internet traffic must pass a firewall before
HTTP/S traffic is terminated at a web server. The web server is separated by a second
firewall from the application server. Finally, there is a third firewall between the entire
DMZ network and the internal networks (the cloud in the lower right-hand corner of
the diagram).
Further, in Figure 3.3, it is clear that only Structured Query Language (SQL) traf-
fic will be allowed from the application server to internal databases. The SQL traffic
originates at the application server and terminates at the internal databases. No other
traffic from the DMZ is allowed onto internal networks. The other resources within
the internal cloud do not receive traffic from the DMZ.
Figure 3.3 is still too high level for analyzing the infrastructure and runtime of the
components. We don’t know what kind of web server, application server, or database
may be implemented. Still, we have a far better idea about the general layout of the
architecture than from, say, Figure 3.1. We certainly know that HTTP and some vari-
ant of SQL protocols are being used. The system supports HTTPS (encrypted HTTP)
up to the first firewall. But communications are not encrypted from that firewall to the
web server. From Figure 3.3, we can tell that the SSL/TLS tunnel is terminated at the
first firewall. The diagram clearly demonstrates that it is HTTP past the firewall into
the DMZ.
We know where the protocols originate and terminate. We can surmise boundaries
of trust* from highly exposed to internally protected. We know that there are functional
tiers. We also know that external users will be involved. Since it’s HTTP, we know that
those users will employ some sort of browser or browser-like functionality. Finally, we
know that the infrastructure demarks a formal DMZ, which is generally restricted
from the internal network.
The security architect needs to understand bits of functionality that can be treated
relatively independently. Unity of any particular piece of the architecture we’ll call
“atomic.” The term “atomic” has a fairly specific meaning in some computer contexts.
It is the third Oxford Dictionary definition of atomic that applies to the art of secur-
ing systems:
* “Boundaries” in this context is about levels of exposure of networks and systems to hostile
networks, from exposed to protected. Th ese are usually called “trust boundaries.” It is gener-
ally assumed that as a segment moves closer to the Internet the less it is trusted. Well protected
from external traffi c has higher trust. We will examine boundaries in greater detail, later.

64 Securing Systems
[O]f or forming a single irreducible unit or component in a larger system15
“Irreducible” in our context is almost never true, until one gets down to the indivi-
dual line of code. Even then, is the irreducible unit a single binary computer instruc-
tion? Probably. But we don’t have to answer this question,* as we work toward the “right”
level of “single unit.” In the context of security assessments of systems, “atomic” may
be taken as treat as irreducible or regard as a “unit or component in a larger system.”16
In this way, the security architect has a requirement for abstraction that is different
from most of the other architects working on a system. As we shall see further along, we
reduce to a unit that presents the relevant attack surfaces. The reduction is dependent
on other factors in an assessment, which were enumerated earlier:
• Active threat agents that attack similar systems
• Infrastructure security capabilities
• Expected deployment model
• Distribution of executables or other deployable units
• The computer programming languages that have been used
• Relevant operating system(s) and runtime or execution environment(s)
This list is essentially synonymous with the assessment “background” knowledge,
or pre-assessment “homework” that has already been detailed. Unfortunately, there is
no single architecture view that can be applied to every component of every system.
“Logical” and “Component” are the most typical.
Depending upon on the security architect role that is described, one of two likely
situations prevail:

1. The security architect must integrate into existing architecture practices, making
use of whatever architecture views other architects are creating.
2 The security architect is expected to produce a “security view” of each architec-
ture that is assessed.†
In the first case, where the organization expects integration, essentially, the assessor
is going to “get what’s on offer” and make do. One can attempt to drive artifacts to some
useful level of detail, as necessary. When in this situation, I take a lot of notes about the
architecture because the diagrams offered are often incomplete for my purposes.
The second case is perhaps the luxury case? Given sufficient time, producing both
an adequate logical and component architecture, and then overlaying a threat model
onto them, delivers a working document that the entire team may consider as they
* I cannot remember a single instance of needing to go down to the assembly or binary code
level during a review.
† Th e author has personally worked under each of these assumptions.

Security Architecture of Systems 65
architect, design, code, and test. Such an artifact (diagram, or better, layered diagram)
can “seed” creative security involvement of the entire team.
Eoin Carroll, when he worked as a Senior Quality Engineer at McAfee, Inc., inno-
vated exactly this practice. Security became embedded into Agile team consideration
to the benefit of everyone involved with these teams and to the benefit of “building
security in from the start.” As new features were designed,* teams were able to consider
the security implications of the feature and the intended design before coding, or while
iterating through possible algorithmic solutions.
If the security architect is highly shared across many teams, he or she will likely not
have sufficient time to spend on any extensive diagramming. In this situation, because
diagramming takes considerable time to do well, diagramming a security architecture
view may be precluded.
And, there is the danger that the effort expended to render a security architecture
may be wasted, if a heavyweight document is only used by the security architect dur-
ing the assessment. Although it may be useful to archive a record of what has been
considered during the assessment, those building programs will want to consider cost
versus benefit carefully before mandating that there be a diagrammatic record of every
assessment. I have seen drawings on a white board, and thus, entirely ephemeral, suffice
for highly complex system analysis. Ultimately, the basic need is to uncover the security
needs of the system—the “security requirements.”
The decision about exactly which artifacts are required and for whose consumption
is necessarily an organizational choice. Suffice it to note that, in some manner, the secu-
rity architect who is performing a system analysis will require enough detail to uncover
all the attack surfaces, but no more detail than that. We will explore “decomposing”
and “factoring” architectures at some length, below. After our exploration, I will offer a
few guidelines to the art of decomposing an architecture for security analysis.
Let’s turn our attention for a moment to the “mental” game involved in understand-
ing an architecture in order to assess the architecture for security.
It has also been said that architecture is a practice of applying patterns. Security pat-
terns are unique problems that can be described as arising within disparate systems and
whose solutions can be described architecturally (as a representation).
Patterns provide us with a vocabulary to express architectural visions, as well as
examples of representative designs and detailed implementations that are clear and to
the point. Presenting pieces of software in terms of their constituent patterns also allows
us to communicate more effectively, with fewer words and less ambiguity.17
For instance, the need for authentication occurs not just between users, but wherever
in a software architecture a trust boundary occurs. This can be between eCommerce
* In SCRUM Agile, that point in the process when user stories are pulled from the backlog for
implementation during a Sprint.

66 Securing Systems
tiers (say, web to application server) or between privilege boundaries among executables
running on top of an operating system on a computer. The pattern named here is the
requirement of proof that the calling entity is not a rogue system, perhaps under control
of an attacker (say, authentication before allowing automated interactions). At a very
gross level, ensuring some level of trust on either side of a boundary is an authentication
pattern. However, we can move downwards in specificity by one level and say that all
tiers within a web stack are trust boundaries that should be authenticated. The usual
authentication is either bidirectional or the less trusted system authenticates to those
of higher trust. Similarly, any code that might allow attacker access to code running
at a higher privilege level, especially across executable boundaries, presents this same
authentication pattern.
That is, entities at higher trust levels should authenticate communication flows from
entities of lower trust. Doing so prevents an attacker from pretending to be, that is,
“spoofing,” the lower trust entity. “Entity” in this discussion is both a web tier and an exe-
cutable process. The same pattern expresses itself in two seemingly disparate archite ctures.
Figure 3.4 represents the logical Web architecture for the Java application develop-
ment environment called “AppMaker.”* AppMaker produces dynamic web applications
without custom coding by a web developer. The AppMaker application provides a plat-
Figure 3.4 AppMaker Web architecture.
* AppMaker is not an existing product. Th ere are many off erings for producing web applica-
tions with little or no coding. Th is example demonstrates a typical application server and
database architecture.

Security Architecture of Systems 67
form for creating dynamic web applications drawing data from a database, as needed,
to respond to HTTP requests from a user’s browser. For our purposes, this architecture
represents a classic pattern for a static content plus dynamic content web application.
Through this example, we can explore the various logical components and tiers of a
typical web application that also includes a database.
The AppMaker architecture shows a series of arrows representing how a typical HTTP
request will be handled by the system. Because there are two different flows, one to
return static content, and an alternate path for dynamic content built up out of the data-
base, the return HTTP response flow is shown (“5” from database server to AppMaker,
and then from AppMaker through the webserver). Because there are two possible flows
in this logical architecture, there is an arrow for each of the two response flows.
Quite often, an HTTP response will be assumed; an architecture diagram would
only show the incoming request. If the system is functioning normally, it will generate
a response; an HTTP response can be assumed. HTTP is a request/response protocol.
But in this case, the program designers want potential implementers to understand
that there are two possible avenues for delivering a response: a static path and a dynamic
path. Hence, you can see “2a” being retrieved from the disk available to the Web server
(marked “Static Content”). That’s the static repository.
Dynamic requests (or portions of requests) are delivered to the AppMaker web
application, which is incoming arrow “2b” going from the Web server to the applica-
tion server in the diagram. After generating the dynamic response through interactions
with custom code, forms, and a database server (arrows 3 and 4), the response is sent
back in the outgoing arrows, “5.”
Digging a little further into Figure 3.4, you may note that there are four logical
tiers. Obviously, the browser is the user space in the system. You will often hear secu-
rity architects exclude the browser when naming application tiers, whereas the browser
application designers will consider the browser to be an additional web application tier,
for their purposes. Inclusion of the browser as a tier of the web application is especially
common when there is scripting or other application-specific code that is downloaded
to the browser, and, thus, a portion of the system is running in the context of the user’s
browser. In any case, whether considering the browser as a tier in the architecture or
not, the user’s browser initiates a request to the web application, regardless of whether
there is server-supplied code running in the browser.
This opposing viewpoint is a function of what can be trusted and what can be pro-
tected in a typical Web application. The browser must always be considered “untrusted.”
There is no way for a web application to know whether the browser has been compro-
mised or not. There is no way for a web application to confirm that the data sent as
HTTP requests is not under the control of an attacker.* By the way, authentication
of the user only reduces the attack surface. There is still no way to guarantee that an
* Likewise, a server may be compromised, thus sending attacks to the user’s browser. From the
user’s perspective, the web application might be considered untrusted.

68 Securing Systems
attacker hasn’t previously taken over the user’s session or is otherwise misusing a user’s
login credentials.
Manipulating the variables in the URL is simple. But attackers can also manipulate
almost all information going from the client to the server like form fields, hidden fields,
content-length, session-id and http methods.18
Due to the essential distrust of everything coming into any Web application, security
architects are likely to discount the browser as a valid tier of the application. Basically,
there is very little that a web application designer can do to enhance the protection of
the web browsers. That is not to say that there aren’t applications and security controls
that can’t be applied to web browser; there most certainly are. Numerous security ven-
dors offer just such protections. However, for a web application that must serve content
to a broad population, there can be no guarantees of browser protection; there are
no guarantees that the browser hasn’t already been compromised or controlled by an
attacker. Therefore, from a security perspective, the browser is often considered outside
the defensible perimeter of a web application or web system. While in this explanation
we will follow that customary usage, it must be noted that there certainly are applica-
tions where the browser would be considered to lie within the perimeter of the web
application. In this case, the browser would then be considered as the user tier of the
Returning then to Figure 3.4, from a defensible perimeter standpoint, and from the
standpoint of a typical security architect, we have a three-tier application:
1. Web server
2. Application server
3. Database
For this architecture, the Web server tier includes disk storage. Static content to be
served by the system resides in this forward most layer. Next, further back in the sys-
tem, where it is not directly exposed to HTTP-based attacks (which presumably will be
aimed at the Web server?), there is an application server that runs dynamic code. We
don’t know from this diagram what protocol is used between the Web server and the
application server. We do know that messages bound for the application server originate
at the Web server. The arrow pointing from the Web server to the application server
clearly demonstrates this. Finally, as requests are processed, the application server inter-
acts with the database server to construct responses. Figure 3.4 does not specify what
protocol is used to interact with the database. However, database storage is shown as a
separate component from the database server. This probably means that storage can be
separated from the actual database application code, which could indicate an additional
tier, if so desired.
What security information can be harvested from Figure 3.4? Where are the obvious
attack surfaces? Which is the least-trusted tier? Where would you surmise that the

Security Architecture of Systems 69
greatest trust resides? Where would you put security controls? You will note that no
security boundaries are depicted in the AppMaker logical architecture.
In Chapter 6, we will apply our architecture assessment and threat modeling
methodology to this architecture in an attempt to answer these questions.
Figure 3.5 represents a completely different type of architecture compared to a web
application. In this case, there are only two components (I’ve purposely simplified the
architecture): a user interface (UI) and a kernel driver. The entire application resides
on some sort of independent computing device (often called an “endpoint”). Although
a standard desktop computer is shown, this type of architecture shows up on laptops,
mobile devices, and all sorts of different endpoint types that can be generalized to most
operating systems. The separation of the UI from a higher privileged system function is
a classic architecture pattern that crops up again and again.
Under most operating systems where there is some user-accessible component that
then opens and perhaps controls a system level piece of code, such as a kernel driver, the
kernel portion of the application will run at a higher privilege level than the user inter-
face. The user interface will run at whatever privilege level the logged-in user’s account
runs. Generally, pieces of code that run as part of the kernel have to have access to all
system resources and must run at a much higher privilege level, usually the highest
privilege level available under the operating system. The bus, kernel drivers, and the
like are valuable targets for attackers. Once an attacker can insert him or herself into
the kernel: “game over.” The attacker has the run of the system to perform whatever
actions and achieve whatever goals are intended by the attack. For system takeover, the
kernel is the target.
For system takeover, the component presents a valuable and interesting attack sur-
face. If the attacker can get at the kernel driver through the user interface (UI) in some
Figure 3.5 Two-component endpoint application and driver.

70 Securing Systems
fashion, then his or her goals will have been achieved. Whatever inputs the UI portion
of our architecture presents (represented in Figure 3.5) become critical attack surfaces
and must be defended. If Figure 3.5 is a complete architecture, it may describe enough
of a logical architecture to begin a threat model. Certainly, the key trust boundary is
obvious as the interface between user and system code (kernel driver). We will explore
this type of application in somewhat more depth in a subsequent chapter.
3.4 Seeing and Applying Patterns
A pattern is a common and repeating idiom of solution design and architecture. A
pattern is defined as a solution to a problem in the context of an application.19
Through patterns, unique solutions convert to common patterns that make the task
of applying information security to systems much easier. There are common patterns
at a gross level (trust/distrust), and there are recurring patterns with more specificity.
Learning and then recognizing these patterns as they occur in systems under assess-
ment is a large part of assessing systems for security.
Identifying patterns is a key to understanding system architectures. Understanding
an architecture is a prerequisite to assessing that architecture. Remediating the security
of an architecture is a practice of applying security architecture patterns to the system
patterns found within an architecture. Unique problems generating unique solutions do
crop up; one is constantly learning, growing, and maturing one’s security architecture
practice. But after a security architect has assessed a few systems, she or he will start to
apply security patterns as solutions to architectural patterns.
There are architectural patterns that may be abstracted from specific architectures:
• Standard e-commerce Web tiers
• Creating a portal to backend application services
• Database as the point of integration between disparate functions
• Message bus as the point of integration between disparate functions
• Integration through proprietary protocol
• Web services for third-party integration
• Service-oriented architecture (SOA)
• Federated authentication [usually Security Assertion Markup Language (SAML)]
• Web authentication validation using a session token
• Employing a kernel driver to capture or alter system traffic
• Model–view–controller (MVC)
• Separation of presentation from business logic
• JavaBeans for reusable components
• Automated process orchestration
• And more

Security Architecture of Systems 71
There are literally hundreds of patterns that repeat, architecture to architecture. The
above list should be considered as only a small sample.
As one becomes familiar with various patterns, they begin to “pop out,” become
obvious. An experienced architect builds solutions from these well-known patterns.
Exactly which patterns will become usable is dependent upon available technologies
and infrastructure. Typically, if a task may be accomplished through a known or even
implemented pattern, it will be more cost-effective than having to build an entirely new
technology. Generally, there has to be a strong business and technological motivation
to ignore existing capabilities in favor of building new ones.
Like architectural patterns, security solution patterns also repeat at some level of
abstraction. The repeatable security solutions are the security architecture “patterns.”
For each of the architectural patterns listed above, there are a series of security controls
that are often applied to build a defense-in-depth. A security architect may fairly rapidly
recognize a typical architecture pattern for which the security solution is understood.
To the uninitiated, this may seem mysterious. In actuality, there’s nothing mysterious
about it at all. Typical architectural patterns can be generalized such that the security
solution set also becomes typical.
As an example, let’s examine a couple of patterns from the list above.
• Web services for third-party integration:
ο Bidirectional, mutual authentication of each party
ο Encryption of the authentication exchange
ο Encryption of message traffic
ο Mutual distrust: Each party should carefully inspect data that are received for
anomalous and out-of-range values (input validation)
ο Network restrictions disallowing all but intended parties
• Message bus as a point of integration:
ο Authentication of each automated process to the message bus before allowing
further message traffic
ο Constraint on message destination such that messages may only flow to
intended destinations (ACL)
ο Encryption of message traffic over untrusted networks
ο In situations where the message bus crosses the network trust boundaries, access
to the message bus from less-trusted networks should require some form of
access grant process
Hopefully, as may be seen, each of the foregoing patterns (listed) has a fairly well-
defined security solution set.* When a system architecture is entirely new, of course, the
* Th e security solutions don’t include specifi c technology; the implementation is undefi ned—
lack of specifi city is purposive at this level of abstraction. In order to be implemented, these
requirements will have to be designed with specifi c technologies and particular semantics.

72 Securing Systems
security assessor will need to understand the architecture in a fairly detailed manner (as
we will explain in a later chapter). However, architectural patterns repeat over and over
again. The assessment process is more efficient and can be done rapidly when repeating
architectural patterns are readily recognized. As you assess systems, hopefully, you will
begin to notice the patterns that keep recurring?
As you build your catalog of architectural patterns, so you will build your catalog
of security solution patterns. In many organizations, the typical security solution sets
become the organization’s standards.
I have seen organizations that have sufficient standards (and sufficient infrastructure
to support those standards in an organized and efficient manner) to allow designs that
strictly follow the standards to bypass security architecture assessment entirely. Even
when those standard systems were highly complex, if projects employed the standard
architectural patterns to which the appropriate security patterns were applied, then the
organization had fairly strong assurance that there was little residual risk inherent in
the new or updated system. Hence, the AR A could be skipped. Such behavior is typi-
cally a sign of architectural and security maturity. Often (but not always), organizations
begin with few or no patterns and little security infrastructure. As time and complex-
ity increase, there is an incentive to be more efficient; every system can’t be deployed
as a single, one-off case. Treating every system as unique is inefficient. As complexity
increases, so does the need to recognize patterns, to apply known solutions, and to
make those known solutions standards that can then be followed.
I caution organizations to avoid attempting to build too many standards before the
actual system and security patterns have emerged. As has been noted above, there are clas-
sic patterns that certainly can be applied right from the start of any program. However,
there is a danger of specifying capabilities that will never be in place and may not even
be needed to protect the organization. Any hints of “ivory tower,” or other idealized but
unrealistic pronouncements, are likely to be seen as incompetence or, at the very least,
misunderstandings. Since the practice of architecture is still craft and relatively relation-
ship based, trust and respect are integral to getting anything accomplished.
When standards reflect reality, they will be observed. But just as importantly, when
the standards make architectural and security sense, participants will implicitly under-
stand that a need for an exception to standards will need to be proved, not assumed.
Hence, blindly applying industry “standards” or practices without first understanding
the complexities of the situation at hand is generally a mistake and will have costly
Even in the face of reduced capabilities or constrained resources, if one understands
the normal solution to an architectural pattern, a standard solution, or an industry-
recognized solution, one can creatively work from that standard. It’s much easier to start
with something well understood and work towards an implementable solution, given
the capabilities at hand. This is where a sensible risk practice is employed. The architect
must do as much as possible and then assess any remaining residual risk.
As we shall see, residual risk must be brought to decision makers so that it can either
be accepted or treated. Sometimes, a security architect has to do what he or she can

Security Architecture of Systems 73
within the limits and constraints given, while making plain the impact that those limits
are likely to generate. Even with many standard patterns at hand, in the real world,
applying patterns must work hand-in-hand with a risk practice. It has been said that
information security is “all about risk.”
In order to recognize patterns—whether architectural or security—one has to have
a representation of the architecture. There are many forms of architectural representa-
tion. Certainly, an architecture can be described in a specification document through
descriptive paragraphs. Even with a well-drawn set of diagrams, the components and
flows will typically need to be documented in prose as well as diagramed. That is,
details will be described in words, as well. It is possible, with sufficient diagrams and
a written explanation, that a security assessment can be performed with little or no
interaction. In the author’s experience, however, this is quite rare. Inevitably, the dia-
gram is missing something or the descriptions are misleading or incomplete. As you
begin assessing systems, prepare yourself for a fair amount of communication and dia-
logue. For most of the architects with whom I’ve worked and who I’ve had the privilege
to train and mentor, the architectural diagram becomes the representation of choice.
Hence, we will spend some time looking at a series of diagrams that are more or less
typical. Like Figure 3.3, let’s try to understand what the diagram tells us, as well as from
a security perspective, what may be missing.
3.5 System Architecture Diagrams and Protocol Interchange
Flows (Data Flow Diagrams)
Let’s begin by defining what we mean by a representation. In its simplest form, the
representation of a system is a graphical representation, a diagram. Unfortunately, there
are “logical” diagrams that contain almost no useful information. Or, a diagram can
contain so much information that the relevant and important areas are obscured.
A classic example of an overly simplified view would be a diagram containing a
laptop, a double-headed arrow from the laptop to the server icon with, perhaps, a brick
wall in between representing a firewall (actual, real-world “diagrams”). Figure 3.1 is
more less this simple (with the addition of some sort of backend server component).
Although it is quite possible that the system architecture is really this simple (there are
systems that only contain the user’s browser and the Web server), we still don’t know a
key piece of information without asking, namely, which side, laptop or server, opens the
connection and begins the interaction. Merely for the sake of understanding authenti-
cation, we have to understand that one key piece of the communication flow.* And for
most modestly complex systems, it’s quite likely that there are many more components
* Given the ubiquity of HTTP interactions, if the protocol is HTTP and the content is some
form of browser interaction (HTML+dynamic content), then origination can safely be
assumed from the user, from the user’s browser, or from an automated process, for example,
a “web service client.”

74 Securing Systems
involved than just a laptop and a server (unless the protocol is telnet and the laptop is
logging directly into the server).
Figure 3.6 represents a conceptual sample enterprise architecture. Working from the
abovementioned definition given by Godinez et al. (2010)20 of a conceptual architec-
ture, Figure 3.6 then represents the enterprise architect’s view of the business relation-
ships of th e architecture. What the conceptual architecture intends to represent are the
business functions and their interrelationships; technologies are typically unimportant,
We start with an enterprise view for two reasons:
1. Enterprise architecture practice is better described than system architecture.
2. Each system under review must fit into its enterprise architecture.
Hence, because the systems you will review have a place within and deliver some part
of the intent of the enterprise architecture, we begin at this very gross level. When one
possesses some understanding of enterprise architectures, this understanding provides
a basis for the practice of architecture and, specifically, security architecture. Enterprise
architecture, being a fairly well-described and mature area, may help unlock that which
is key to describing and then analyzing all architectures. We, therefore, begin at the
enterprise level.
Figure 3.6 Conceptual enterprise architecture.

Security Architecture of Systems 75
In a conceptual enterprise architecture, a very gross level of granularity is displayed
so that viewers can understand what business functions are at play. For instance, in
Figure 3.6, we can understand that there are integrating services that connect func-
tions. These have been collapsed into a single conceptual function: “Integrations.”
Anyone who has worked with SOA knows that, at the very least, there will be clients
and servers, perhaps SOA managing software, and so on. These are all collapsed, along
with an enterprise message bus, into a single block. “Functions get connected through
integrations” becomes the architecture message portrayed in Figure 3.6.
Likewise, all data has been collapsed into a single disk. In an enterprise, it is highly
unlikely that terrabytes of data could be delivered on a single disk icon. Hence, we know
that this representation is conceptual: There is data that must be delivered to applica-
tions and presentations. The architecture will make use of “integrations” in order to
access the data. Business functions all are integrated with identity, data, and metadata,
whereas the presentations of the data for human consumption have been separated out
from the business functions for a “Model, View, Controller” or MVC separation. It is
highly unlikely that an enterprise would use a single presentation layer for each of the
business functions. For one thing, external customers’ presentations probably shouldn’t
be allowed to mix with internal business presentations.
In Figure 3.6, we get some sense that there are technological infrastructures that
are key to the business flows and processes. For instance, “Integrations” implies some
sort of messaging bus technology. Details like a message bus and other infrastructures
might be shown in the conceptual architecture only if the technologies were “stan-
dards” within the organization. Details like a message bus might also be depicted if
these details will in some manner enhance the understanding of what the architecture
is trying to accomplish at a business level. Mostly, technologies will be represented at a
very gross level; details are unimportant within the conceptual architecture. There are
some important details, however, that the security architect can glean from a concep-
tual architecture.
Why might the security architect want to see the conceptual architecture? As I wrote
in Chapter 9 of Core Software Security,21 early engagement of security into the Secure
Development Lifecycle (SDL) allows for security strategy to become embedded in the
architecture. “Strategy” in this context means a consideration of the underlying secu-
rity back story that has already been outlined, namely, the organization’s risk tolerance
and how that will be implemented in the enterprise architecture or any specific portion
of that architecture. Security strategy will also consider the evolving threat landscape
and its relation to systems of the sort being contemplated. Such early engagement will
enhance the conceptual architecture’s ability to account for security. And just as impor-
tantly, it will make analysis and inclusion of security components within the logical
architecture much easier, as architectures move to greater specificity.
From Figure 3.6 we can surmise that there are “clients,” “line of business systems,”
“presentations,” and so on who must connect through some sort of messaging or other
exchange semantic [perhaps file transfer protocol (FTP)] with core business services. In
this diagram, two end-to-end, matrix domains are conceptualized as unitary:

76 Securing Systems
• Process Orchestrations
• Security and privacy services
This is a classic enterprise architect concept of security; security is a box of ser-
vices rather than some distinct services (the security infrastructure) and some security
Figure 3.7 Component enterprise architecture.

Security Architecture of Systems 77
capabilities built within each component. It’s quite convenient for an enterprise archi-
tect to imagine security (or orchestrations, for that matter) as unitary. Enterprise archi-
tects are generally not domain experts. It’s handy to unify into a “black box,” opaque,
singular function that one needn’t understand, so one can focus on the other services. (I
won’t argue that some security controls are, indeed, services. But just as many are not.)
Figure 3.6 also tells us something about the integration of the systems: “service-
oriented.” This generally means service-oriented architecture (SOA). At an enterprise
level, these are typically implemented through the use of Simple Object Access protocol
(SOAP) services or Web services. The use of Web services implies loose coupling to
any particular technology stack. SOAP implementation libraries are nearly ubiquitous
across operating systems. And, the SOAP clients and servers don’t require program-
ming knowledge of each other’s implementation in order to work: loosely coupled. If
mature, SOA may contain management components, and even orchestration of services
to achieve appropriate process stepping and process control.
You might take a moment at this point to see what questions come up about this
diagram (see Figure 3.6). What do you think is missing? What do you want to know
more of? Is it clear from the diagram what is external to the organization and what lies
within possible network or other trust boundaries?
Figure 3.7 represents the same enterprise architecture that was depicted in Figure
3.6. Figure 3.6 represents a conceptual view, whereas Figure 3.7 represents the compo-
nent view.
3.5.1 Security Touches All Domains
For a moment, ignore the box second from the left titled “Infrastructure Security
Component” found in the conceptual diagram (Figure 3.6). For enterprise architects,
it’s quite normal to try and treat security as a black box through which communications
and data flow. Somehow the data are “magically” made secure. If you work with enough
systems, you will see these “security” boxes placed into diagrams over and over again.
Like any practice, the enterprise architect can only understand so many factors
and so many technologies. Usually, anyone operating at the enterprise level will be an
expert in many domains. The reason they depend upon security architects is because
the enterprise architects are typically not security experts. Security is a matrix function
across every other domain. Some security controls are reasonably separate and distinct,
and thus, can be placed in their own component space, whereas other controls must
be embedded within the functionality of each component. It is our task as security
architects to help our sister and brother architects understand the nature of security a s
a matrix domain.*
* Annoying as the treatment of security as a kind of unitary, magical transformation might be,
I don’t expect the architects with whom I work to be security experts. Th at’s my job.

78 Securing Systems
In Figure 3.7, the security functions have been broken down into four distinct
1. Internet facing access controls and validation
2. External to internal access controls and validation
3. Security monitoring
4. A data store of security alerts and events that is tightly coupled to the security
monitoring function
This component breakout still hides much technological detail. Still, we can see
where entrance and exit points are, where the major trust boundaries exist. Across the
obvious trust boundary between exposed networks (at the top of the diagram) and the
internal networks, there is some sort of security infrastructure component. This com-
ponent is still largely . Still, placing “access controls and validation” between
the two trust zones allows us to get some feel for where there are security-related com-
ponents and how these might be separated from the other components represented in
Figure 3.7. The security controls that must be integrated into other components would
create too much visual noise in an already crowded representation. Another security-
specific view might be necessary for this enterprise architecture.
3.5.2 Component Views
Moving beyond the security functions, how is the component view different from the
conceptual view?
Most obviously, there’s a lot more “stuff ” depicted. In Figure 3.7, there are now
two very distinct areas—“external” and “internal.” Functions have been placed such
that we can now understand where within these two areas the function will be placed.
That single change engenders the necessity to split up data so that co-located data will
be represented separately. In fact, the entire internal data layer has been sited (and thus
associated to) the business applications and processing. Regarding those components
for which there are multiple instances, we can see these represented.
“Presentations” have been split from “external integrations” as the integrations are
sited in a special area: “Extranet.” That is typical at an enterprise, where organizations
are cross-connected with special, leased lines and other point-to-point solutions, such
as virtual private networks (VPN). Access is granted based upon business contracts and
relationships. Allowing data exchange after contracts are confirmed is a different rela-
tionship than encouraging interested parties to be customers through a “presentation”
of customer services and online shopping (“eCommerce”). Because these two modes
of interaction are fundamentally different, they are often segmented into different
zones: web site zone (for the public and customers) and Extranet (for business partners).
Typically, both of these will be implemented through multiple applications, which are

Security Architecture of Systems 79
usually deployed on a unitary set of shared infrastructure services that are sited in the
externally accessible environment (a formal “DMZ”).
In Figure 3.7 you see a single box labeled, “External Infrastructures,” which cuts
across both segments, eCommerce and Extranet. This is to indicate that for economies
of scale, there is only one set of external infrastructures, not two. That doesn’t mean
that the segments are not isolated from each other! And enterprise architects know
full well that infrastructures are complex, which is why the label is plural. Still, at this
granularity, there is no need to be more specific than noting that “infrastructures” are
separated from applications.
Take a few moments to study Figures 3.6 and 3.7, their similarities and their dif-
ferences. What functions have been broken into several components and which can be
considered unitary, even in the component enterprise architecture view?
3.6 What’s Important?
The amount of granularity within any particular architecture diagram is akin to the
story of Goldilocks and the Three Bears. “This bed is too soft! This bed is too hard! This
bed is just right.” Like Goldilocks, we may be presented with a diagram that’s “too
soft.” The diagram, like Figure 3.1, doesn’t describe enough, isn’t enough of a detailed
representation to uncover the attack surfaces.
On the other hand, a diagram that breaks down the components that, for the pur-
poses of analysis, could have been considered as atomic (can be treated as a unit) into
too many subcomponents will obscure the attack surfaces with too much detail: “This
diagram is too hard!”
As we shall see in the following section, what’s “architecturally interesting” is depen-
dent upon a number of factors. Unfortunately, there is no simple answer to this problem.
When assessing, if you’re left with a lot of questions, or the diagram only answers one
or two, it’s probably “too soft.” On the other hand, if your eyes glaze over from all the
detail, you probably need to come up one or two levels of granularity, at least to get
started. That detailed diagram is “too hard.” There are a couple of patterns that can help.
3.6.1 What Is “Architecturally Interesting”?
This is why I wrote “component functions.” If the interesting function is the operat-
ing system of a server, then one may think of the operating system in an atomic man-
ner. However, even a command-line remote access method such as telnet or secure
Shell (SSH) gives access to any number of secondary logical functions. In the same
way, unless a Web server is only sharing static HTML pages, there is likely to be an
application, some sort of processing, and some sort of data involved beyond an atomic
web server. In this case, our logical system architecture will probably need a few more

80 Securing Systems
components and the methods of communication between those components: Web
server, application, data store. There has to be a way for the Web server to instantiate
the application processing and then return the HTTP response from that processing.
And the application will need to fetch data from the data store and perhaps update the
data based on whatever processing is taking place. We have now gone from two compo-
nents to five. We’ve gone from one communication flow to three. Typical web systems
are considerably more complex than this, by the way.
On the other hand, let’s consider the web tier of a large, commercial server. If we
know with some certainty that web servers are only administered by security savvy,
highly trained and highly trusted web masters, then we can assume a certain amount
of restriction to any attacker-attractive functionality. Perhaps we already know and
have approved a rigorous web server and operating environment hardening standard.
Storage areas are highly restricted to only allow updates from trusted sources and to
only allow read operations from the web servers. The network on which these web
servers exist is highly restricted such that only HTTP/S is allowed into the network
from untrusted sources, only responses from the web servers can flow back to untrusted
sources, and administrative traffic comes only from a trusted source that has consider-
able access restrictions and robust authorization before grant of access. That adminis-
trative network is run by security savvy, highly trusted individuals handpicked for the
role through a formal approval process, and so forth.*
In the website case outlined above, we may choose to treat web servers as atomic
without digging into their subcomponents and their details. The web servers inherit
a great deal of security control from the underlying infrastructure and the established
formal processes. Having answered our security questions once to satisfaction, we don’t
need to ask each web project going into the environment, so long as the project uses the
environment in the intended and accepted manner, that is, the project adheres to the
existing standards. In a security assessment, we would be freed to consider other factors,
given reasonably certain knowledge and understanding of the security controls already
in place. Each individual server can be considered “atomic.” In fact, we may even be
able to consider an entire large block of servers hosting precisely the same function as
atomic, for the purposes of analysis.
Besides, quite often in these types of highly controlled environments, the application
programmer is not given any control over the supporting factors. Asking the application
team about the network or server administration will likely engender a good deal of
frustration. Also, since the team members actually don’t have the answers, they may be
encouraged to guess. In matters relating to security due diligence, guessing is not good
enough. An assessor must have near absolute certainty about everything about which
certainty can be attained. All unknowns must be treated as potential risks.
Linked libraries and all the different objects or other modular interfaces inside an
executable program usually don’t present any trust boundaries that are interesting. A
* We will revisit web sites more thoroughly in later chapters.

Security Architecture of Systems 81
single process (in whatever manner the execution environment defines “process”) can
usually be considered atomic. There is generally no advantage to digging through the
internal software architecture, the internal call graph of an executable process space.
The obvious exception to the guideline to treat executable packages as atomic are
dynamically linked executable forms,* such as DLLs under the Microsoft operating sys-
tems or dynamic link libraries under UNIX. Depending upon the rest of the architec-
ture and the deployment model, these communications might prove interesting, since
certain attack methods substitute a DLL of the attacker’s choosing.
The architecture diagram needs to represent the appropriate logical components. But,
unfortunately, what constitutes “logical components” is dependent upon three factors:
1. Deployment model
2. Infrastructure (and execution environment)
3. Attack methods
In the previous chapter, infrastructure was mentioned with respect to security capa-
bilities and limitations. Alongside the security capabilities that are inherited from the
infrastructure and runtime stack, the very type of infrastructure upon which the system
will run influences the level at which components may be considered atomic. This
aspect is worth exploring at some length.
3.7 Understanding the Architecture of a System
The question that needs answering in order to factor the architecture properly for attack
surfaces is at what level of specificity can components be treated as atomic? In other
words, how deep should the analysis decompose an architecture? What constitutes
meaningless detail that confuses the picture?
3.7.1 Size Really Does Matter
As mentioned above, any executable package that is joined to a running process after
it’s been launched is a point of attack to the executable, perhaps to the operating system.
This is particularly true where the attack target is the machine or virtual machine itself.
Remember that some cyber criminals make their living by renting “botnets,” networks
of attacker-controlled machines. For this attack goal, the compromise of a machine
has attacker value in and of itself (without promulgating some further attack, like key-
stroke logging or capturing a user session). In the world of Advanced Persistent Threats
(APT), the attacker may wish to control internal servers as a beachhead, an internal
* We will examine another exception below: Critical pieces of code, especially code that handles
secrets, will be attacked if the secret protects a target suffi ciently attractive.

82 Securing Systems
machine from which to launch further attacks. Depending upon the architecture of
intrusion detection services (IDS), if attacks come from an internal machine, these
internally originating attacks may be ignored. Like botnet compromise, APT attackers
are interested in gaining the underlying computer operating environment and subvert-
ing the OS to their purposes.
Probing a typical computer operating system’s privilege levels can help us delve into
the factoring problem. When protecting an operating environment, such as a user’s lap-
top or mobile phone, we must decompose down to executable and/or process boundaries.
The presence of a vulnerability, particularly an overflow or boundary condition vulner-
ability that allows the attacker to execute code of her or his choosing, means that one
process may be used against all the others, especially if that process is implicitly trusted.
As an example, imagine the user interface (UI) to an anti-virus engine (AV).
Figure 3.4 could represent an architecture that an AV engine might employ. We could
add an additional process running in user space, the AV engine. Figure 3.8 depicts this
change to the architecture that we examined in Figure 3.4. Many AV engines employ
system drivers in order to capture file and network traffic transparently. In Figure 3.8,
we have a generalized anti-virus or anti-malware endpoint architecture.
The AV runs in a separate process space; it receives commands from the UI, which
also runs in a separate process. Despite what you may believe, quite often, AV engines
do not run at high privilege. This is purposive. But, AV engines typically communicate
or receive communications from higher privilege components, such as system drivers
and the like. The UI will be running at the privilege level of the user (unless the security
architect has made a big mistake!).
Figure 3.8 Anti-virus endpoint architecture.

Security Architecture of Systems 83
In this situation, a takeover of the UI process would allow the attacker to send com-
mands to the AV engine. This could result in a simple denial of service (DOS) through
overloading the engine with commands. But perhaps the UI can turn off the engine?
Perhaps the UI can tell the engine to ignore malicious code of the attacker’s choosing?
These scenarios suggest that the communication channel from UI to AV needs some
protection. Generally, the AV engine should be reasonably suspicious of all communica-
tions, even from the UI.
Still, if the AV engine does not confirm that the UI is, indeed, the one true UI
component shipped with the product, the AV engine presents a much bigger and more
dangerous attack surface. In this case, with no authentication and validation of the UI
process, an attacker no longer needs to compromise the UI! Why go to all the trouble of
reverse-engineering the UI, hunting for possible overflow conditions, and then building
an exploit for the vulnerability? That’s quite a bit of work compared to simply supplying
the attacker’s very own UI. By studying the calls and communications between the UI
and the AV engine, the attacker can craft her or his own UI component that has the
same level of control as the product’s UI component. This is a lot less work than reverse
engineering the product’s UI component. This attack is made possible when the AV
engine assumes the validity of the UI without verification. If you will, there is a trust
relationship between the AV engine and the UI process. The AV process must establish
trust of the UI. Failure to do so allows the attacker to send commands to the AV engine,
possibly including, “Stop checking for malware.”
The foregoing details why most anti-virus and malware programs employ digital sig-
natures rendered over executable binary files. The digital signature can be validated by
each process before communications commence. Each process will verify that, indeed,
the process attempting to communicate is the intended process. Although not entirely
foolproof,* binary signature validation can provide a significant barrier to an attack to a
more trusted process from a less than trusted source.
Abstracting the decomposition problem from the anti-virus engine example, one
must factor an independently running endpoint architecture (or subcomponent) down
to the granularity of each process space in order to establish trust boundaries, attack
surfaces, and defensible perimeters. As we have seen, such granular depth may be
unnecessary in other scenarios. If you recall, we were able to generally treat the user’s
browser atomically simply because the whole endpoint is untrusted. I’ll stress again: It is
the context of the architecture that determines whether or not a particular component
will need to be factored further.
* It is beyond the scope of this book to delve into the intricacies of signature validations. Th ese
are generally performed by the operating system in favor of a process before load and execu-
tion. However, since system software has to remain backward compatible, there are numerous
very subtle validation holes that have become diffi cult to close without compromising the
ability of users to run all of the user’s software.

84 Securing Systems
For the general case of an operating system without the presence of significant, addi-
tional, exterior protections, the system under analysis can be broken down into execut-
able processes and dynamically loaded libraries. A useful guideline is to decompose
the architecture to the level of executable binary packages. Obviously, a loadable “pro-
gram,” which when executed by the operating system will be placed into whatever
runtime space is normally given to an executable binary package, can be considered
an atomic unit. Communications with the operating system and with other executable
processes can then be examined as likely attack vectors.
3.8 Applying Principles and Patterns to Specifi c Designs
How does Figure 3.9 differ from Figure 3.8? Do you notice a pattern similarity that exists
within both architectures? I have purposely named items in the drawing using typical
mobile nomenclature, rather than generalizing, in the hope that you will translate these
details into general structures as you study the diagram. Before we explore this typical
mobile anti-virus or anti-malware application architecture, take a few moments to look
at Figure 3.8, then Figure 3.9. Please ponder the similarities as well as differences. See if
you can abstract the basic underlying pattern or patterns between the two architectures.
Obviously, I’ve included a “communicate” component within the mobile architec-
ture. Actually, there would be a similar function within almost any modern endpoint
Figure 3.9 Mobile security application endpoint architecture.

Security Architecture of Systems 85
security application, whether the software was intended for consumers, any size orga-
nization, or enterprise consumption. People expect their malware identifications to get
updated almost in real time, let’s say, “rapidly.” These updates* are often sent from a
central threat “intelligence” team, a threat evaluation service via centralized, highly
controlled Web services to the endpoint.†
In addition, the communicator will likely send information about the state of the
endpoint to a centralized location for analysis: Is the endpoint compromised? Does it
store malware? What versions of the software are currently running? How many evil
samples have been seen and stopped? All kinds of telemetry about the state of the end-
point are typically collected. This means that communications are usually both ways:
downwards to the endpoint and upwards to a centralized server.
In fact, in today’s mobile application market, most applications will embed some
sort of communications. Only the simplest application, say a “flashlight” that turns on
the camera’s light, or a localized measuring tool or similar discreet application, will not
require its own server component and the necessary communications flows. An embed-
ded mobile communications function is not unique to security software; mobile server
communications are ubiquitous.
In order to keep things simple, I kept the communications out of the discussion of
Figure 3.8. For completeness and to represent a more typical mobile architecture, I have
introduced the communicator into Figure 3.9. As you may now see, the inclusion of the
communicator opens up all kinds of new security challenges. Go ahead and consider
these as you may. We will take up the security challenges within a mobile application
in the analyses in Part II. For the moment, let’s restrict the discussion to the mobile
endpoint. Our task at this point in the journey is to understand architectures. And,
furthermore, we need to understand how to extract security-related information from
an architecture diagram so that we have the skills to proceed with an architecture risk
assessment and threat model.
The art of architecture involves the skill of recognizing and then applying abstract
patterns while, at the same time, understanding any local details that will be ignored
through the application of patterns. Any unique local circumstances are also important
and will have to be attended to properly.
It is not that locally specific details should be completely ignored. Rather, in the
interest of achieving an “architectural” view, these implementation details are over-
looked until a broader view can be established. That broader view is the architecture.
As the architecture proceeds to specific design, the implementation details, things like
specific operating system services that are or are not available, once again come to the
fore and must receive attention.
* Th ese updates are called “DAT” fi les or updates. Every endpoint security service of which the
author knows operates in this manner.
† For enterprises, the updated DAT will be sent to an administrative console from which admin-
istrators can then roll out to large numbers of endpoints at the administrator’s discretion

86 Securing Systems
I return to the concept of different architecture views. We will stress again and again
how important the different views are during an assessment. We don’t eliminate the
details; we abstract the patterns in order to apply solutions. Architecture solutions in
hand, we then dive into the detail of the specifics.
In Figure 3.8, the trust boundary is between “user” space and “kernel” execution
area. Those are typical nomenclature for these execution areas in UNIX and UNIX-
like and Windows™ operating systems. In both the Android™ and iOS™ mobile plat-
forms, the names are somewhat different because the functions are not entirely same:
the system area and the application environment. Abstracting just what we need from
this boundary, I think it is safe to declare that there is an essential similarity between
kernel and system, even though, on a mobile platform, there is a kernel beneath the sys-
tem level (as I understand it). Nevertheless, the system execution space has high privi-
leges. System processes have access to almost everything,* just as a kernel does. These
are analogous for security purposes. Kernel and system are “high” privilege execution
spaces. User and application are restricted execution environments, purposely so.
A security architect will likely become quite conversant in the details of an operat-
ing system with which he or she works on a regular basis. Still, in order to assess any
architecture, one needn’t be a “guru.” As we shall see, the details change, but the basic
problems are entirely similar. There are patterns that we may abstract and with which
we can work.
Table 3.1 is an approximation to illuminate similarities and, thus, must not be taken
as a definitive statement. The makers of each of these operating systems may very well
violently disagree. For instance, much discussion has been had, often quite spirited,
about whether the Linux system is a UNIX operating system or not. As a security
architect, I purposely dodge the argument; a position one way or the other (yes or no) is
irrelevant to the architecture pattern. Most UNIX utilities can be compiled to run on
Linux, and do. The configuration of the system greatly mirrors other UNIX systems,
that is, load order, process spaces, threading, and memory can all be treated as similar
to other UNIX variants. For our purposes, Linux may be considered a UNIX variant
without reaching a definitive answer to the question, “Is Linux a UNIX operating sys-
tem?” For our purposes, we don’t need to know.
Hence, we can take the same stance on all the variants listed in Table 3.1—that is,
we don’t care whether it is or is not; we are searching for common patterns. I offer the
following table as a “cheat sheet,” if you will, of some common operating systems as of
this writing. I have grossly oversimplified in order to reveal similarities while obscur-
ing differences and exceptions. The list is not a complete list, by any means. Experts in
each of these operating systems will likely take exception to my cavalier treatment of
the details.
* System processes can access processes and services in the system and user spaces. System pro-
cesses will have only restricted access to kernel services through a formal API of some sort,
usually a driver model and services.

Security Architecture of Systems 87
Table 3.1 Common Operating Systems and Their Security Treatment
Name Family
Privilege? User Space
BSD UNIX UNIX1 Kernel2 User3
Posix UNIX UNIX Kernel User
System V UNIX Kernel User
Mac OS™ UNIX (BSD) Kernel Administrator4 User
iOS™ Mac OS Kernel System Application
Linux5 UNIX-like Kernel User
Android™ Linux Kernel System Application8
Windows™6 Windows NT Kernel System User
Mobile™ (variants)
Windows7 Kernel System Application
1. There are far more UNIX variants and subvariants than listed here. For our purposes, these
variations are essentially the same architecture.
2. The superuser or root, by design, has ultimate privileges to change anything in every UNIX
and UNIX-like operating system. Superuser has god-like powers. The superuser should be
considered essentially the same as kernel, even though the kernel is an operating environ-
ment and the superuser is a highly privileged user of the system. These have the same
privileges: everything.
3. In all UNIX and UNIX descendant systems, users can be configured with granular read/
write/execute privileges up to and including superuser equivalence. We ignore this for the
moment, as there is a definite boundary between user and kernel processes. If the super-
user has chosen to equate user with superuser, the boundary has been made irrelevant from
the attacker’s point of view.
4. Mac OS introduced a preconfigured boundary between the superuser and an administra-
tor. These do not have equivalent powers. The superuser, or “root” as it is designated in
Mac OS documentation, has powers reserved to it, thus protecting the environment from
mistakes that are typical of inexperienced administrators. Administrator is highly privileged
but not god-like in the Mac OS.
5. There are also many variants and subvariants of Linux. For our purposes, these may be
treated as essentially the same operating system.
6. I do not include the Windows-branded operating systems before the kernel was ported to
the NT kernel base. These had an entirely different internal architecture and are completely
obsolete and deprecated. There are many variants of the Windows OS, too numerous for
our purposes. There have been many improvements in design over the years. These varia-
tions and improvements are all descendants of the Windows NT kernel, so far as I know. I
don’t believe that the essential driver model has changed since I wrote drivers for the sys-
tem in the 1990s.
7. I’m not conversant with the details of the various Windows mobile operating systems. I’m
making a broad assumption here. Please research as necessary.
8. Android employs OS users as application strategy. It creates a new user for each application
so that applications can be effectively isolated, called a “sandbox.” It is assumed that there
is only a single human user of the operating system since Android is meant for personal
computing devices, such as phones and tablets.

88 Securing Systems
It should be readily apparent, glancing through the operating system cheat sheet
given in Table 3.1, that one can draw some reasonable comparisons between operat-
ing systems as different as Windows Server™ and Android™. The details are certainly
radically different, as are implementation environments, compilers, linkers, testing,
deployment—that is, the whole panoply of development tooling. However, an essential
pattern emerges. There are higher privileged execution spaces and spaces that can have
their privileges restricted (but don’t necessarily, depending upon configuration by the
superuser or system administrator).
On mobile platforms especially, the application area will be restricted on the deliv-
ered device. Removing the restrictions is usually called “jail breaking.” It is quite pos-
sible to give applications the same privileges as the system or, rather, give the running
user or application administrative or system privileges. The user (or malware) usually
has to take an additional step*: jail breaking. We can assume the usual separation of
privileges rather than the exception in our analysis. It might be a function of a mobile
security application to ascertain whether or not the device has been jail broken and,
based upon a positive result, take some form of protective action against the jail break.
If you now feel comfortable with the widespread practice of dividing privileges for
execution on operating systems, we can return to consideration of Figure 3.9, the mobile
security application. Note that, like the endpoint application in Figure 3.8, there is a
boundary between privileges of execution. System-level code has access to most com-
munications and most services, whereas each application must be granted privileges as
necessary. In fact, on most modern mobile platforms, we introduce another boundary,
the application “sand box.” The sand box is a restriction to the system such that system
calls are restricted across the privilege boundary from inside the sandbox to outside.
Some system calls are allowed, whereas other calls are not, by default. The sand box
restricts each application to its own environment: process space, memory, and data.
Each application may not see or process any other application’s communications and
data. The introduction of an execution sand box is supposed to simplify the application
security problem. Applications are by their very nature, restricted to their own area.†
Although the details of mobile security are beyond this book, in the case of a secu-
rity application that must intercept, view, and perhaps prevent other applications from
executing, the sand box is an essential problem that must be overcome. The same might
be said for software intended to attack a mobile device. The sand box must be breached
in both cases.
For iOS and, most especially, under Android, the application must explicitly request
privileges from the user. These privilege exceptions are perhaps familiar to iPhone™
users as the following prompt: “Allow push notifications?” The list of exceptions
* Th ere are Linux-based mobile devices on which the user has administrative privileges. On
these and similar systems, there is no need for jail breaking, as the system is not restricted as
† Th ere are many ways to create an isolating operating environment. At a diff erent level, sand-
boxes are an important security tool in any shared environment.

Security Architecture of Systems 89
presented to an Android user has a different form but it’s essentially the same request
for application privileges.
Whether a user can appropriately grant privileges or not is beyond the scope of this
discussion. However, somehow, our security application must be granted privileges to
install code within the system area in order to breach the application sand box. Or,
alternatively, the security application must be granted privileges to receive events gener-
ated by all applications and the system on the device. Mobile operating systems vary in
how this problem is handled. For either case, the ultimate general pattern is equivalent
in that the security system will be granted higher privileges than is typical for an appli-
cation. The security application will effectively break out of its sandbox so that it has a
view of the entire mobile system on the device. For the purposes of this discussion (and
a subsequent analysis), we will assume that, in some manner, the security application
manages to install code below the sandbox. That may or may not be the actual mecha-
nism employed for any particular mobile operating system and security application.
Take note that this is essentially a solution across a trust-level boundary that is
similar to what we saw in the endpoint software discussion. In Figure 3.8, the AV
engine opens (or installs) a system driver within the privileged space. In Figure 3.9,
the engine must install or open software that can also intercept application actions
from every application. This is the same problem with a similar solution. There is
an architecture pattern that can be abstracted: crossing an operating system privilege
boundary between execution spaces. The solution is to gain enough privilege such that
a privileged piece of code can perform the necessary interceptions. At the same time,
in order to reduce security exposure, the actual security engine runs as a normal appli-
cation in the typical application environment, at reduced privileges. In the case of the
endpoint example, the engine runs as a user process. In the case of the mobile example,
the engine runs within an application sand box. In both of these cases, the engine runs
at reduced privileges, making use of another piece of code with greater privileges but
which has reduced exposure.
How does the high-privilege code reduce its exposure? The kernel or system code
does as little processing as possible. It will be kept to absolute simplicity, usually deliver-
ing questionable events and data to the engine for actual processing. The privileged
code is merely a proxy router of events and data. In this way, if the data happens to be
an attack, the attack will not get processed in the privileged context but rather by the
engine, which has limited privileges on the system. As it happens, one of the archi-
tectural requirements for this type of security software is to keep the functions of the
privileged code, and thus its exposure to attack, to an absolute minimum.
In fact, on an operating system that can instantiate granular user privilege levels,
such as UNIX and UNIX-like systems, a user with almost no privileges except to run
the engine might be created during the product installation. These “nobody” users
are created with almost complete restriction to the system, perhaps only allowed to
execute a single process (the engine) and, perhaps, read the engine configuration file.
If the user interface reads the configuration file instead of the engine, then “nobody”
doesn’t even need a file privilege. Such an installation and runtime choice creates strong

90 Securing Systems
protection against a possible compromise of the engine. Doing so will give an attacker
no additional privileges. Even so, a successful attack may, at the very least, interrupt
malware protection.
As in the endpoint example, the user interface (UI) is a point of attack to the engine.
The pattern is exactly analogous between the two example architectures. The solution
set is analogously the same, as well.
Figure 3.9, the mobile malware protection software, shows an arrow originating
from the engine to the interceptor. This is the initialization vector, starting the intercep-
tor and opening the communication channel. The flow is started at the lower privilege,
which opens (begins communications) with the code running at a higher privilege.
That’s a typical approach to initiate communications. Once the channel is open and
flowing, as configured between the interceptor and the engine, all event and data com-
munications come from higher to lower, from interceptor to engine. In this manner,
compromise of the engine cannot adversely take advantage of the interceptor. This
direction of information flow is not represented on the diagram. Again, it’s a matter of
simplicity, a stylistic preference on the part of the author to keep arrows to a minimum,
to avoid the use of double-headed arrows. When assessing this sort of architecture, this
is one of the questions I would ask, one of the details about which I would establish
absolute certainty. If this detail is not on the diagram, I make extensive notes so that
I’m certain about my architectural understanding.
We’ve uncovered several patterns associated with endpoints—mobile and otherwise:
– Deploy a proxy router at high privilege to capture traffic of interest.
– Run exposed code at the least privileges possible.
– Initialize and open communications from lower privilege to higher.
– Higher privilege must validate the lower privileged code before proceeding.
– Once running, the higher privilege sends data to the lower privilege; never the
– Separate the UI from other components.
– Validate the UI before proceeding.
– UI never communicates with highest privilege.
– UI must thoroughly validate user and configuration file input before processing.
As you may see, seemingly quite disparate systems—a mobile device and a laptop—
actually exhibit very similar architectures and security solutions? If we abstract the
architecture patterns, we can apply standardized solutions to protect these typical pat-
terns. The task of the architecture assessment is to identify both known and unknown
architecture patterns. Usual solutions can be applied to the known patterns. At the
same time, creativity and innovation can be engaged to build solutions for situations
that haven’t been seen before, for that which is exceptional.
When considering the “architecturally interesting” problem, we must consider
the unit of atomicity that is relevant. When dealing with unitary systems running
on an independent, unconnected host, we are dealing with a relatively small unit: the

Security Architecture of Systems 91
endpoint.* The host (any computing device) can be considered as the outside boundary
of the system. For the moment, in this consideration, ignore the fact that protection
software might be communicating with a central policy and administrative system.
Irrespective of these functions, and when the management systems cannot be reached,
the protection software, as in our AV example, must run well and must resist attack or
subversion. That is a fundamental premise of this type of protection (no matter whether
on a mobile platform, a laptop, a desktop, etc.) The protections are supposed to work
whether or not the endpoint is connected to anything else. Hence, the rule here is as
stated: The boundary is constrained to the operating environment and hardware on
which it runs. That is, it’s an enclosed environment requiring architectural factoring
down to attackable units, in this case, usually, processes and executables.
Now contrast the foregoing endpoint cases with a cloud application, which may
exist in many points of presence around the globe. Figure 3.10 depicts a very high-level,
cloud-based, distributed Software as a Service (SaaS) application. The application has
several instances (points of presence and fail-over instances) spread out around the globe
Figure 3.10 A SaaS cloud architecture.
* An endpoint protection application must be capable of sustaining its protection services when
running independently of any assisting infrastructure.

92 Securing Systems
(the “cloud”). For this architecture, to delve into each individual process might be “too
hard” a bed, too much information. Assuming the sorts of infrastructure and adminis-
trative controls listed earlier, we can step away from process boundaries. Indeed, since
there will be many duplicates of precisely the same function, or many duplicates of the
same host configuration, we can then consider logical functions at a much higher level
of granularity, as we have seen in previous examples.
Obviously, a security assessment would have to dig into the details of the SaaS
instance; what is shown in Figure 3.10 is far too high level to build a thorough threat
model. Figure 3.10 merely demonstrates how size and distribution change the granu-
larity of an architecture view. In detail, each SaaS instance might look very much like
Figure 3.4, the AppMaker web application.
In other words, the size and complexity of the architecture are determiners of decom-
position to the level of granularity at which we analyze the system. Size matters.
Still, as has been noted, if one can’t make the sorts of assumptions previously listed,
if infrastructure, runtime, deployment, and administration are unknown, then a two-
fold analysis has to be undertaken. The architecture can be dealt with at its gross logi-
cal components, as has been suggested. And, at the same time, a representative server,
runtime, infrastructure, and deployment for each component will need to be analyzed
in detail, as well. AR A and threat modeling then proceed at a couple of levels of granu-
larity in parallel in order to achieve completeness.
Analysis for security and threat models often must make use of multiple views of
a complex architecture simultaneously. Attempts to use a single view tend to produce
representations that become too crowded, too “noisy,” representations that contain too
much information with which to work economically. Instead, multiple views or layers
that can be overlaid on a simple logical view offer a security architect a chance to
unearth all the relevant information while still keeping each view readable. In a later
chapter, the methodology of working with multiple views will be explored more fully.
Dynamically linked libraries are a special case of executable binary. These are not
loaded independently, but only when referenced or “called” by an independently loaded
binary, a program or application. Still, if an attacker can substitute a library of attack
code for the intended library (a common attack method), then the library can easily be
turned into an attack vector, with the calling executable becoming a gullible method
of attack execution. Hence, dynamic libraries executing on an endpoint should be con-
sidered suspiciously. There is no inherent guarantee that the code within the loaded
library is the intended code and not an attack. Hence, I designate any and all forms of
independently packaged (“linked”) executable forms as atomic for the purpose of an
endpoint system. This designation, that is, all executables, includes the obvious load-
able programs, what are typically called “applications.” But the category also extends to
any bit of code that may be added in, that may get “called” while executing: libraries,
widgets, gadgets, thunks, or any packaging form that can end up executing in the same
chain of instructions as the loadable program.
“All executables” must not be confined to process space! Indeed, any executable that
can share a program’s memory space, its data or, perhaps, its code must be considered.

Security Architecture of Systems 93
And any executable whose instructions can be loaded and run by the central process-
ing unit (CPU) during a program’s execution must come under assessment, must be
included in the review. Obviously, this includes calls out to the operating system and its
associated libraries, the “OS.”
Operating systems vary in how loosely or tightly coupled executable code must be
packaged. Whatever packages are supported, every one of those packages is a potential
“component” of the architecture. The caveat to this rule is to consider the amount of
protections provided by the package and/or the operating environment to ensure that
the package cannot be subverted easily. If the inherent controls provide sufficient pro-
tection against subversion (like inherent tampering and validity checks), then we can
come up a level and treat the combined units atomically.
In the case of managed server environments, the decomposition may be different.
The difference depends entirely upon the sufficiency of protections such that these
protections make the simple substitution of binary packages quite difficult. The admin-
istrative controls placed upon such an infrastructure of servers may be quite stringent:
• Strong authentication
• Careful protection of authentication credentials
• Authorization for sensitive operations
• Access on a need-to-know basis
• Access granted only upon proof of requirement for access
• Access granted upon proof of trust (highly trustworthy individuals only)
• Separation of duties between different layers and task sets
• Logging and monitoring of sensitive operations
• Restricted addressability of administrative access (network or other restrictions)
• Patch management procedures with service-level agreements (SLAs) covering the
timing of patches
• Restricted and verified binary deployment procedures
• Standard hardening of systems against attack
The list given above is an example of the sorts of protections that are typical in
well-managed, commercial server environments. This list is not meant to be exhaustive
but, rather, representative and/or typical and usual. The point being that when there
exist significant exterior protections beyond the operating system that would have to be
breached before attacks at the executable level can proceed, then it becomes possible to
treat an entire server, or even a server farm, as atomic, particularly in the case where all
of the servers support the same logical function. That is, if 300 servers are all used as
Java application servers, and access to those servers has significant protections, then an
“application server” can be treated as a single component within the system architecture.
In this case, it is understood that there are protections for the operating systems, and
that “application server” means “horizontally scaled,” perhaps even “multitenant.” The
existing protections and the architecture of the infrastructure are the knowledge sets
that were referred to earlier in this chapter as “infrastructure” and “local environment.”

94 Securing Systems
If assumptions cannot be made about external protections, then servers are just
another example of an “endpoint.” Decomposition of the architecture must take place
down to the executing process level.
What about communications within an executable (or other atomic unit)? With
appropriate privileges and tools, an attacker can intercept and transform any executing
code. Period. The answer to this question, as explained above, relies upon the attacker’s
access in order to execute tools at appropriate privileges. And the answer depends upon
whether subverting execution or intra-process communications returns some attacker
value. In other words, this is essentially a risk decision: An attack to running executa-
bles at high privilege must return something that cannot be achieved through another,
easier means.
There are special cases where further decomposition is critically important, such as
encryption routines or routines that retrieve cryptographic keys and other important
credentials and program secrets. Still, a working guideline for most code is that com-
munications within an executing program can be ignored (except for certain special
case situations). That is, the executable is the atomic boundary of decomposition. Calls
between code modules, calls into linked libraries, and messages between objects can be
ignored during architecture factoring into component parts. We want to uncover the
boundaries between executable packages, programs, and other runtime loadable units.
Further factoring does not produce much security benefit.*
Once the atomic level of functions has been decided, a system architecture of “com-
ponents”—logical functions—can be diagrammed. This diagram is typically called
a “system architecture” or perhaps a “logical architecture.” This is the diagram of the
system that will be used for an analysis. It must include every component at the appro-
priate atomic level. Failure to list everything that will interact in any digital flow of
communication or transaction leads to unprotected attack vectors. The biggest mis-
take that I’ve made and that those whom I’ve coached and mentored typically make
is not including every component. I cannot stress this enough: Keep questioning until
the system architecture diagram includes every component at its appropriate level of
decomposition. Any component that is unprotected becomes an attack vector to the
entire system. A chain is only as strong as its weakest link.
Special cases that require intra-executable architectural decomposition include:
• Encryption code
• Code that handles or retrieves secrets
• Digital Rights Management (DRM) code
• Software licensing code
• System trust boundaries
• Privilege boundaries
* Of course, the software design will necessarily be at a much fi ner detail, down to the compila-
tion unit, object, message, and application programming interface (API) level.

Security Architecture of Systems 95
While it is generally true that executables can be treated atomically, there are some
notable exceptions to this guideline. Wherever there is significant attack value to iso-
lating particular functions within an executable, then these discreet functions should
be considered as atomic functions. Of course, the caveat to this rule must be that an
attacker can gain access to a running binary such that she or he has sufficient privileges
to work at the code object or gadget level. As was noted above, if the “exceptional”
code is running in a highly protected environment, it typically doesn’t make sense to
break down the code to this level (note the list of protections, above). On the other
hand, if code retrieving secrets or performing decryption must exist on an unprotected
endpoint, then that code will not, in that scenario, have much protection. Protections
must be considered then, at the particular code function or object level. Certain DRM
systems protect in precisely this manner; protections surround and obscure the DRM
software code within the packaged executable binary.
Factoring down to individual code functions and objects is especially important
where an attacker can gain privileges or secrets. Earlier, I described as having no attack
value a vulnerability that required high privilege in order to exploit. That is almost
always true, except in a couple of isolated cases. That’s because once an attacker has
high privileges, she or he will prosecute the goals of the attack.
Attackers don’t waste time playing around with compromised systems. They have
objectives for their attacks. If a compromise has gained complete control of a machine,
the attack proceeds from compromise of the machine to whatever further actions have
value for the attacker: misuse of the machine to send spam; participation in a botnet;
theft of credentials, data, or identity; prosecuting additional attacks on other hosts on
the network; and so forth. Further exploit of another vulnerability delivering the same
level of privilege holds no additional advantage.* However, in a couple of interesting
cases, a high-privilege exploit may deliver attacker value.
For example, rather than attempting to decrypt data through some other means,
an attacker might choose to let an existing decryption module execute, the results of
which the attacker can capture as the data are output. In this case, executing a running
program with debugging tools has an obvious advantage. The attacker doesn’t have
to figure out which algorithm was used, nor does the attacker have to recover key-
ing material. The running program already performs these actions, assuming that the
attacker can syphon the decrypted data off at the output of the decryption routine(s).
This avenue may be easier than a cryptographic analysis.
If the attacker is after a secret, like a cryptographic key, the code that retrieves the
secret from its hiding place and delivers the key to the decryption/encryption routines
may be a worthy target. This recovery code will only be a portion, perhaps a set of
* Th e caveat to this rule of thumb is security research. Although not intentionally malicious,
for some organizations security researchers may pose a signifi cant risk. Th e case of researchers
being treated as potential threat agents was examined previously. In this case, the researcher
may very well prosecute an exploit at high privilege for research purposes. Since there is no
adversarial intent, there is no need to attain a further objective.

96 Securing Systems
distinct routines, within the larger executable. Again, the easiest attack may be to let
the working code do its job and simply capture the key as it is output by the code. This
may be an easier attack than painstakingly reverse engineering any algorithmic, digital
hiding mechanism. If an attacker wants the key badly enough, then she or he may be
willing to isolate the recovery code and figure out how it works. In this situation, where
a piece of code is crucial to a larger target, that piece of code becomes a target, irre-
spective of the sort of boundaries that we’ve been discussing, atomic functions, binary
executables, and the like. Instances of this nature comprise the precise situation where
we must decompose the architecture deeper into the binary file, factoring the code
into modules or other boundaries within the executable package. Depending upon the
protections for the executable containing the code, in the case in which a portion of
the executable becomes a target, decomposing the architecture down to these critical
modules and their interfaces may be worthwhile.
3.8.1 Principles, But Not Solely Principles
[T]he discipline of designing enterprises guided with principles22
Some years ago, perhaps in 2002 or 2003, I was the Senior Security Architect respon-
sible for enterprise inter-process messaging, in general, and for Service Oriented
Architectures (SOA), in particular. Asked to draft an inter-process communications
policy, I had to go out and train, coach, and socialize the requirements laid out in the
policy. It was a time of relatively rapid change in the SOA universe. New standards
were being drafted by standards organizations on a regular basis. In my research, I
came across a statement that Microsoft published articulating something like, “observe
mutual distrust between services.”
That single principle, “mutual distrust between services,” allowed me to articulate
the need for services to be very careful about which clients to allow, and for clients to
not assume that a service is trustworthy. From this one principle, we created a standard
that required bidirectional authentication and rigorous input validation in every service
that we deployed. Using this principle (and a number of other tenets that we observed),
we were able to drive security awareness and security control throughout the expanding
SOA of the organization. Each principle begets a body of practices, a series of solutions
that can be applied across multiple architectures.
In my practice, I start with principles, which then get applied to architectures as
security solutions. Of course, the principles aren’t themselves solutions. Rather, princi-
ples suggest approaches to an architecture, ideals for which to strive. Once an architec-
ture has been understood, once it has been factored to appropriate levels to understand
the attack surfaces and to find defensible boundaries, how do we apply controls in order
to achieve what ends? It is to that question that principles give guidance. In a way, it
might be said that security principles are the ideal for which a security posture strives.
These are the qualities that, when implemented, deliver a security posture.

Security Architecture of Systems 97
Beyond uncovering all the attack surfaces, we have to understand the security archi-
tecture that we are trying to build. Below is a distillation of security principles. You may
think of these as an idealized description of the security architecture that will be built
into and around the systems you’re trying to secure.
The Open Web Application Security Project (OWASP) provides a distillation of several
of the most well known sets of principles:
– Apply defense in depth (complete mediation).
– Use a positive security model (fail-safe defaults, minimize attack surface).
– Fail securely.
– Run with least privilege.
– Avoid security by obscurity (open design).
– Keep security simple (verifiable, economy of mechanism).
– Detect intrusions (compromise recording).
– Don’t trust infrastructure.
– Don’t trust services.
– Establish secure defaults.23
Given the above list, how does one go about implementing even a single one of these
principles? We have spent some time in this chapter examining architectural patterns.
Among these are security solution patterns that we’ve enumerated as we’ve examined
various system architectures. Solution patterns are the implementation of the security
It should be noted that running with least privilege or failing securely are part of a
defense-in-depth. The boundaries between the principles aren’t discrete. Not trusting
an infrastructure is a part of minimizing attack surfaces, which is a part of the defense-
in-depth. A single security control can relate to a number of security principles and
supply a solution to one or more attack surfaces. Concomitantly, any attack surface may
require several orthogonal security controls. Remember, specification of a defense is a
matrix problem across many domains and technologies. There are few, “if this, then do
that” solutions. Solutions are more often like, “if this, then do this, and that, and maybe
a little of this other, too, if you can.”
Hence, using security principles is a starting point not an ending. It’s a matter of
applying solutions to achieve the principles. If you see the principles begin to emerge out
of the security requirements that you specify, then you have achieved an understanding
of security architecture as well as having achieved a practice of applying information
security to systems.
Contrary to what most users and even many developers assume, security functionality
does not necessarily provide genuine security; security is a systems property emerging
from the totality of system behavior.24

98 Securing Systems
As we have seen in this chapter, in order to assess systems for security, the assessor has
to have a grounding in the practice of system architecture. Specifically, he or she must
understand logical and component architectures well enough to uncover attack sur-
faces, trust boundaries, and defensible units of composition. For convenience, we call
this process “decomposing in architecture” to a functional granularity such that these
units can be factored into security significant components.
Rarely will a single architectural view suffice, especially for complex system archi-
tectures. Often, a security analysis will make use of multiple views. Each view typically
serves for a particular stakeholder group or community. Enterprise architects have one
view, business architects another view, solutions architects yet another view of a system,
and infrastructure architects have a different view entirely. Each of these views usually
holds some of the security information required. And thus, for a review, each stake-
holder view has some validity.
By abstracting general architectural patterns from specific architectures, we can
apply known effective security solutions in order to build the security posture. There
will be times, however, when we must be creative in response to architecture situations
that are as yet unknown or that are exceptional. Still, a body of typical patterns and
solutions helps to cut down the complexity when determining an appropriate set of
requirements for a system under analysis.
Our security principles will be the ideal towards which we architect security solu-
tions. As solution patterns are applied and requirements specified, the security principles
should be the emergent properties of the solution set. There is no one-to-one mapping.
Still, the set of solutions will together enable a system architecture to achieve a security
posture that exhibits the set of security principles from which the security architect is
working. In this way, the art of securing systems is the craft of applying information
security such that the system’s architecture will exhibit the qualities described by a set
of security principles.
1. Zachman, J. A. (2007). “Foreword.” In Handbook of Enterprise Systems Architecture in
Practice, p. xv, Saha, P., ed. IGI Global.
2. Ibid.
3. Saha, P. (2007). “A Synergistic Assessment of the Federal Enterprise Architecture
Framework against GER AM (ISO15704:2000)” (Ch. 1). In Handbook of Enterprise
Systems Architecture in Practice, p. 1, Saha, P., ed. IGI Global.
4. Merriam-Webster’s Collegiate Dictionary. (2004). 11th ed. Merriam-Webster.
5. Sherwood, J., Clark, A., and Lynas, D.. “Enterprise Security Architecture.” SABSA White
Paper, SABSA Limited, 1995–2009. Retrieved from
members/sites/default/inline-fi les/SABSA_White_Paper .

Security Architecture of Systems 99
6. Zachman, J. A. (2007). “Foreword.” In Handbook of Enterprise Systems Architecture in
Practice, pp. xv–xvi, Saha, P., ed. IGI Global.
7. Ibid., p. xvi.
8. Godinez, M., Hechler, E., Koenig, K., Lockwood, S., Oberhofer, M., and Schroeck, M.
(2010). “Introducing Enterprise Information Architecture” (Ch. 2). In Th e Art of Enterprise
Information Architecture: A Systems-Based Approach for Unlocking Business Insight, p. 33.
IBM Press.
9. Ibid.
10. Berners-Lee, T. (1989). “Information Management: A Proposal” CERN. Retrieved from
11. Godinez, M., Hechler, E., Koenig, K., Lockwood, S., Oberhofer, M., and Schroeck, M.
(2010). “Introducing Enterprise Information Architecture” Ch. 2). In Th e Art of Enterprise
Information Architecture: A Systems-Based Approach for Unlocking Business Insight, p. 74.
IBM Press.
12. Zachman, J. A. (2007). “Foreword.” In Handbook of Enterprise Systems Architecture in
Practice, Saha, P., ed., pp. xv–xvi. IGI Global.
13. Godinez, M., Hechler, E., Koenig, K., Lockwood, S., Oberhofer, M., and Schroeck, M.
(2010). “Introducing Enterprise Information Architecture” (Ch. 2). In Th e Art of Enterprise
Information Architecture: A Systems-Based Approach for Unlocking Business Insight, p. 74.
IBM Press.
14. Schoenfi eld, B. (2011). “How to Securely Process and Allow External HTTP Traffi c to
Terminate on Internal Application Servers.” ISA Smart Guide. SANS Institute.
15. Oxford Dictionary of English. (2010). 3rd ed. UK: Oxford University Press.
16. Ibid.
17. Buschmann, F., Henney, K., and Schmidt, D. C. (2007). Pattern-Oriented Software
Architecture: On Patterns and Pattern Languages, Vol. 5, p. xxxi. John Wiley & Sons.
18. Open Web Application Security Project (OWASP). (November 2013). OWASP
Application Security FAQ, Retrieved from
php/OWASP_ Application_ SecuriFAQ#W hy_can.27t_I_trust_the_information_
19. Ramachandran, J. (2002). Designing Security Architecture Solutions, p. 43. John Wiley
& Sons.
20. Godinez, M., Hechler, E., Koenig, K., Lockwood, S., Oberhofer, M., and Schroeck, M.
(2010). “Introducing Enterprise Information Architecture” (Ch. 2). In Th e Art of Enterprise
Information Architecture: A Systems-Based Approach for Unlocking Business Insight, p. 33.
IBM Press.
21. Schoenfi eld, B. (2014). “Applying the SDL Framework to the Real World” (Ch. 9). In
Core Software Security: Security at the Source, pp. 255–324. Boca Raton (FL): CRC Press.
22. Saha, P. (2007). “A Synergistic Assessment of the Federal Enterprise Architecture
Framework against GER AM (ISO15704:2000)” (Ch. 1). In Handbook of Enterprise
Systems Architecture in Practice, p. 1, Saha, P., ed. IGI Global.
23. Open Web Application Security Project (OWASP). (2013). Some Proven Application
Security Principles. Retrieved from Category:Principle.
24. Redwine, S. T., Jr. and Davis, N., eds. (2004). “Processes to Produce Secure Software:
Towards more Secure Software.” Software Process Subgroup, Task Force on Security across
the Software Development Lifecycle, National Cyber Security Summit, March 2004.

Chapter 4
Information Security Risk
It’s about contextual risk.
– Anurag Agrawal, in conversation with the author, 2014
The success of the assessment depends greatly upon the assessor’s ability to calculate or
rate the risk of the system. There is the risk of the system as it’s planned at the moment
of the assessment. And there’s the risk of each attack vector to the security posture of the
system. Most importantly, the risk from the system to the organization must be deter-
mined in some manner. If computer security risk cannot be calculated in a reasonable
fashion and consistently over time, not only does any particular assessment fail, but the
entire assessment program fails. An ability to understand, to interpret, and, ultimately,
to deliver risk ratings is an essential task of the architecture risk assessment (AR A) and
threat modeling.
The word “risk” is overloaded and poorly defined. When discussing it, we usu-
ally don’t bother to strictly define what we mean; “risk” is thrown around as though
everyone has a firm understanding of it. But usage is often indiscriminate. A working
definition for the purposes of security assessment must be more stringent. “Risk,” for
our purposes, will be defined more formally, below. For the moment, let’s explain
“risk” as Jack Jones does: “the loss exposure associated with the system.”1 This work-
ing definition encompasses both the likelihood of a computer event occurring and its
negative impact.
4.1 Rating with Incomplete Information
It would be extraordinarily helpful if the standard insurance risk equation could be
calculated for information security risks.

102 Securing Systems
Probability * Annualized Loss = Risk
However, this equation requires data that simply are not available in sufficient quan-
tities for a statistical analysis comparable to actuarial data that are used by insurance
companies to calculate risk. In order to calculate probability, one must have enough sta-
tistical data on mathematically comparable events. Unfortunately, generally speaking,
few security incidents in the computer realm are particularly mathematically similar.
Given multivariate, multidimensional events generated by adaptive human agents, per-
haps it wouldn’t be too far a stretch to claim that no two events are precisely the same?*
Given the absence of actuarial data, what can a poor security architect do?
4.2 Gut Feeling and Mental Arithmetic
Security architects generate risk ratings on a regular basis, perhaps every day, depending
on job duties. Certainly, most practitioners make risk ratings repeatedly and regularly.
There’s a kind of mental arithmetic that includes some or all of the elements of risk, fac-
tors in organizational preferences and capabilities, the intentions of the system, and so
on, and then outputs a sense of high, medium, or low risk. For many situations referenc-
ing many different types of systems and attack vectors, a high, medium, or low rating
may be quite sufficient. But how does one achieve consistency between practitioners,
between systems, and over time?
This is a thorny and nontrivial problem.
I don’t mean to suggest that one cannot approach risk calculation with enough data
and appropriate mathematics. Jack Jones, the author of Factor Analysis of Information
Risk (FAIR), told the author that the branch of mathematics known as casino math
may be employed to calculate probability in the absence of sufficient, longitudinal actu-
arial data. Should you want it, the FAIR “computational engine” is a part of the stan-
dard.†2 For our purposes, it is sufficient to note that the probability term is difficult to
calculate without significant mathematical investment. It isn’t too hard to calculate
likely loss in dollars, in some situations, although it may be tricky to annualize an
information security loss.
Annualizing loss may not be an appropriate impact calculation for information secu-
rity, anyway. Annualizing a loss obviously works quite well for buying insurance. It’s
also useful for comparing and prioritizing disparate risks. But we’re not talking about
* A significant amount of study has been devoted to data breaches. In particular, please see the
breach reports from the Poneman Institute ( However, by the time
a breach has occurred, we are at a very different point in the computer security cycle. AR A is
meant to prevent a breach. And a data breach is only one of the many types of possible suc-
cessful impacts.
† A computational engine for FAIR is protected by patent, however.

Information Security Risk 103
an ability to directly insure the risk of a computer system attack in this book.* In order
to create some risk treatment, some protection, periodic premiums are usually paid to a
trusted third party, the insurer.
Generally, the cost of security technology occurs one time (with perhaps main-
tenance fees). It may be difficult to predict the useful life of technology purchases.† The
personnel needed to run the technology are a “sunk cost.” Unless services are entirely
contracted, people are typically hired permanently, not on a yearly subscription basis
(though some security firms are attempting a subscription model). In any event, this
book is about applying computer security to systems such that they will be well-enough
defended. The insurance, such as it may be, comprises the capabilities and defenses that
protect the systems. Although logically analogous to insurance, the outlay structures for
preventative system security follow a different cost model than paying for insurance.
The problem with sophisticated math is that it may take too long to calculate for
the purpose of assessment. Generally, during an analysis, the security assessor must
repeatedly calculate risk over multiple dimensions and over multiple items. If there are
numerous attack surfaces, then there are numerous risk calculations. It would be oner-
ous if each of these calculations required considerable effort, and worse if the calcula-
tions each took an extended period. Any extended time period beyond minutes may be
too long. If there’s significant residual risk‡ left over after current protections and future
treatments are factored into the calculation, then the assessor must come up with an
overall risk statement based upon the collection of risk items associated with the system.
Taking a few hours to produce the overall risk is usually acceptable. But taking weeks
or even months to perform the calculations is usually far too long.
Experienced security architects do these “back of the napkin” calculations fairly
rapidly. They’ve seen dozens, perhaps hundreds, of systems. Having rated risk for hun-
dreds or perhaps many more attack vectors, they get very comfortable delivering risk
pronouncements consistently. With experience comes a gut feeling, perhaps an intuitive
grasp, of the organization’s risk posture. Intimacy with the infrastructure and security
capabilities allows the assessor to understand the relative risk of any particular vulnera-
bility or attack vector. This is especially true if the vulnerability and attack vector are
well understood by the assessor. But what if one hasn’t seen hundreds of systems? What
does one do when just starting out?
* It is possible to underwrite liability from computer security incidents. Th at is one risk treat-
ment. However, the point of architecture analysis for security is to try and reduce the possibil-
ity of loss in the fi rst place. Either are treatments for the risk. Th ese are not mutually exclusive.
† Digital technology changes rapidly. On the other hand, the investment in building upon
and around core systems may discourage change, thus extending the useful far beyond
‡ Residual risk” is that risk that remains after all treatments and/or mitigations have been
applied. It is the risk that cannot or will not be adequately mitigated.

104 Securing Systems
Indeed, the computer security language around risk can be quite muddled.
Vulnerabilities are treated as risks in and of themselves. Tool vendors often do this in
order to produce some kind of rating based upon vulnerability scans. Typically, the
worst possible scenario and impact is taken as the likely scenario and impact. And
unfortunately, in real systems, this is far from true.
As an example, take a cross-site scripting (XSS) error that lies within a web page.
As was noted previously, depending upon the business model, an organization may or
may not care whether its website contains XSS errors.* But how dangerous is a XSS error
that lies within an administrative interface that is only exposed to a highly restricted
manage ment network? Even further, if that XSS can only be exercised against highly
trained and highly trusted, reasonably sophisticated system administrators, how likely
are they to fall for an email message from an unknown source with their own adminis-
trative interface as the URL? Indeed, the attacker must know that the organization uses
the vulnerable web interface. They have to somehow induce technically savvy staff to
click a faulty URL and further hope that staff are logged into the interface such that the
XSS will fire. Indeed, on many administrative networks, external websites are restricted
such that even if the XSS error did get exercised, the user couldn’t be redirected to a
malicious page because of network and application restrictions. Since no URL payload
can be delivered via the XSS, the exploit will most likely fail. In this scenario, there exist
a number of hoops through which an attacker must jump before the attacker’s goal can
be achieved. What’s the payoff for the attacker? Is surmounting all the obstacles worth
attempting the attack? Is all of this attacker effort worthwhile when literally millions of
public websites reaching hundreds of millions of potential targets are riddled with XSS
errors that can be exploited easily?
I would argue that a XSS error occurring in a highly restricted and well-protected
administrative interface offers considerably less risk due to factors beyond the actual
vulnerability: exposure, attack value, and difficulty in deriving an impact. However, I
have seen vulnerability scanning tools that rate every occurrence of a particular variation
of XSS precisely the same, based upon the worst scenario that the variation can produce.
Without all the components of a risk calculation, the authors of the software don’t have
a complete enough picture to calculate risk; they are working with only the vulnerability
and taking the easiest road in the face of incomplete information: assume the worst.
In fact, what I’ve described above are basic parts of a information security risk cal-
culation: threat agent, motivation, capability, exposure, and vulnerability. These must
come together to deliver an impact to the organization in order to have risk.
A threat is not a risk by itself. A vulnerability is not a risk in and of itself. An
exploita tion method is not a risk when the exploit exists in the absence of a threat agent
* An attack against the session depends upon other vulnerabilities being present (it’s a com-
bination attack) or it depends upon the compromise of the administrator’s browser and/or
machine, which are further hurdles not relevant to this example.

Information Security Risk 105
that is capable and motivated to exercise the exploit and in the presence of a vulnerabil-
ity that has been exposed to that particular threat agent’s methodology. This linking
of dependencies is critical to the ability of a successful attack. And, thus, understand-
ing the dependency of each of the qualities involved becomes key to rating the risk of
occurrence. But even the combination so far described is still not a risk. There can be no
risk unless exploitation incurs an impact to the owners or users of a computer system.
“Credible attack vector” was defined in Chapter 2:
Credible attack vector: A credible threat exercising an exploit on an exposed
4.3 Real-World Calculation
For the purposes of architecture assessment for security, risk may be thought of as:
Credible Attack Vector * Impact = Risk Rating
Credible Attack Vector (CAV) = 0 < CAV > 1
Impact = An ordinal that lies within a predetermined range such that 0 < Impact >
Predetermined limit (Example: 0 < Impact > 500)3
I do not claim that guessing at or even calculating in some manner the credible
attack vector (CAV) will calculate a risk probability. It’s merely one of any number of
risk rating systems. The following explanation is one approach to risk that rates a collec-
tion of dependent conditions that must be taken together as a whole. One cannot sim-
plistically grab a vulnerability and assume the other factors. The following is one way to
decompose the elements of probability, an approach to impact that has been used over
hundreds, perhaps even more than a thousand risk assessments over a number of years.
It is certainly not the only way, nor the “True Way.” Credible attack vector is presented
as an example for a cyber-risk calculation rather than a recipe to follow precisely.
The point of describing this one approach is for you to understand the complexity
of computer risk, especially when the risk ensues from the activities of human attackers.
In order to assess computer systems, one must have a reasonable understanding of the
component attributes of risk and one must be facile in applying that risk understanding
to real-world attackers and actual vulnerabilities within real computer systems, where a
successful attack is likely to cause some harm. Credible attack vector rating is a simple
and proven method to account for the many connected and dependent factors that,
taken together in some measure, make up the probability calculation for cyber risk. Use

106 Securing Systems
CAV as a starting point for your understanding and your organization’s methodology,
if it helps to get a handle on this thorny problem.
If we can consistently compute CAV and also rate Impact within a chosen scale,
their multiplication will result in a risk rating. This is precisely how the Just Good
Enough Risk Rating (JGERR) computes risk. The rating will always be some ratio
of 500. JGERR users must choose what portion of 500 will be low, what the medium
risk range will cover (presumably, centered on 250?), and the remainder will be high.
Or, as we did at Cisco Systems, Inc., the range might be divided up into five buckets:
low, medium-low, medium, medium-high, and high. Depending upon the needs of the
organization, the buckets need not be symmetrical. Greater emphasis might be placed
on higher risk by skewing towards high risk through expansion of the high bucket.
Simply start the high classification from a lower number (say, “300”). Or an organiza-
tion with a more risk-tolerant posture might decide to expand low or medium at the
expense of high. Skewing around a particular bucket guarantees that the security needs
of the organization are reflected within the rating system’s buckets.
We will come back to the terms making up any similar calculation at a later point.
First, let’s define what risk means in the context of computer security.
As in the entire assessment process, there is significant craft and art involved in risk
rating. Because of this, a key tool will be the assessor’s mind. Each of us unique human
beings is blessed (or cursed) with her or his own risk tolerance. Effectively rating risk
can only be done when the assessor understands his or her personal risk tolerance.
Effectively managing information risk and security, without hindering the organization’s
ability to move quickly, will be key to business survival.4
4.4 Personal Security Posture
Personal risk predilection will have to be factored out of any risk calculations per-
formed for an organiza tion’s systems. The analyst is not trying to make the system
under analysis safe enough for him or herself. She is trying to provide sufficient security
to enable the mission of the organization. “Know thyself ” is an important maxim with
which to begin.
Faced with the need to deliver risk ratings for your organization, you will have
to substitute the organization’s risk preferences for your own. For, indeed, it is the
organization’s risk tolerance that the assessment is trying to achieve, not each asses-
sor’s personal risk preferences. What is the risk posture for each particular system as
it contributes to the overall risk posture of the organization? How does each attack
surface—its protections if any, in the presence (or absence) of active threat agents and
their capabilities, methods, and goals through each situation—add up to a system’s
particular risk posture? And how do all the systems’ risks sum up to an organization’s
computer security risk posture?

Information Security Risk 107
4.5 Just Because It Might Be Bad, Is It?
What is “risk”?
An event with the ability to impact (inhibit, enhance or cause doubt about) the mission,
strategy, projects, routine operations, objectives, core processes, key dependencies and/or
the delivery of stakeholder expectations.5
At its core, risk may be thought of as “uncertainty about outcome.” In the risk defi-
nition quoted above, the author focuses on “impact” to an organization’s goals, and
impact to the processes and systems that support the mission of the organization. In
this generalized approach, “impact” can be either enhancing or detracting, although, as
Jack Jones says, risk equates more or less with general “uncertainty.”6 The focus in clas-
sic risk management is on uncertainty in how events outside the control of an organiza-
tion may affect the outcomes of the efforts of the organization. The emphasis in this
definition is on uncertainty—events beyond the sphere of control—and the probability
of those events having an effect.
Given certain types of attacks, there is absolute certainty in the world of computer
security: Unprotected Internet addressable systems will be attacked. The uncertainty
lies in the frequency of successful attacks versus “noise,” uncertainty in whether the
attacks will be sophisticated or not, how sophisticated, and which threat agents may get
to the unprotected system first. Further, defenders won’t necessarily know the objec-
tives of the attackers. Uncertainty lies not within a probability of the event, but rather
in the details of the event, the specificity of the event.
Taking any moment within the constant barrage of Internet-based system pro-
gramming and deployment, we know with some certainty that sophisticated attackers
are active in pursuing objectives against a wide range of targets. The question is not
whether an attack will occur or not, so much as how soon, using what methods, and
for what purposes?
Because we are concerned with the assessment of systems that are likely to be
attacked and specify ing protections to prevent success of the attacks, we may constrain
our definition of risk. For the purposes of this book, we are not concerned with the
positive possibilities from risk. We focus here on negative events as they relate to human
actors attempting to misuse or abuse systems for goals not intended by the owners of
those systems. In other words, we are interested in preventing “credible attack vectors”
from success, whatever the goals of the attackers may be. We are constraining our defi-
nition of risk to:
• Human threat agents
• Attacks aimed at computer systems
• Attack methods meant to abuse or misuse a system

108 Securing Systems
Just for the purposes of this book, we exclude nonhuman events (though certainly,
in many information security practices, nonhuman events would also be considered).
We also exclude any event whose impact may unexpectedly enhance the desired objec-
tives of a computer system. Although those factors that we have excluded are, indeed,
a part of the risk to an organization, these are generally not under consideration in a
“risk” assessment of a computer system. So, just for the purposes of this book, we may
safely exclude these.
It is worth noting once again that many* human attackers focused on computer
systems are:
• Creative
• Innovative
• Adaptive
It is in the adaptive nature of computer-focused threat agents where a great deal of
the uncertainty lies in gauging security risk. It has been said that, “whatever can be
engineered by humans can be reverse engineered by humans.” That is, in this context,
whatever protections we build can ultimately, with enough resources, time, and effort,
be undone. This is an essential piece of the probability puzzle when calculating or rat-
ing computer security risk. The fact that the attackers can learn, grow, and mature,
and that they will rapidly shift tactics, indicates a level of heuristics to the defense of
systems: Expect the attacks to change, perhaps dramatically. How the attackers will
adapt, of course, is uncertain.
Calculating risk may seem a daunting task. But perhaps a little understanding of the
components of information security risk can help to ease the task?
4.6 The Components of Risk
[W]ithout a solid understanding of what risk is, what the factors are that drive risk,
and without a standard nomenclature, we can’t be consistent or truly effective in using
any method.7
* Th ere are attackers who only run well-known, automated attacks. Th ese “script kiddies,” as
they are commonly called, have little skill. Attack methods regularly move from innovative
to automated, thus becoming available to the less skilled attacker. Defenses must account for
automated, well-known attacks as well as the creative and new ones. Defending against well-
established attacks is a slightly easier task, however, since security vendors regularly update
their products to account for these well-known attack methods.

Information Security Risk 109
There is a collection of conditions* that each must be true in order for there to be any
significant computer security risk.† If any one of the conditions is not true, that is, the
condition doesn’t exist or has been interrupted, then that single missing condition can
negate the ability of an attack to succeed.
“Negate” may be a strong term? Since there is no absolute protection in computer
security, there can be no surety that any particular exploit against a known vulner-
ability will not take place. Although Internet attack attempts are certain, there is no
100% surety about which will be tried against a particular vulnerability and when that
particular attempt will take place.
However, even in the face of intrinsic uncertainty, we can examine each of the
conditions that must be true and thus gain a reasonable understanding of whether any
particular attack pattern is likely to succeed. “True” in this context can be thought of as
a Boolean true or false. Treating the sub-terms as Boolean expressions gives us a simple
way of working with the properties. But please bear in mind that because there is no
absolute surety, these conditions are not really binary but, rather, the conditions can be
sufficiently protected to an extent that the probability of a particular attack succeeding
becomes significantly less likely. When there are enough hoops to jump through, many
threat agents will move on to the next target or be sufficiently delayed that they either
give up or are discovered before harm can take place.
To illustrate how network defenders can act on their knowledge of their adversaries’
tactics, the paper lays out the multiple steps an attacker must proceed through to plan
and execute an attack. These steps are the “kill chain.” While the attacker must complete
all of these steps to execute a successful attack, the defender only has to stop the attacker
from completing any one of these steps to thwart the attack.8
In short, for the purposes of AR A, we can treat each term as Boolean, though we
rationally understand that no strict Boolean true or false state actually exists. We are
simplifying for the purpose of rapid rating. Treating the sub-terms as Boolean terms
allows for rapid risk ratings. Treating the sub-terms as Boolean terms allows us to pro-
ceed with a more easily practiced methodology. In the words of the “kill chain” analysis
quoted above, the defender must interrupt the kill chain. The risk rating system pro-
posed (actually in use by some organizations) provides a taxonomy for a general purpose
kill chain risk model.
* Th e work presented here is based upon Just Good Enough Risk Rating (JGERR). JGERR is
based upon “Factor Analysis of Information Risk” (FAIR), by Jack Jones. Th e author had the
privilege of attending a number of in-depth sessions with Jack Jones during the later develop-
ment of FAIR.
† In this context, we are only concerned with risk from attacks originating from humans.

110 Securing Systems
Surety in the computer security risk arena is much like the proverbial story of the
bear and two backpackers (“trekkers”). In this archetypical story, one backpacker asks
the other why he’s carrying a pair of running shoes. He replies, “for the bear.” The first
backpacker responds, “You can’t outrun a bear.” “I don’t need to outrun the bear. I just
need to outrun you,” quips the second backpacker.
In the same way, if exercising an attack method becomes too expensive for many
attackers, or the exploit exceeds the attacker’s work factor, that is, the attack exceeds
the amount of effort that the attacker is willing to put in to achieve success, then the
attacker will move on to an easier target, to a less well-defended system. The system
with the best “running shoes” will remain uncompromised or unbreached.
There are certain types of attackers who possess far more resources and patience
than most organiza tions are willing to muster for their security protection. As we saw
earlier in describing various threat agents, certain types of attackers are incredibly deter-
mined and persistent. Each organization will have to decide whether it will try to out-
last this type of attack, to “outrun” such a determined bear. It may not be worth it
to the organization to expend that much energy on its security posture. Or only the
organization’s most critical systems may get the appropriate resources to withstand such
persistent and sophisticated attacks. This will have to be an organizational risk decision.
No generalized book can presume to make such an important decision for any person
or any organization.
4.6.1 Threat
Whatever the organization’s stance towards advanced persistent threats (APT) and
their ilk, the first condition that must be met is that there must be active, motivated
threat agents who are interested in attacking systems of the sort that are under assess-
ment. The “threat agent” is an individual or group that attacks computers. The term
“threat” is scattered about in the literature and in parlance among practitioners. In some
methodologies, threat is used to mean some type of attack methodology, such as spoof-
ing or brute force password cracking. Under certain circumstances, it may make sense
to conflate all of the components of threat into an attack methodology. This approach
presumes two things:
• All attack methodologies can be considered equal.
• There are sufficient resources to guard against every attack methodology.
If one or both of the conditions above are not true, it will make more sense to develop
a more sophisticated understanding of the “threat” term. A threat can be thought of as
consisting of three qualities that could be considered independently and then combined
into the “threat” term.

Information Security Risk 111
• Threat agent: an individual or group that attacks computers.
• Threat goal: the usual and typical value that the threat agent hopes to achieve
through their attack.
• Threat capability: the typical attack methodologies that the threat agent employs.
You may have encountered one or more of the more sensational threat agents in the
news media. Certainly, at the time of this writing, certain types of cyber attacks gain
a fair amount of attention. Still, many threat agents labor in relative obscurity. It is no
longer newsworthy when a new computer virus is released. In fact, hundreds of new
varieties appear daily; most variations are used only a few times. There are literally
hundreds of millions of malware variations. Perhaps, by now, there may be billions?
Who writes these viruses and for what purpose? When we pose this question, we walk
into the components of threat. Who are the threat agents? What are their goals? What
are their methods?
I would add two more significant dimensions to threat: How much work are they
willing to put into achieving their goals? I will call this the “work factor.” And what is
the threat agent’s risk tolerance? What chances is the attacker willing to take in order to
achieve his or her goals? Does the attacker care if the attack is discovered? What are the
likely consequences of getting caught for these particular attacks and goals? Are these
consequences a deterrent? These are key questions, I believe, in order to understand
how relevant any particular threat agent is to a particular attack surface, impact or loss
to the organization, and the level of protection required to dissuade that particular type
of attacker.
• Threat agent
• Threat goals
• Threat capabilities
• Threat work factor
• Threat risk tolerance
When we put all these terms together, we have a picture of a particular threat agent
as well as their relevance vis-à-vis any particular system and any particular organiza-
tion. Ultimately, if we’re not going to treat every attack as equal (the easiest course),
then we have to come up with some yardstick (risk!) in order to prioritize some attacks
over others. That is the point of the risk calculation. We are trying to build an under-
standing of which attackers and which attacks are relevant and which are less relevant
or even irrelevant. This will vary from system to system. And this will vary significantly
depending upon organization. Luckily, there are patterns.
In Chapter 2, we examined three different threat agents. Table 4.1 summarizes the
attributes that can be associated with cyber criminals. Focusing for a moment on cyber
crime, we can dig a little deeper into the questions posed above.

112 Securing Systems
Cyber crime is a for-profit business. The goal is monetary gain from attacking systems
and system users. Cyber criminals go to jail when prosecuted; the goal is to take, that is,
steal (in one way or another, scam, swindle, confidence game, black mail, outright theft)
assets or money without getting caught and then successfully prosecuted. In other words,
theft without punishment. As Kamala Harris wrote, “cybercrime is largely opportunistic.”9
Like any for-profit business, there is more profit given less work to generate revenue.
For that reason, cyber criminals tend to use proven methods and techniques.*
Hopefully, at this point, you have some sense of how to analyze a particular threat in
terms of its components. It will be important for you to construct a reasonably holistic
picture of each type of attacker whose goals can be met by attacking your organiza-
tion’s computer systems. Then, in light of the attackers’ methodologies, consider which
systems are exposed to those attack methodologies. In this way, you will begin to build
a threat catalog for your organization and your systems. Again, no two threat catalogs
are identical. Threat catalogs tend to be very organization and system dependent.
4.6.2 Exposure
Now let’s consider the “exposure” portion of a risk calculation. Exposure in this context
may be defined as the ability of an attacker to apply the attacker’s methodologies† to
a vulnerability. The attacker must have access to a vulnerability in order to exercise it,
to “exploit” the vulnerability. At its most basic, if a vulnerability is not exposed to an
attacker that’s interested in that vulnerability, then the likelihood of that vulnerability
being exploited goes down. As has been said, there is no zero possibility of exploitation.
Still, when exposure can only be achieved through a significant amount of effort, hope-
fully too much effort, then the probability of exploitation can be considered to be low,
or even very low, depending upon the protections. An attack cannot be promulgated
unless a vulnerability is exposed to that particular methodology. The attacker needs
access. And the attacker’s methodology has to be matched to the type of vulnerability
that has been exposed. There are many classes of vulnerability. Attacks tend to be quite
specific. Many attackers specialize in a limited set of attack methodologies. Only the
* It’s important to remember that cyber criminals may tend towards the well known, but that
doesn’t mean that they, along with other threat agents, won’t also be creative.
† Attacker methods are often called “exploits.” It is said that a computer attacker exploits a
vulnerability. Th e specifi c technical steps necessary to successfully exercise a vulnerability are
termed an “exploit.”
Table 4.1 Cyber Criminal Threat Attributes
Threat Agent Goals Risk Tolerance Work Factor Methods
Cyber criminals Financial Low Low to medium Known proven

Information Security Risk 113
most sophisticated attackers have at their disposal a wide range of methodologies, the
complete gamut of attack possibilities. The more difficult a vulnerability is to reach, the
harder it is going to be to exploit. Thus, “exposure” is a key component of the likelihood
of an attack succeeding.
As an example, highly trusted administrative staff often have very privileged access.
Certainly within their trust domain, a database administrator can generally perform
a great many tasks on a database. In situations in which the database administrators
are not being monitored (their activities and tasks on the databases not independently
watched), they may be able to pretty much do anything with the data: change it, steal it,
delete it. They may even have sufficient access to wipe away their “tracks,” that is, delete
any evidence that the database administrators have been tampering with the data.
In organizations that don’t employ any separation of duties between roles, admin-
istrative staff may have the run of backend servers, databases, and even applications.
In situations like this, the system administrators can cause catastrophic damage. As an
example, I bring up the case of a network administrator for the City and County of San
Francisco, California, USA. For whatever personal reasons, he became disgruntled with
his employer. He changed the administrative password to the network routers for the
internal network. Only he knew to what he had changed the password. No other admin-
istrative staff had access to work on the internal routers. Nothing could be changed. No
problem that occurred could be fixed. All network administrative activity ceased. It was
an unmitigated disaster. The employee refused to disclose this single, unlocking pass-
word for some number of weeks. He even endured a jail stay while refusing to comply.
The City and County of San Francisco were held hostage for weeks on end because a
single administrator had the power to stop all useful network administrative activity.
Even in mature and well-run shops, administrative staff will have significant power
to do damage. The excepted protections against misuse of this power are:
• Strict separation of duties
• Independent monitoring of the administrative activities to identify abuse of
administrative access
• Restriction of outbound capabilities at the time when and on the network where
administrative duties are being carried out
• Restriction of inbound vectors of attack to administrative staff when they are
carrying out their duties
It is beyond the scope of this book to detail how these security controls would be
implemented. There are numerous ways to solve each of these problems. Still, despite
the deep trust most organizations place in their administrative staff, the possibility of
privileged insiders taking advantage of their access is usually met with a series of con-
trols. The larger and more complex the organization, the more formal these controls
are likely to be. The important point to take away from this example is that insiders,
especially trusted insiders, have a lot of access. Vulnerabilities are exposed to the trusted

114 Securing Systems
insiders; if they so choose, they have the capability to exploit many, if not all, vulner-
abilities. We can say that the “exposure” term* has a high value for the trusted insider,
at least in the domain for which insiders have access.
Now let’s contrast insider exposure to an external threat agent. An external attacker
must somehow exploit a weakness to attain the intended goals. The weakness might
be a computer vulnerability. But it can just as well be a human vulnerability: social
engineering. Social engineering, the manipulation of a person or persons to gain access
in order to eventually exploit computer vulnerabilities, is part of the one, two (three,
four, . . .) punch that many sophisticated attacks employ. Much of the time, exploita-
tion of a single vulnerability is not enough for the attacker to achieve her or his goal.
Single vulnerability exploitation only occurs in the instance where the attack value is
directly achieved through that single vulnerability: SQL injection or complete system
takeover through remote code execution vulnerability, to name a couple of single vul-
nerability instances. In the case of executing SQL injection vulnerabilities that were
thought to be protected by the authentication, if an attacker is not using any data that
actually points back to the attacker—the attacker is using a false name, false address,
false identity, and stolen credit card—the attacker can proceed to carry out injection
attacks as long as the account remains open. There will be no consequences for the
attacker if the attack is essentially anonymous.
The data is the goal. If the SQL injection is exposed to the attacker, as in cases in
which an injection can be promulgated through a public Web interface that requires
no authentication then, of course, a single vulnerability is all that the attacker requires.
Occasionally, various runtimes will have vulnerabilities that allow an unauthenticated
remote attack to be promulgated. The PHP web programming language has had a
number of these that could be exploited through a PHP coded webpage. Certain Java
application servers have had such vulnerabilities from time to time.
But, oftentimes, in order to get through a layer of defenses, like requiring an inter-
nal user name and password for privileged access, the privileged user becomes the tar-
get. This is where social engineering comes into play. The attacker delivers a payload
through some media, email, phone call, malicious website, or any combination of two
or more of these, such that the privileged user is tricked into giving up their credentials
to the attacker.
In the world of highly targeted phishing attacks, where a person’s social relations,
their interests, even their patterns of usage, can be studied in detail, a highly targeted
“spear-phishing” attack can be delivered that is very difficult to recognize. Consequently,
these highly targeted spear-phishing techniques are much more difficult to resist. The
highly targeted attacks are still relatively rare compared to a “shotgun” approach. If you,
the reader, maintain a more or less public Web persona with an email address attached
to that persona, you will no doubt see your share of untargeted attacks every day—that
is, email spam or phishing attacks.
* Th e controls in the list above are mitigations to the high exposure of vulnerabilities to insider
staff .

Information Security Risk 115
Certainly, if Nigerian prince scams did not work, we would not see so many in our
email inboxes. Unfortunately, there are individuals who are willing to believe that some
distant prince or government official has indeed, left them an exorbitant sum of money.
In order to become instantly rich, a person has only to be willing to give bank account
details to a complete stranger. And the same may be true for buying diet preparations
and other “medicines” through unsolicited email. People do buy that stuff. People do
respond to emails from “friends” who say they are in trouble and without funds and are,
therefore, stuck in some distant city. You may delete every one of those emails. But rest
assured, someone does respond.
Trusted insiders who are sophisticated system administrators are not likely to
respond. These people are typically not the targets of the general level of spam email
that you may see in your inbox day after day. An attacker is going to have to socially
engineer, that is, fairly tightly target a sophisticated user in order to succeed. In today’s
interconnected work environment, most staff expect to receive and send email, both
work-related and personal. Every connected person is exposed to generalized social
engineering through email and social networks. Depending upon your job role, you
may also be exposed to more targeted attacks. This is the human side of exposure in
the digital age. We all visit websites on a fairly regular basis. At least occasionally, one
or more search engine hits will be to a malicious website rather than a genuine service.
We are all exposed to social engineering to some extent or other.
[I]t was revealed that the Target hackers managed to sneak their way into the company’s
systems by  stealing credentials from a contractor. From there, they planted malicious
code targeting the retailer’s payment terminals. In the wake of the attack, some Target
customers have been hit with fraudulent charges, forcing banks to replace millions of
credit and debit cards.10
In the computer sense, an attacker must be able to reach the vulnerability. Obviously,
taking the foregoing explanation into account, if the attacker can trick someone into
giving up credentials, the attacker can bypass access restrictions. But, in contrast, let’s
examine a common approach to restricting access for publicly available Software as a
Service (SaaS) products.
Where the service is being offered for free but requires a valid credit card in order
to register, there may be some illusion that the account and resulting access links to a
legitimate individual. The last time I had information (some years ago), valid credit
card numbers cost $.25 apiece on the black market. Email addresses are readily avail-
able for free. It is not too much of a leap to believe that a cyber criminal might tender
a credit card that’s still valid (briefly) with a free email address and, in this manner, get
access to many “freemium”* services. Even if the access exists only until the stolen credit
* “Fremium” services off er a basic or limited package for free. Additional features require a
subscription (the premium off ering). A combination of the words “free” and “premium.”

116 Securing Systems
card fails, the attacker has unfettered access during the window before the fraudulent
account is closed.
Occasionally, a legitimate user will carry out attacks as well; these may be thought
of as a variation of the trusted insider.
“Exposure” is the ability of an attacker to make contact with the vulnerability. It is
the availability of vulnerabilities for exploitation. The attacker must be able to make use
of whatever media the vulnerability expresses itself through. As a general rule, vulner-
abilities have a presentation. The system presents the vulnerability through an input to
the system, some avenue through which the system takes in data. Classic inputs are:
• The user interface
• A command-line interface (CLI)
• Any network protocol
• A file read (including configuration files)
• Inter-process communication
• A system driver interface
• And more
Really, any input to a system may offer a channel for an attacker to reach the vul-
nerability. There are other, more subtle attack channels, to be sure. Among the more
sophisticated methods are reverse engineering and tight control of code execution paths
within a running binary. Let’s constrain ourselves to inputs, as the vast majority of
attack surfaces involve programmatic or user inputs to the system.
Even if a vulnerability is presented through an input, it still may not be exposed to
a particular threat agent. Some inputs may be purposely exposed to a wide range of
attackers. Think of a public, open website like a public blog. In fact, when the author
opened his very first website, the web designer used the administrative user ID as the
password. On Friday night, a first version of the site was completed. By Sunday morn-
ing, the website was defaced. Again, on the public Internet, attack is certain.
Some inputs, however, may be available to only a very restricted audience. Highly
restricted manage ment networks—unconnected machines in tightly controlled physi-
cal environments (which might be someone’s home, depending on circumstances)—
come to mind. If the input is not in the presence of an attacker who has the capabilities,
knows about, and sees value in attacking that particular input, then the vulnerability
is not exposed.
The exposure of a vulnerability, then, needs to be factored into our calculation of
“credible attack vector.” Without the necessary exposure, the vulnerability cannot be
exercised by the threat agent. Thus, our credible attack vector term contains a false con-
dition greatly lowering any risk rating that we might make. A vulnerability that is easily
accessed by a threat agent interested in exploiting that particular vulnerability would
then greatly raise the risk of exploitation. Exposure is a key contributing factor in our
ability to assess the likelihood of a successful attack.

Information Security Risk 117
4.6.3 Vulnerability
Considering our constrained definition of risk (above), “vulnerability” should be defined
with respect to computer security risk.
Vulnerability is any characteristic of the system that allows an attacker to commit
As may be apparent from the definition above, we are particularly concerned here
with software and design errors and flaws that have relevance to human attackers mis-
using a system in some manner. There are other definitions of vulnerability that are
broader. And depending upon the role of the security architect and the context of AR A,
a broader definition may be in order. For the purposes of this book, we confine ourselves
to the simpler and more direct case: any weakness in the system that allows a human
attacker an opportunity to use the system in ways not intended by the designers of the
system, not in line with the objectives of the owners of the system, and, perhaps, not in
line with the objectives or safety of users of the system. The techniques outlined in this
book can be applied to a broader definition of threats or vulnerabilities. The definition
is constrained in order to provide a more thorough explanation and examples around
the most typical attack vectors and vulnerabilities that will be part of most assessments.
An enumeration of all the different types of vulnerabilities and their many varia-
tions is beyond the scope of this book. One only need to roll through the Common
Weakness Enumeration database ( to understand how many varia-
tions there are in the many classes of vulnerabilities. From the author’s experience, an
encyclopedic understanding of vulnerabilities is typically not required. Although an
in-depth understanding of at least one variation in each class of vulnerability comes
in handy during assessments, an in-depth understanding of many variations doesn’t
enhance the assessment a great deal. Treatments to protect against the vulnerability
tend to apply to many variations of that vulnerability. Hence, the security architect
performing assessments must know the classes of vulnerability that can occur for that
kind of system. Understanding each variation of that class of vulnerability isn’t neces-
sary. Instead, what is required is the understanding of how those vulnerabilities occur
and how they may be protected. And that can usually be done at a higher level of
abstraction—a grosser level of granularity working with classes of vulnerability rather
than specific vulnerability variat ions.
For instance, The OWASP “XSS Filter Evasion Cheat Sheet” contains 97 XSS varia-
tions, with some variations containing several to many subvariations.12 The security
architect might make a study of one of more of these to understand the mechanisms
required of an attacker in order to exploit an XSS. What the architect must know is that
the attacker needs access to a vulnerable page. The attacker must also induce the target
user to load the page with browser scripting enabled. The language used in the attack
must be specifically enabled in the user’s browser. The attacker must include in the

118 Securing Systems
URL attack the script that is the initial attack. All of these high-level conditions must be
true irrespective of the details of the actual method used to execute the attacker’s exploit
script. A security architect who is assessing a web front end should understand XSS at
this level in order to effectively design controls and in order to understand precisely
what the target is and what is at risk from the successful exploitation of the vulnerability.
And finally, developers are more likely to respond to a request for code changes if the
security architect can speak authoritatively about what the coder must implement.
The treatments that are typically applied to XSS vulnerabilities are input validation
and, where possible, some assurance that output does not include any encoding that
allows an attacker to express any sort of scripting semantics. Additionally, defenders
typically restrict access such that the system can’t be probed by the millions of attacks
hunting for weakness on the Internet. Although authentication by itself isn’t a pro-
tection against XSS, it can reduce the attack surface somewhat for many, but not all
situations. The amount of attack surface reduction depends upon the amount of restric-
tion to a more trustable population that the authentication achieves. We explored this
somewhat above.
Similarly, exploitation of any heap overflow will be unique to the organization of
memory in an executable, on a particular operating system. The techniques can be
generalized; the exploitation is always particular to a particular vulnerability. There
are literally thousands of exploits. Anley et al. (2007)13 include three examples of heap
overflows in their introductory chapter and several chapters of samples under major
operating systems, such as Windows, Linux, and Mac OS X. Again, a security archi-
tect assessing a system that is written in a language that must handle memory must
understand the nature and attackability of heap overflows and should understand the
generalized value gained by the attacker through exploitation. But an encyclopedic
understanding of every possible heap overflow isn’t necessary in order to proceed with
a system analysis.
To prevent heap overflows, coders must handle system memory and heap memory
with great care by allocating the size that will be needed, releasing memory properly,
and never reusing the memory. In addition, restricting access to input channels may
reduce the attack surface, as well.
I noted above that it’s useful to understand the workings of at least one variation of
each class of vulnerability. In order to apply the right treatments or the right security
controls, which, ultimately, will be the requirements for the system, one needs to under-
stand just how vulnerabilities are exercised and what exercise “buys” the attacker, that
is, what is gained through exploitation.
As has been noted previously, resources are always limited. Therefore, it almost
never makes sense to specify security controls that provide little additional security
value. Each control should be applied for specific reasons: a credible attack vector that,
when exercised, delivers something of attacker value. Since there is no one-to-one map-
ping between control and attack vector, one control may mitigate a number of vectors,
and, concomitantly, it may take several treatments, or defenses, to sufficiently mitigate
a single attack vector. It’s an M:N problem.

Information Security Risk 119
Furthermore, it is typical for smart engineers to question requirements. This is par-
ticularly true if a security assessment has been mandated from outside the develop-
ment team. The assessor, you, will need ammunition to build credibility with the
development team. Your influence to make requirements and recommendations will
be enhanced, once they have confidence that you actually know what you’re talking
about, understand the relevant threats and how real they are, and how they will attack
the system. In the author’s experience, engineers can be quite suspicious of security
pronouncements and ivory tower requirements. Sometimes, the security assessors must
prove their mettle. There’s nothing like describing how a system can be misused, and
stating some statistics about how often that happens, to clear the air and create a sense
of common purpose. As usual, it’s a mistake to ignore the people side of information
security. Security architecture is probably as much about relationships as about any-
thing technical.
Vulnerabilities can be introduced during the coding of the system. Issues such as
XSS and the various types of overflows are almost always, at least in part, coding
errors—that is, they are bugs.
Importantly, there’s another type of miss that creates vulnerability in the system.
These are the missed security features and design elements (sometimes architectural
elements) that don’t make it into the architecture and are not then designed into the
system. Gary McGraw, Chief Technical Offi cer of Cigital, Inc., and author of Software
Security: Building Security In, told me that “flaws” are 50% of the errors found in sys-
tems.* In my experience, when working with mature architecture practices, the number
of architecture and design misses tends to be much lower. I can only speak anecdotally,
because I have not kept any metrics on the distribution.
It may be worth noting that terming architecture and design misses as “flaws” might
not serve a security architect integrating with other non-security architects. If com-
paring a system under analysis to the ideal security architecture, missing key security
capabilities is certainly a flaw in that architecture. However, the term “flaws” might be
received as a comment upon the maturity of practices, especially with senior manage-
ment. But I’m pretty sure it certainly won’t make any friends among one’s sister and
brother architects, one’s peers. If I begin working with a development team and I then
tell the hard-working members of that team that they had created a bunch of flaws in
their design, I probably wouldn’t be invited back for another session. “Flaw” might not
be the most tactful term I could employ? Maybe this is a difference between consulta-
tion services and having spent a career working with and integrating into enterprise-
level architecture practices?
* I don’t know where Gary gets this number. However, the number of design misses or “fl aws,”
in Gary’s parlance, is not really important for our purposes. A single security requirement
that has not been put into the system can open that system to misuse, sometimes catastrophic
misuse. AR A is not about cumulative numbers, but rather, thoroughness.

120 Securing Systems
I prefer the term “requirement.” Architects proceed from requirements. This is a
common term within every architecture practice with which I’ve worked. During the
assessment, one builds the list of security requirements. Then, during the architecture
phase, the security architect will likely be engaged in specifying how those require-
ments will be met. Finally, during the design phase, the architecture is turned into
specific designs that can be implemented and coded. Delivering a set of architecture
requirements fits neatly into the overall architecture process. Your “mileage,” the needs
of your organization, may, of course, be different.
The design makes the architecture buildable. Programmers work from the design.14
As may be seen, there must be some sort of weakness—whether it’s a missing secu-
rity feature such as authentication or encryption, or a coding error—a bug that exists,
before an attacker can misuse the system as per our definition above. Thus, vulnerabil-
ity is a key component of our credible attack vector term.
Hopefully, it’s obvious that vulnerability is not the only term for calculating risk?
Vulnerabilities are key. Without them, an attacker may not proceed, but, in and of
themselves, vulnerabilities do not equate to risk. The other factors combined into our
term, credible attack vector, must each be in place. And therein lays one of the keys
to building a defense. We will take this up in a subsequent chapter. In short, inter-
rupting the terms of the credible attack vector is a way to disrupt the ability of the
attacker to promulgate a successful compromise. Restricting or removing terms with
a credible attack vector delivers defense. Hence, not only is it useful to understand the
nature of credible attack vectors in order to rate risk, it’s also essential to building a
Obviously, if there are no vulnerabilities, we might require less robust and layered
cyber defenses. It’s important to note that even if a system contains no vulnerabilities,
it still might need security features and controls in order to meet system objectives.
For example, in an online banking system, each user must only access her or his own
account details. Each user must not have access to any other user’s details. This bank-
ing system requires an authorization model of some sort. Although it may be argued
that failure to implement an authorization model actually introduces a vulnerability,
failure to implement authorization properly also misses one of the key objectives of the
banking system: safety and privacy of each user’s financial details. The intertwining
of vulnerability and necessary security features points back to our list of assessment
prerequisites. Particularly, the assessor must understand the system’s intended purpose,
objectives, and goals. And it is good practice to understand how the system’s purpose
contributes to the organization’s mission. It’s probably a mistake to chase vulnerability
in and of itself. Although important, vulnerability is not everything to the assessment,
to the objectives of the system, or to risk calculation.
We have explored the subcomponents, the contributing factors, to a credible attack
vector. A credible attack vector may substitute nicely in everyday risk assessments for

Information Security Risk 121
the probability term. (For more in-depth information on the aforementioned, please
refer to the following: Brook Schoenfield, “Just Good Enough Risk Rating,” SANS
Institute Smart Guide, released 2012.) Because risk calculation without sufficient data
can be quite difficult and perhaps mathematically lengthy, we build a technique that is
sufficient to deliver reasonable and consistent risk ratings—namely, a technique that is
lightweight enough and understandable enough to be useful “in the trenches.” I make
no claim as to the mathematical provability of using the term credible attack vector as a
substitute for a risk proba bility term. However, to its credit, there are literally thousands
of system assessments that have used this calculation. There exist longstanding teams
that have been recording these values over an extended period of many years in order to
manage risk. I believe that credible attack vector is based upon sound concepts gleaned
from the FAIR risk methodology.
I encourage you to break down the elements that make up the likelihood of the
chain of events for successful compromise in whatever terms or scheme will work for
you and your organization. As long as your terms represent, in some manner, the fac-
tors involved in connecting attackers to vulnerabilities through the attacker’s exploits,
any complete set of terms will probably suffice. I offer credible attack vector as only one
possible approach, perhaps overly simplistic? My intention is to seed consideration on
the nature of the probability problem. And, as has been mentioned, we can use the sub-
terms of credible attack vector as a starting point for building defenses.
However your organization and you personally choose to calculate probability,
please bear in mind that it’s a complex of various elements, each of which should be
considered before you can arrive at a probability of an attacker succeeding. Failure to
honor this complexity will be a failure of your risk rating approach.
4.6.4 Impact
There is one term remaining in our pseudo-risk calculation. This is the loss value, or
“impact” of a successful attack. If a monetary loss can be calculated, I believe that is
generally preferable. However, when we consider losses whose monetary value may be
more difficult to calculate, I use the term “impact” to reach beyond monetary calcu-
lations into the arena of general harm. Impact to a system, a process, a brand, or an
organization is meant to be a slightly broader term. I use “impact” to encode all of the
CIA elements: confidentiality, integrity, and availability. Impact may include the good
will of customers and partners, which is notoriously difficult to estimate. Although it
may be possible to associate monetary values to some losses, there are situations where
that may be more difficult, or even impossible. However, understanding an impact as
high, medium, or low is often sufficient.
As a simple example, consider an organization that maintains thousands of servers
for its Internet accessible services. In this scenario, loss of a single server may be fairly
insignificant. However, let’s say that customer goodwill is exceedingly important to the

122 Securing Systems
business model. Given the importance of customers trusting an organization, should
the compromised server get used to attack customers, or to display inappropriate mes-
sages, such a situation might result in a more significant loss. What if that server has
become a base of operations for attackers to get at more sensitive systems? In any of the
foregoing scenarios, a single compromised server among thousands that are untouched
may be seen as a much greater loss.
Once again, we see that accounting for more than the vulnerability by itself yields
different risk results. I cannot stress enough the importance of accounting for all the
sub-terms within the credible attack vector. Once accounted for, the impact of exer-
cising CAV depends, as we have seen, upon the risk posture of the system within an
organizational context. To divorce these various component items from each other may
yield no more than a “finger to the wind” gauge of possible risk values.
I reiterate that many vendors’ products, vulnerability scanners, Global Risk and
Compliance (GRC) systems, and the like often offer a “risk” value for vulnerability or
noncompliance findings. Be suspicious of these numbers or ratings. Find out how these
are calculated. I have sometimes found upon asking that these ratings are based upon
the worst case scenario or upon the Common Vulnerabilities and Exposures (CVE) rat-
ing of similar issues.* Without context and relative completeness, these numbers cannot
be taken at face value. This is why the Common Vulnerability Scoring System (CVSS)
adds an “environmental” calculation in order to account for the local circumstances.
There is no shortcut to gauging all the terms within CAV (or similar) and rating organi-
zational impact in order to derive a reasonable sense of risk to a specific organization at
a particular time, and in light of the organization’s current capabilities.
4.7 Business Impact
There is obviously a technical impact that occurs from the exercise of most vulner-
abilities. In our XSS examples, the technical impact is the execution of a script of the
attacker’s choosing in the context of the target’s browser. The technical impact from a
heap overflow might be the execution of code of the attacker’s choosing in the context
of an application at whatever operating system privileges that application is running.
These technical details are certainly important when building defenses against these
attacks. Further, the technical impact helps coders understand where the bug is in the
code, and technical details help to understand how to fix the issue. But the technical
impact isn’t typically important to organizational risk decision makers. For them, the
impact must be spelled out in terms of the organization’s objectives. We might term this
“business impact,” as opposed to “technical impact.”
* A simple search of CVE returns XSS vulnerabilities rated in a range from 1.9 (very low) to 9.3

Information Security Risk 123
Continuing with the two examples that we’ve been considering, let’s examine a cou-
ple of business impacts. As was explained earlier, a XSS attack is usually an attack via a
web site targeting the user of that web site. The goal of the attacker will be something of
value that the user has: his identity information, her account details, the user’s machine
itself (as a part of a botnet, for instance). The attacker is attempting to cause the user
to unwittingly allow the attacker to do something to the user or to the user’s machine.
There are too many possibilities to list here. For a security architect who is trying to
assess the risk of an XSS to an organization, it is probably sufficient to understand that
the user of the web site is being attacked (and possibly hurt) through a mechanism
contained on the web site. The organization’s web site is the vector of attack, linking
attacker to target.
From the web site owner’s perspective, a web site becomes a tool in service to an
attacker. The attacks are directed at the web site’s users, that is, the organization’s users.
For example, consider an organization offering an open source project, providing a set
of executable binaries for various operating systems (the “application”) and the source
code to which developers may contribute. Most open source project sites also foster a
sense of community, a web site for interaction between users and coders. So, there’s
usually some sort of messaging, discussion forums, and search function. All the above
functions are offered besides static and dynamic web content about the project.
Open source projects often don’t have “leaders” so much as leadership. But imagine,
if you will, that those who take responsibility for the project’s web site must consider
XSS vulnerability, which I’m sure, many must. What is the business impact of letting
attackers target the users of the project’s web site?
I would guess that such a site might be able to sustain an occasional user being
targeted, since success of the attack is not guaranteed, given an up-to-date, security
sandboxed browser. Further, if the site does not require the user to turn on site scripting
in their browser, users are free to use the site in a more secure manner, less vulnerable
to XSS.
But if I were responsible for this site, I would not want to squander my user confi-
dence upon XSS attacks from my web site. Put yourself, just for a moment, in the shoes
of the people dedicating their time, usually volunteered, to the project. I can well ima-
gine that it is precisely user confidence that will make my project a success. “Success”
in the open source world, I believe, is measured by user base, namely, the number of
users who download and use the application. And success is also measured through the
number of code contributions and contributors, and by the level of activity of code and
design discussion. If users do not feel safe coming to the project’s web site, none of the
foregoing goals can be met.
Hence, the business impact of a XSS to an open source project is loss of user and
contributor confidence and trust. Further, if the web site is buggy, will users have confi-
dence that the application works properly? Will they trust that the application does not
open a vulnerability that allows an attack on the user’s machine? I’m guessing that user
confidence and user trust are paramount and must be safeguarded accordingly.

124 Securing Systems
Now, we can assess the risk of a XSS bug within the context of the organzation’s goals.
And we can express the impact of a successful attack in terms that are applicable to a
given open source project: “business impact.”
Let us now consider the heap overflow case above. Like XSS, an overflow allows
the attacker to run code or direct an application’s code to do something unintended
(such as allowing the attacker to write to the password store, thus adding the attacker
as a legitimate user on the system). Heap overflows occur most often in programs
created with languages that require the programmer to directly handle memory.* The
word “application” covers a vast array of software scenarios, from server-side, backend
systems to individual programs on an endpoint, laptop, desktop, smart phone, what-
have-you. Apple’s iOS phone operating system, a derivative of the Mac OS X, is really
a UNIX descendant, “under the covers”; iOS applications are written in a derivative of
the C programming language. Because the universe of applications written in memory
handling languages is far too vast to reasonably consider, let us constrain our case down
to an endpoint application that executes some functionality for a user, has a broad
user base (in the hundreds of millions), and is produced by a company whose mission
is to produce broadly accepted and purchased software products. Examples might be
Microsoft or Adobe Systems. Each of these companies produces any number of discreet
applications for the most popular platforms.
A heap overflow that allows an attacker to misuse a widely distributed application to
attack a user’s machine, collect the user’s data, and ultimately, to perhaps control a user’s
machine really is not much different, in a technical aspect, from XSS. The attacker
misuses an application to target the user’s endpoint (and, likely, the user’s data). The
difference is one of scale. Scripts usually run in the security sandboxed context of the
user’s browser. Applications run directly on the operating system at the privilege level
of the logged-in user. For broadly distributed applications, privilege might be restricted
(typically on enterprise-supplied computers). But most users run personally owned
computers at “administrator” or some similar high level of privilege. If the attacker can
execute her or his code at this level, she or he has the “run of the box,” and can do with
the computer whatever is desired. Users are thus at great risk if they use their computers
for sensitive activities, such as finances and medical information. In addition, at this
level of privilege, the computer might be used by the attacker to target other comput-
ers, to send spam (perhaps through the email accounts of the user?), or to store illegal
digital materials.
I believe that it isn’t too much of a leap to imagine the business impact to an
Independent Software Vendor (ISV) from a widely distributed heap overflow that
* Th ough it must be remembered that even languages that protect application programmers
from memory handling typically program the runtime environment with a memory handling
language such as C/C++, at least partially. Th erefore, although the application programmer
may not have to worry about overfl ow issues, the runtime must certainly be coded carefully.

Information Security Risk 125
allows execution of code of the attacker’s choosing. Again, the ISV is staring at a loss of
customer confidence, in the software, in the brand, in the company itself.
4.7.1 Data Sensitivity Scales
Chapter 2 introduced data sensitivity as a determinant of system criticality. Uncovering
the sensitivity of the data flowing and stored by a system is a key part of an architectural
assessment and threat model. Sensitive data is likely to be a target of attack and may
offer value, sometimes great value, to potential attackers, the threat agents. But just as
importantly, the reason data is classified as sensitive should be due to the potential harm
to the organization if the data is disclosed before the organization intends to disclose it.
Obviously, the customer’s personal, health, and financial data must be protected,
for customer trust and goodwill and by law, at least in the United States.* But there are
numerous other reasons for declaring data sensitive. In the United States, insider trad-
ing laws prohibit the sale of publicly traded stock with knowledge of financial results
for the company before the public has the information. These laws were put into effect
in order to provide a fair trading environment in which everyone has the same infor-
mation. The laws also prevent the manipulation of stocks based upon prior, “inside”
knowledge. Because of these laws, anyone who has access to such information before
it is disclosed must be considered a financial insider, whether they are aware of their
status or not.
That awareness is important because a database administrator who has access to
pre-announce financial information, even casually, while doing other unrelated tasks,
is still bound by the insider trading rules. If she or he inadvertently and uninten-
tionally sells company stock during a quiet (no trading) period, this administrator
might have broken the insider trading law. Due to the broad applicability of the law,
pre-announcement financials are typically considered to be among a public company’s
deepest secrets. The moment the financial results are announced, however, they become
part of the public domain.
Patented prescription drug formulas have a similar shape. They are financially key
company intellectual property that must remain proprietary and protected so long
as they are in development. Once the formula receives approval for marketing, it is
patented and thus protected by law. The formula becomes part of the patent’s public
record and need not be held secret at that point.
For a software maker, the algorithms that make up the unique properties of the
code that produces the company’s revenue will typically be a closely guarded secret.
Even when the algorithm is protected under patent law, the algorithm’s implementation
may be considered secret, since a competitor, given knowledge of the source code, could
* Many other countries have more stringent personal data protection laws than the United
States, of course. But privacy protection law is beyond the scope of this work.

126 Securing Systems
potentially build a similar product without incurring the research and development
cost for an original design. Security software, especially cryptography implementations,
tend to be held quite closely even though the algorithms are public. This is so that if
there is an error in the implementation, attackers will not find out before the imple-
menter can fix the error.
These are a few examples of data that may be considered “secret” or “proprietary,” or
closely held by an organization. Each organization must decide which types of data, if
lost, will represent a significant loss or impact to the company. There will also be infor-
mation that may incur impact if disclosed, but which won’t be key or critical. And most
organizations have public information that is intended for disclosure.
A mature security architecture practice will understand the data sensitivity rating
scale of the organiza tion and how to apply it to different data types. By classifying the
sensitivity of data, the assessor has information about the required security posture
needed to protect the data to the level that is required. Further to the point of this sec-
tion, loss or impact can be expressed in business terms by noting which data are targets
and by understanding the potential effects on the system and the organization when
particular data are disclosed or tampered with. Data sensitivity, then, becomes a short-
hand tool for expressing the business impact of a risk.
Understanding the business impact of the exercise of a credible attack vector is the
goal of a risk rating. Although severity is one dimension, ultimately, it is the amount of
effect that can radiate from a compromise that is important. With that in hand, deci-
sions about how much risk to tolerate, or not, are properly informed.
4.8 Risk Audiences
There are different audiences, different stakeholders, who need to understand risk
through unique, individualized perspectives. It’s a good practice to craft risk messages
that can be understood from the perspectives of each stakeholder group. As has been
noted, decision makers, namely, organization leaders, typically prefer that risk be stated
in business terms, what I’ve termed “business impact.” Business impact is the effect that
the successful exercise of a credible attack vector will have on the organization’s opera-
tions and goals.
But as we’ve already seen, there also exists a purely technical impact. Defenders need
to understand the sorts of things that can happen to the attacked computer system.
Without this understanding, defending the system adequately is not possible. Security
defense must be matched to attack method and is always implemented to protect some-
thing of value. Further, one can be more precise when one understands the technical
targets of particular attack methods.
There are other audiences beyond the purely technical. Each audience may need its
own expression of impact. For instance, program and project managers will want to
understand how successful compromise will affect their deliverables: implementation
phases, schedules, resources, and budgets. Project managers are also likely to want to

Information Security Risk 127
know whether they have missed some step in the development lifecycle. Architects will
require an architectural view of impacts: which components are affected, which data
can no longer be considered trustworthy, and what communication flow will need pro-
tection. Developers will want to know what features should have been delivered or what
the correct behavior should have been.
Of course, not every impact will have relevance to each of these perspectives. And
some stakeholders will only want impacts (executives, usually), while others (engineers)
may need to understand the attack vector, as well. The wise practitioner couches risk
communication in the terms best understood by the different viewpoints. “Impact”
may be expressed in multiple ways so that each person can understand a risk and why
it’s important to address the risk. Impact can be tailored specifically, depending upon
which decisions are being made surrounding a risk. Usually, people need to understand
why a risk is important to them before they will be willing to make changes in order to
mitigate the risk.
4.8.1 The Risk Owner
Which brings us to the “risk owner.” Depending upon the organization’s structure
and mission, there may be people who are held accountable for decisions that accept
or mitigate organizational risk. In my years of very unscientific surveys of enterprise
information security professionals, my sense is that many enterprise-sized organizations
let business leaders (usually, a vice president or above) decide about what risk to take,
how much residual risk to accept rather than treat, and which attack vectors can remain
unmitigated. But certainly, even in my casual (though lengthy) observation, organiza-
tions big, medium, and small differ, sometimes dramatically, with respect to exactly
who can make these decisions.
Since one of the purposes of AR A is to uncover risks from digital systems, naturally
the process is going to search for and find risk. Not all of that risk can be mitigated.
And in some organizations, none of it will be mitigated until a decision maker chooses
to apply a treatment.
In some organizations, the risk assessor may be empowered to make decisions,
anywhere from making all the computer risk decisions to only those that fall within
organi zation guidance, standards, or policies. A decision split that I have seen numerous
times is constrained to where a risk can be entirely treated by following an organization
standard or industry standard, or similar. The assessor is empowered to decide upon a
design to fulfill the requirement. However, if the risk cannot be treated to at least an
industry standard approach, then it must be “raised.”
Raising risk means bringing the untreated or residual risk to a decision maker for a
risk decision. These decisions typically take one of three mutually exclusive forms:
1. Assumption of the risk: “proceed without treatment,” that is, the organization
agrees to bear the burden of the consequences, should an impact occur.

128 Securing Systems
2. Craft an exception to treating the risk immediately, that is, “fix the risk later, on
an agreed-upon schedule.”
3. Treat the risk immediately.
In order to raise a risk for a decision, one must know to whom to raise the risk. The
person who can make this decision for an organization is the “risk owner.” This is the
person or persons who have sufficient responsibility to the organization that matches
the scope of the risk.
[R]isk owner: person or entity with the accountability and authority to manage a RISK15
In large organizations, there may be an escalation path based upon the impact of
the risk, from team, to group, to division, to enterprise. Depending upon how much of
the entire organization may be impacted, the risk owner might escalate from the pro-
ject team (project delayed or over budget), to the director for a group (operations of the
group are impacted), to a vice president of a division, or even to the top tier of manage-
ment for risks involving the reputation of the entire enterprise. In short, it is the impact
of the risk that dictates at what level a decision can be made. But of course, there is sub-
jectivity when scoping impact. Although this subjectivity needs to be acknowledged, it
is still usually possible to ascertain the scope of impact in terms of organizational levels
and boundaries. If in doubt, go up a level. It never hurts to have clear decision-making
power. On the other hand, if a decision is made by those without the authority to make
it, they put the organization at risk. Risk decisions made at a level insufficient to the
scope of the impact will then likely be hidden from those that do have the authority.
Impact surprises from risks that had previously been discovered but have not been made
known to decision makers are rarely “fun.”
Before any assessments are performed, the assessor should have a firm grasp on just
which roles have decision-making power over particular impact scopes. At each level
(whether a single level or many), the role with the decision-making authority will be
the risk owner. Having a clear understanding of just who is capable of making which
decisions is critical so that any residual risk that is uncovered will be dealt with appro-
priately. Whether decisions are made collectively by all participants, or the organization
has a strict hierarchy (with all decisions made at the top level), whatever the form, the
assessor must understand the form and the roles. Given the difficult and changeable
nature of cyber risk, there is sure to be residual risk for which hard decisions will need
to be made, and risk assumed or treated.
Along with risk ownership is the escalation path of decision making. This is also an
important prerequisite to assessment. Of course, fully collective organizations, where
decisions are made by all the participants, have no formal, hierarchical escalation path.
But that doesn’t mean that there’s no formal escalation path at all. The escalation might
be from a working group to the governing body. In organizations that make decisions
in a fully participatory manner, the escalation will be to a time and place where every-
one can participate, where all can be a part of the decision.

Information Security Risk 129
In enterprises that make some decisions through some form of informal consensus,
there is usually a deciding “vote” in case of a deadlock at each peer level. Typically,
that structure will be the escalation path. And in the typical, hierarchical corporate or
govern ment organization, the decision making structure is usually laid out clearly. In
these more hierarchical organizations, the security architect must understand just how
much or how little each level may decide, to what amount of harm, and for what organi-
zational scope of impact. This might be given in dollar amounts: Managers may decide
for $15,000 of harm, which is confined to their team. Directors may have $50,000
discretion, with impact bounded strictly to the director’s group. And vice presidents
might have $250,000 discretion, which is confined to their division. These numbers
are merely examples, not a recipe. Each organization decides these things locally. The
key is to find out some measure of risk discretion confined to impact to an organization
boundary before having to escalate a risk. In other words, know who the risk owners are
within the organization and for how much and how wide the risk owners may decide.
4.8.2 Desired Security Posture
The ultimate goal of an AR A for the security of any system is to bring that system to
a desired security posture. The operative term is “desired” or “intended.” Since there
is no possibility of “100% secure” (since the world is full of unknowns), and particu-
larly since merely connecting systems together and interacting through automation is
fraught with cyber risk and cyber attacks against vulnerable software, a certain level of
defense is almost always called for. But what is that “level of defense”?
There is no easy prescription or recipe to determine the desired risk posture. One
can turn to the organization’s security policy and standards as a starting point. In
organi zations whose cyber-security function is relatively mature, there may exist stan-
dards that point the way to the controls that must be implemented.
Experienced practitioners may have a good “gut feeling” for what level of risk is
acceptable and what is not. A mature GRC function may have conducted research into
the organization’s risk tolerance and concerns. Desired posture may be calculated as a
percentage of system cost or expected revenue. Or any combination of the foregoing
may provide sufficient clues to derive a security posture.
In the absence of any of the above, it may come down to conducting interviews and
listening to what is acceptable or not among the decision makers. In any event, it helps
mitigate the influence of one’s personal risk tolerance to understand what the organi-
zation seeks from risk assessments, how much security needs to be implemented, and
what risk can be tolerated.
4.9 Summary
The calculation of risk is fundamental to security assessment and threat modeling.
Ultimately, some reliable and repeatable risk methodology will have to be adopted in

130 Securing Systems
order for priority decisions to be made about which attack surfaces will receive mitiga-
tion, how much mitigation to build, and which risks can be tolerated without unduly
impinging on an organization’s mission.
In an effort to simplify risk calculation such that it can be performed rapidly during
security assessments, we’ve proposed a rather simple approach: All of the terms in our
“credible attack vector” must be true in order for a threat agent to be able to exercise a
vulnerability. Even if there is a credible attack vector, the impact of the exploit must be
significant or there is no risk.
The terms in a credible attack vector are:
• Threat (exploit)
• Exposure
• Vulnerability
Each of these terms is further broken down so that the aspects of a successful attack
can be assessed in separate and distinct terms. In fact, we propose substituting credible
attack vector for the probability term in the standard, well-known insurance risk equa-
tion that was presented at the beginning of this chapter.
When we build security defenses, we can use a simplified Boolean approach to each
of the terms in credible attack vector. To interrupt any single term is to prevent an
attack. This simplified approach allows us to more precisely specify security controls as
we build our defense-in-depth.
In this chapter, we have narrowed the scope of the term “risk” to precisely fit the pur-
pose of security assessment and threat modeling. We have proposed one method ology as
an example of how risk can be understood and rated fairly easily. Whatever methodol-
ogy is used, it will have to be repeatable by the analysts who’ll provide security assess-
ments, build threat models, and provide requirements for a system’s security posture.
1. Jones, J. A. (2005). “An Introduction to Factor Analysis of Information Risk (FAIR).”
Risk Management Insight LLC. Retrieved from
media/documents/FAIR_Introduction .
2. Ibid.
3. Schoenfi eld, B. (2012). “Just Good Enough Risk Rating.” Smart Guide. SANS Institute.
4. Harkins, M. (2013). Managing Risk and Information Security: Protect to Enable, p. xv.
Apress Media, LLC.
5. Hopkin, P. (2012). Fundamentals of Risk Management: Understanding, Evaluating and
Implementing Eff ective Risk Management, 2nd ed., p. 14. Institute of Risk Management
(IRM). Kogan Page.
6. Jones, J. A. (2005). “An Introduction to Factor Analysis of Information Risk (FAIR).”
Risk Management Insight LLC. Retrieved from
media/documents/FAIR_Introduction .

Information Security Risk 131
7. Ibid.
8. U.S. Senate Committee on Commerce, Science, and Transportation. (March 26, 2014).
A “Kill Chain” Analysis of the 2013 Target Data Breach. Majority Staff Report For
Chairman Rockefeller.
9. Harris, K. D. (February 2014). “Cybersecurity in the Golden State.” California
Department of Justice.
10. Welch, C. (February 14, 2014). “Target’s Cybersecurity Team Raised Concerns Months
Before Hack.” Th e Verge. Retrieved from
11. Mansourov, N. and Campara, D. (2011). System Assurance: Beyond Detecting Vulnerabilities.
p. xv. Morgan Kaufmann Publishers.
12. Hansen, R. (2013). “XSS Filter Evasion Cheat Sheet.” Retrieved from
13. Anley, C., Heasman, J., Lindner, F., and Richarte, G. (2007). Th e Shellcoder’s Handbook:
Discovering and Exploiting Security Holes, 2nd ed. John Wiley & Sons.
14. Schoenfi eld, B. (2014). “Applying the SDL Framework to the Real World” (Ch. 9). In
Core Software Security: Security at the Source, pp. 255–324. Boca Raton (FL): CRC Press.
15. ISO Technical Management Board Working Group on risk management, ISO
31000:2009, Risk management – Principles and guidelines, 2009-11-15, ICS, 03.100.01.
Available from:

Chapter 5
Prepare for Assessment
In this chapter, we will review the assessment and threat modeling process that has
been introduced in previous chapters. The process has been described at a high level,
though presented piece-by-piece, wherever a particular step of the process was relevant
to fully understand the background material necessary for assessment. In this chapter,
we will go through, in a step-wise fashion, a single example architecture risk assessment
(AR A) and threat model. The goal is to become familiar with the process rather than
to complete the assessment. In Part II, we will apply these steps more thoroughly to six
example architectures in order to fully understand and get some practice in the art and
craft of AR A. The example used in this chapter will be completed in Part II.
5.1 Process Review
At the highest level, an assessment follows the mnemonic, ATASM:
Architecture ➤ Threats ➤ Attack Surfaces ➤ Mitigations
Figure 5.1 shows the ATASM flow graphically. There are architecture tasks that will
help to determine which threats are relevant to systems of the type under assessment.
Figure 5.1 Architecture, threats, attack surfaces, mitigations.

134 Securing Systems
The architecture must be understood sufficiently in order to enumerate the attack sur-
faces that the threats are applied to. Applying specific threats to particular attack sur-
faces is the essential activity in a threat model.
ATASM is meant merely as a very high-level abstraction to facilitate keeping the
assessment process in mind. In addition, ATASM may help beginning assessors order
and retain the details and specifics of the AR A process. There are many steps and
details that must be understood and practiced to deliver a thorough AR A. Having a
high-level sequence allows me to retain these numerous details while I proceed through
an analysis. My hope is to offer you an abstraction that makes this process easier for
you, as well.
5.1.1 Credible Attack Vectors
At some essential level, much of an AR A is focused as an attempt to enumerate the
complete set of credible attack vectors (CAVs). If you recall from Chapter 2, a credible
attack vector was defined as follows:
Credible attack vector: A credible threat exercising an exploit on an exposed
Recalling the risk term discussion from Chapter 4, a CAV encapsulates the three
threat sub-terms into a single expression:
• Threat
• Exposure
• Vulnerability
Each of these terms is likewise composed of details that were explained in Chapter 4.
If you don’t feel comfortable with CAV, in particular, and computer security risk, in
general, you may want to review Chapter 4 before you proceed.
Risk is the critical governing principle that underlies the entire risk assessment and
threat modeling process. Ultimately, we must mitigate those computer attacks that
are likely to impinge upon the use of the system under assessment and upon efforts to
obtain the objectives of the organization. As my friend, Anurag “Archie” Agrawal, says,
“[threat modeling is] all about risk. . . .” Still, you will find that “risk” is not mentioned
as much as priorities.
As you walk through the process, filtering and prioritizing, you will be calculating
risk. Although a formal risk calculation can be, and often is, a marker of a mature secu-
rity architecture practice, by itself, simply understanding the risks is only one goal for
an ARA and threat model. We also need to know which risks can be treated and which
cannot, and produce an achievable set of requirements that will get implemented. Risk is

Prepare for Assessment 135
the information that drives these decisions, but it is not the sole end result of the ATASM
process. For this reason, risk calculation is built into the steps of ATASM and underlies
much of the process, rather than being a separate and distinct calculation exercise.
5.1.2 Applying ATASM
Assume that the analyst (you?) has completed the required homework and has studied
the background subjects sufficiently to understand the context into which the system
will be deployed. The “context” is both technical and organizational and may be bro-
ken down into strategy, structures, and specifications. To reiterate, this knowledge set
includes at least the following:
Table 5.1 Required Background Information
Strategy Threat landscape Risk posture
Structures The set of possible
Existing security limitations
Specifi cations Data sensitivity Runtime and execution
Table 5.1 summarizes the typical background information an assessor brings to an
assessment. These topics have been arranged with the “3 S’s” merely for convenience
and to provide an ordering principle.
No matter how one may choose to order these topics, it remains that before begin-
ning an AR A and threat model, one will need to have examined the current threat
landscape into which the system will be deployed. Further, investigation will have taken
place to understand the risk posture of the organization that will use and deploy the
system as well as what the overall risk tolerance of the organization is. One must under-
stand the current state of security capabilities and infrastructure in order to understand
what controls are easiest to implement and what will be a future state (perhaps only a
wish?). Since no security is perfect, there will be limitations that must be taken into
account. Certain approaches or security technologies may be impossible, given the cur-
rent state of affairs. If the assessor wants to provide real-world mitigations, she or he
will need to know what can and cannot be accomplished in the present, in the short
term, in the medium term, and perhaps longer still. Each execution environment typi-
cally presents a unique set of security services and a unique set of vulnerabilities and
security challenges. A system may take advantage of the services but, likewise, must
take account of the runtime’s weaknesses. Finally, as was noted previously, different
deployment models require different architectural approaches to security. Customer
premise equipment must be capable of implementing the owner’s security posture.
Systems destined for an infrastructure that is controlled by an organization will inherit
that infrastructure’s security weaknesses and strengths.

136 Securing Systems
• Runtime Models
• Deployment
• Deployment Processes
The wise security analyst will arm her or himself with sufficient background know-
ledge to thoroughly assess a system. The enumerated background subjects create a
strong basis for AR A.
Recall from Chapter 2 a list of steps that, taken together, constitute a high-level proce-
dure for conducting an ARA. Figure 5.2 orders those steps by the high-level ATASM
abstraction. That is, architecture steps are followed by threat-related tasks. The results of
these are applied to an enumeration of attack surfaces, which, when prioritized, can then
be defended by building a set of risk mitigations, a defense-in-depth of security controls.
Each of the steps outlined in Figure 5.2 is, of course, what developers call a “non-
trivial” activity. We will tease these steps apart more fully below. In this book, the steps
Figure 5.2 ATASM procedure steps.

Prepare for Assessment 137
are presented as a sequenced set of tasks. The first assessments will proceed in this orderly
manner. However, in the real world, the process can be more like peeling an onion.*
You may factor an architecture into defensible units and then discover that there
are other critical systems that were not diagrammed in the architecture, as presented
up to that point. Or you may be writing what you think is a complete list of security
requirements, only to discover that a security control you thought existed was actually
designed out or had been phased out and is no longer available. The implementation
team may tell you that they absolutely must deploy a technology about which you know
next to nothing. Encountering an unknown technology will normally cause an asses-
sor to research the technology and learn about its security history, its weaknesses and
strengths, before proceeding. During many agile development processes, the design
(and even the architecture) will change during coding, which can entail shifting secu-
rity requirements around to accommodate the changing software architecture. There
are can be delays obtaining necessary technologies and equipment or changes in scope
or schedule.
Any of the above scenarios, in isolation or in various combinations, may cause a
threat model to become “stale,” to be incomplete or even obsolete. The assessor will
have to return to once again achieving an understanding of the architecture and then
move through the succeeding steps to account for new information or other significant
changes. (In other words, the process begins again at the architecture stage in order to
analyze the new material.) The worst case is when there has been no notice of a need
for reassessment, the system is near deployment (or already live and in production!), and
suddenly, changes made during development surface such that the changes invalidate
most or all of the security posture. Such events can and do happen, even in the best
run organizations. The short of it is, learn the steps that must be taken. Proceed in as
orderly and organized a fashion as possible. But don’t expect complex system develop-
ment to be entirely linear. Change is the only constant.
5.2 Architecture and Artifacts
We spent considerable time in Chapter 3 understanding what system architecture does
and why it’s important for security assessment. We’ve looked at a few architectures,
both to understand the architecture and from the perspective of what a security archi-
tect needs to know in order to perform an AR A. We will examine these architectures
in greater depth in Part II. Let us examine what needs to be accomplished with the
architecture activities.
* John Steven, an industry-recognized authority on threat modeling, calls the threat modeling
process “fractal.” Adding to the organic exploration forays to which John refers, I believe that,
in practice, the process tends also to be recursive. John Steven’s biography is available from
OWASP: reat_Modeling_by_John_Steven.

138 Securing Systems
Part of understanding a system architecture is to understand the system’s overall
functions and what objectives deploying the system is hoping to achieve. An assessment
should start at the beginning by determining the system’s intended purpose and how it
fits into the overall goals of the deploying organization and its users. This information
will “ground” the architecture into the real world and will place the architecture within
its intended context. Ascertaining the overall purpose may be enough information to
determine the appropriate security posture of the system, especially when the assessor
is familiar enough with the risk tolerance and security posture of the organization as
a whole.
Try not to start with technologies, if you can avoid it. Instead, steer the conversation
back up to purpose and, perhaps, an overall architecture or how this system will fit into
an enterprise architecture (if the enterprise architecture is known).
5.2.1 Understand the Logical and Component Architecture
of the System
First, of course, one must know what a logical and/or component architecture is.
Hopefully, you have some feel for this gained through reading Chapter 3?
The logical architecture represents each logical function that will be in the sys-
tem. These groupings are irrespective of how many individual units of each function
get instantiated and irrespective of the physical architecture. Generally, that there are
computers (servers?) and operating systems is assumed and not detailed; the physical
network addresses assigned to each network interface are unimportant in the logical
view. Instead, functions like business logic versus the presentation versus back office
accounting systems versus databases are separated out. It is assumed that all of these are
connected by some sort of network architecture. Oftentimes, the system management
function will be detailed as a separate logical function, even if the business logic and
the management module share the same server. If the functions are different and these
functions are separated in some manner (process space, execution environment, what
have you), then in a logical architecture, the functions will be shown as separate units.
In a component architecture, it is the various components that are differentiated.
For instance, if the business logic has a database and the management function also
has separated data, then each of these databases will be detailed. In order to depict the
component relationship, the business database can be shown as a part of the business
component, while the management database can be depicted as a part of the manage-
ment component. This makes sense in the component view, even if in actuality, both
databases reside in a common data layer, or even if these are different tables in the same
database server. The concept is to represent all the units that will make up the architec-
ture, not to represent where and how they will be executing.
As you may see, each of these views, logical and component, has advantages and
disadvantages. No one representation can possibly show everything for even modestly

Prepare for Assessment 139
complex systems. Hence, there are many possible views. If you remember from Chapter
3, an assessment may take several different views in order to uncover all the relevant
information. For more complex systems, you will likely end up stitching together the
information from several views because security is the matrix domain that involves and
is implemented at many levels, often in many, if not all, of the components of a system.
Until details of implementation are considered, the physical architecture will likely
have too many details and may certainly obscure important relationships between
functions. For instance, for many years I worked with an eCommerce architecture
whose physical network architecture actually consisted of a group of seven or eight huge
switches. On the face of it, that might seem a dreadful Internet facing architecture? The
networking equipment is one big block? Where are trust boundaries within that single,
physical group of switches? What if an attacker gained one of those switches? The whole
would be lost, yes? Where are the layers to protect more valuable resources by offering
bastion systems that, if lost, would prevent loss of every system connected to the block
of switches?
The logical architecture was actually a standard three-tier web architecture, with
very strict rules between layers, bidirectional authentications across trust boundaries,
valued resource protected by bastion systems that terminated and then validated traf-
fic before passing across trust boundaries, and similar protections. Looking only at the
physical architecture would not properly represent the way that traffic flowed and the
trust boundaries that existed. It was the logical architecture that appeared to separate
out the firewalls (and restrictions) of each layer and that demonstrated how traffic
flowed or was stopped. All of this was performed by a group of multifunction switches
with different blade inserts for the various functions that implemented the logical archi-
tecture. Although ultimately understanding that physical architecture made me a bet-
ter, more competent system assessor, I didn’t need that information for quite some time,
as long as I understood the logical architecture of the infrastructure.
When deciding precisely where to place network restrictions or administrative secu-
rity controls, the physical architecture will be required. Still, for coming to an under-
standing of a complex system—all of the systems’ functions and the components that
will implement those functions—the logical architecture is usually the right starting
point. As we have seen in Chapter 3, the logical and the component architecture views
often get put together into the same view; for the sake of convenience, let’s call that view
“the logical architecture.” Ultimately, in the process of decomposition and factoring,
an analysis must get down to components, perhaps even modules, and processes. As
has been said, the point of factoring the architecture is to uncover attackable units and
defensible boundaries. And as we have seen, the unit down to which we go depends on
several factors; there is no single rule to apply; architecture decomposition for security
is an art that has to be learned through experience.
As guidance, a logical architecture should start with each function in broad, inclu-
sive categories delineated by functions such that the entire system can be represented.
This representation can be along technological groupings, or the delineation can be

140 Securing Systems
along business functions, or, often, a combination of technological “stack” and business
functions. These two dimensions can be mixed successfully in a “logical” architecture
as long as the architecture representation doesn’t become too crowded to understand.
If the representation, that is the diagram, becomes so busy that it can’t be read or lines
of integration can’t be followed, that’s usually the time to come up a level of granular-
ity and group similar units together, at the same time showing the detail in sub views.
The more complex the system, the more likely it is that a security analysis will
require multiple views of that system. A guiding principle is to start at a gross level that
can be easily understood and then decompose to detail, factoring the architecture in
ever more detailed units until all the attack services and all the defensible boundaries
are understood. The “units” that will be useful depends on the security assumptions
surrounding the system under analysis. As we saw in Chapter 3, a system that is exposed
on an endpoint has a different set of boundaries than a system that inherits the security
of a well-managed infrastructure. You may remember that one of the important know-
ledge sets that is brought to an analysis is the deployment model of the system (or all the
deployment models that are involved)? And, of course, the execution environment and
its boundaries and security capabilities also partially determine just how deep a unit
must be, or at what atomicity the architecture will have to be factored.
Essentially, you must begin somewhere and then investigate. Asking implementa-
tion team members for a “logical” or a “component” architecture will get the conversa-
tion and, thus, the investigation started.
Personally, unless I get a diagram similar to Figures 3.1 and 3.2 (in Chapter 3),
which are too gross to have much security meaning, I try to work with what the team
produces, moving either up or down in granularity, as necessary. That discovery pro-
cess is often best done on a whiteboard, captured, and then diagrammed in some more
permanent form that can be shared and subsequently become a part of the record of the
security assessment. Other experts in threat modeling sometimes provide more direc-
tion by providing an architecture template. That’s a perfectly reasonable approach, as
well. As was explained earlier, I prefer to integrate into any existing processes rather
than imposing a different approach from outside. Depending upon your organization’s
style, either of these approaches can work; the right approach will depend upon the
organization and your relationship to implementation teams and any existing architec-
ture practice.
5.2.2 Understand Every Communication Flow and Any Valuable
Data Wherever Stored
Starting from a logical architecture, as described above, every communications flow,
whether to exchange data or control messages—that is, irrespective of the purpose of
the communications flow—should be diagrammed. Recalling the AppMaker architec-
ture introduced in Chapter 3, Figure 5.3 (reprised from Figure 3.4) can be considered a
representative “Data Flow Diagram” (DFD). The DFD portrays not only communi cation

Prepare for Assessment 141
flows, but their direction, as well. Single-headed (single-pointed) arrows are preferred.
Especially in request/response protocols such as HTTP, a response can be assumed.
Since the ordering of authentication and validation is important, and the origina-
tion across trust boundaries is critical to understand, the single-headed arrow, from
origination pointing to the destination, is by far the preferred representation. This is
for security purposes. Other architects may require other views using bidirectionally
pointed arrows for their purposes. That’s OK. But I caution the security analyst to insist
upon understanding origination and destination, especially in client/server applications
in which the origination of the communication is going to be critical to getting the
security controls correctly placed, and the destination will be the most common attack
surface—the attack coming from client to server, from requester to responder. Building
a defense will start at the server. (This does not necessarily exclude the client. But the
obvious attack surface to be defended is usually the destination side.)
A way to proceed is to start with the logical architecture and add the communica-
tions flows. In Figure 5.3, we have an architecture that is separated, both along func-
tional lines:
• Server for static content
• AppMaker application construction function
Figure 5.3 Data Flow Diagram of AppMaker.

142 Securing Systems
• Database software
• Data repository
• Content repository
And along technology boundaries:
• User’s browser
• Web server
• Application server
• Database server and data repository
Hopefully, the combination of logical function and component can be seen in
Figure 5.3? As was noted in Chapter 3, for reasons of clarity, an HTTP response flow
was added to this DFD. Again, real-world diagrams will contain exceptions to the usual
rules. In this particular case, since the response flow is bifurcated, a response has been
added (5) to demonstrate the bifurcation rather than a more typical response assumption.
Figure 5.4 AppMaker DFD with data types.

Prepare for Assessment 143
Data types have been added to the AppMaker diagram to create Figure 5.4. There
is a co-located repository containing various types of static content to be served to users
(customers). Moving further back in the architecture, every application server must
contain configuration data and metadata about the applications that the server is run-
ning. Usually, but not always, there is local disk storage, or readily available storage for
this application server–specific data. Although some application servers use databases
for the data associated to the application server’s running and to the execution of appli-
cations, let’s assume, in this case, that this application server makes use of local disk
storage or a network disk that appears as a local disk.
Though five different types of data associated with serving customers are shown on
the right of the diagram, there is only one arrow, for simplicity. Let’s assume that the
database server understands how to fetch appropriate data in order to fill in a response
made by AppMaker. Thus, the diagram uses a single arrow to represent all data fetches
for any type of data, each delineated data set being held in a distinct table.
In this particular use of the AppMaker application, customer financial data, profiles
of the customer—that is, the customer’s contact information, interests, order history,
browsing history among products, etc., as well as the customer’s preferences—are kept
in a database (or collection of databases that can functionally be considered a single
unit). In addition, AppMaker itself has data and metadata associated with the various
applications that it builds and instantiates, that is, the applications that are constructed
using the AppMaker software. These constructed applications must also draw from a
data catalog of products and offerings to be presented to customers for their purchase.
A security assessment must enumerate all the targets of value that will be of interest
to the various threat agents who may attack the system. A portion of that enumeration
is uncovering the data stored by and processed by the system. Most importantly, data
are rated for sensitivity and importance to the organization’s goals. For some organiza-
tions, the product catalog may be considered entirely public information. However, if
that catalog contains prices, and prices depend upon the customer, then the pricing
information may be sensitive, even highly sensitive. Data sensitivity rating can be quite
unique to each organization.
Even so, we might take some guesses about the importance of some of the data:
The obviously sensitive data will be the customer financial data. Under many jurisdic-
tions’ laws, financial data is subject to a host of regulations and protections. Any system
handling financial data from or that will be stored within one of these jurisdictions will
have to meet those regulatory protections. This compliance may dictate the sensitivity
of the data. And, in fact, the Payment Card Industry (PCI) standards dictate what can
and cannot be done with payment card financial data. PCI also dictates the protections
that must be in place for these data. It’s probably not too wild a guess to surmise that
customer financial data will be considered sensitive by this organization?
For a moment, consider the static content. Much or all of this content may be pub-
licly available to anyone and everyone. Even so, the static content will constitute at
least a portion of the public image of the organization. Although the data may be
considered “public,” a malicious change to the data would normally affect the image

144 Securing Systems
of the organization adversely. That is, in classic information security language, the
integrity of the data is important to the organization. Even public data may require
some form of protection; most organizations don’t want their publicly available product
catalog to be changed, nor do organizations typically tolerate their public marketing
materials being surreptitiously changed.
We will examine all of the data types and their protections in this AppMaker archi-
tecture in Chapter 6. For the moment, let’s remain focused on the steps of the process
that need to be completed in an assessment. One of those steps is discovering the data
that is processed and communicated by the system under analysis. Part of that discov-
ery process is rating the data for sensitivity—rating the impact of data loss, malicious
change, or unavailability. Data sensitivity is a rating system that codifies the impact on
the system and the overall goals of the organization of failure to guard the confidential-
ity, integrity, or availability (CIA) of the organization’s data.
One hint about the data sensitivity step is that in any collection of data there may
be multiple levels of sensitivity. It’s actually fairly rare that all the data in a particular
store or transaction are of the same sensitivity rating. It is the highest sensitivity that
will “rule” the collection of data sensitivities. If the high-sensitivity data are scattered
among lower-sensitivity data items, the entire collection will have to be protected to
the highest sensitivity that is included in the data set. Always ask, “What is the highest
sensitivity of data stored in this location?” Or ask, “What is the highest sensitivity of
data communicated in this flow?” That will allow you to easily uncover the needs for
data protection rather than being taken down rabbit holes of the nature of, “Most of the
data are public.” The obvious assessor response to a statement about “most of the data”
is, “Of what sensitivity is the remaining data?”
It is also important to realize that aggregation of data may raise the sensitivity or
lower it, depending on the aggregation. Anonymizing the data, the removal of identify-
ing personal identifying information, will tend to lower the sensitivity of data, at least
for privacy purposes.*
On the other hand, there are financial data that, when taken separately or in pieces,
don’t provide enough information to make the overall financial condition of an organi-
zation readily understood. However, when those data pieces are collected with other
information to create a complete picture, the data become far more sensitive because
then the collected data describe a complete, or perhaps even an “insider,” financial
picture. One aspect of investigating the data sensitivity for any particular system is to
examine the data not only in pieces but also explore how sensitivity may change when
data are aggregated or presented together.
Experienced security architects may question why I do not make a formal asset list?
Lists of every asset within a system are a part of many approaches to threat modeling.
* Businesses that monetize aggregate consumer behavior, such as Facebook and Google, might
consider their anonymized, aggregate consumer data sensitive because of the data’s relation-
ship to revenue.

Prepare for Assessment 145
I prefer to focus on assets that are at risk of attack rather than building a list of every
object or datum that might possibly be considered an asset. These lists can be quite
long. And, I believe such long lists may obscure important prioritization that will have
to be made in the face of limited resources.
Instead, I try to understand generally what data are moving through each flow and
get stored. After I’ve uncovered attack surfaces, I can apply prioritized credible attack
vectors to assets that will be made available through the vectors. The attackable assets
that are not yet sufficiently protected will become a part of the impact calculation that
then feeds a risk rating.
By waiting until I have credible attack vectors, I avoid the extra work of listing pos-
sibly irrelevant and well-protected assets. I try to focus on those assets that are actu-
ally attackable and, thus, whose compromise will most likely affect the system and
organization. This is probably just a stylistic preference of mine, rather than a formal
methodology? If in your practice, you would prefer or have a need to list all assets, then
I would suggest that this is best done during the “architecture” phrase of ATASM.
The AppMaker diagrams, Figures 5.3 and 5.4, do not describe any security com-
ponent or trust boundary. If security controls exist, these would need to be added to
the diagram. As we explore the process for threat enumeration and attack surfaces
(ATASM), we will add trust boundaries to the AppMaker logical architecture.
5.3 Threat Enumeration
Once the architecture has been represented in a manner conducive to security analysis,
the next step in the ATASM process is the enumeration and examination of threats that
will be relevant to systems of the type under analysis. In Chapter 2, we examined a few
types of threat agents in order to understand the general background knowledge that
security architects bring to the process of system assessment for security. In Chapter 4,
we explored how a single threat agent—cyber criminals—contributes to the calculation
of information security risk. Each of these threat investigations can be thought of as
background to the assessment process. We need to understand the attackers in order to
know which of them apply to any particular system. And in order to prioritize attack
surfaces and attack targets, we need to be able to calculate the risk of any particular
attack occurring. Since it is likely that we won’t be able to defend against every attack
equally, there has to be a way to prioritize those attacks that will have the most impact
and de-prioritize those attacks that are likely to produce an insignificant result.
These higher-priority areas are characterized by the existence of threat agents who have
the motivation and capabilities to take advantage of likely methods that will lead them
to their objectives – and will in turn cause unacceptable losses . . .1
If the assessor has a firm grasp on the threat agents, the capabilities of the threats,
and the exposed attack surfaces of the system, then the enumeration of the threats for

146 Securing Systems
the system in question is almost complete. However, if threat research and understand-
ing is incomplete, then the threat enumeration must be completed as a part of the sys-
tem assessment and threat modeling process.
5.3.1 List All the Possible Threat Agents for This Type of System
Using Figure 5.4 as the target architecture, who are the threat agents who will be most
interested in attacking Web applications created through AppMaker? Since we know
that this particular instance of the AppMaker implements a customer-facing store that
processes financial transactions, how does that influence which threat agents may be
interested in attacking it?
As explained earlier, any system that is open to public Internet traffic will be
attacked continuously. The vast majority of these attacks are undirected or untargeted.
Attackers are merely snooping around trying to figure out what might be vulnerable.
Information security people call this “doorknob rattling.” The vast majority of this
traffic comprises unsophisticated, automated scripts attempting well-known attacks
against possibly unpatched systems. The motives of these undirected attack sweeps are
varied; many of these sweeps are conducted by criminal organizations or independent
attackers. But there are many other reasons for running constant vulnerability sweeps,
as well. Increasing one’s skill as a security tester is certainly among these reasons. Every
sweep is not conducted by criminals, though, in many jurisdictions, conducting such
a sweep is illegal.
Obviously, there are threat agents interested in stealing the financial information of
the customers. Not so obvious, perhaps, are the threat agents interested in various forms
of cyber fraud: denial of service attacks (DoS) coupled to blackmail extortion requests.
If the website has not been careful in the handling of financial transactions, another
fraud scam might be the purchase of products for little or no price, or the theft of
products through warranty replacement scams, and the like. If a warrantee replacement
product can be delivered to the thief, the thief may resell the product at nearly 100%
profit. Identity thieves are going to be interested in the personal details of customers.
Attackers who run botnets (large collections of compromised hosts) for rental will
be interested in the servers underlying this web application. Those who sell illegal digi-
tal products, such as pirated movies, pirated software programs, or other illegal digital
products, will be interested in the web application’s storage as a place to keep such
illegal digital content. And of course, we must not forget those who would use the
web applications as a way to attack customers who will believe they’re going to a trust-
worthy website and instead are being attacked through the website [for example, cross-
site scripting (XSS)].
Interestingly, though each of these threat agents has slightly different targets, I believe
they can all be lumped together under the class “cyber criminal.” As a class, they more
or less collectively exhibit the same gross attributes, which were listed in Chapter 2 and
revisited in Chapter 4. Ultimately, each of these goals is financial reward. However, the

Prepare for Assessment 147
system-level targets differ widely, from raw machines and storage, to information that
is financial and/or personal.
Will an activist group be interested in attacking our cyber store? Probably not unless
the company fielding the web application is perceived as taking a political stand on a
controversial subject. As long as the company that owns and operates our AppMaker
web application is seen as relatively neutral, the likelihood of an activist attack is fairly
small. Let’s assume that our example company stays out of political controversy as best
as they can.
How about nation-state or industrial espionage? We don’t know what the products
are that are being sold through this website. I imagine that if the products are military
hardware, the site most definitely would be a nation-state cyber target. In the same
vein, if the products are particularly proprietary, industrial espionage is a distinct pos-
sibility. However, for a company selling fashionable socks, these two threat agent types
are likely to be much less interested in the site or the organization. Let’s assume, just
for simplicity’s sake, that this web site sells socks and nothing else—“Web-Sock-A-
Rama”—which leaves us with the various flavors of cyber criminal and the ever-present
vulnerability sweeps of the Internet.
There is one additional threat agent with which most public sites and products must
contend. Calling security researchers “threats” is somewhat of a misnomer and might
be perceived by researchers as an insult? No insult is intended. Still, if an organization
is on the receiving end of vulnerability research, the impact to reputation and brand
may be approximately the same as what can occur following a successful compromise
of systems or data.
Let me explain. It’s important to understand that the vast majority of vulnerability
researchers have no interest in hurting the systems they probe. These are honest people,
in general (just like any other affinity group). A single tricky or difficult to discover
vulnerability found within a high-profile product or service can significantly enhance
the career and remuneration prospects of the finder. That is a fact.
In addition, once one becomes known for discovering vulnerabilities, there is a bit of
a treadmill; the researcher must continue to produce significant results at a level com-
mensurate with expectations or the researcher’s career opportunities begin to diminish.
It’s a bit like being an actor or musician. After a celebrity enjoys a bit of success, a failure
to consistently follow up with more successes kills one’s opportunities, and ultimately,
one’s career. In order to maintain a position as a leader, the researcher must find ever
more difficult vulnerabilities that must get published, hopefully, through presentations
at one or more of the leading conferences. Or the researcher’s “hack” must achieve plenty
of media attention. In short, there is pressure to continue to uncover vulnerabilities.
Any product or service (or site) that achieves some amount of notoriety (such that
vulnerabilities will be seen as significant) is going to receive researcher attention. And
these people are smart, savvy, and dogged.*
* For clarity, vulnerability researchers number among my personal friends.

148 Securing Systems
My friend, Chris Valecek, calls many of the more difficult to promulgate research
attacks (“hacks”) “stunt hacks.” That is, these attacks aren’t really practical for most
attack scenarios. The attacks require highly privileged access; many sophisticated
attacks even require physical control of the machine; and special tools may be involved,
not to mention lots of time and effort in order for the attack to be carried out.
Stunt hacks make great presentations, and even, sometimes terrific theatre (in a com-
puter-oriented, geeky sort of way). And these difficult and tricky exploitations are most
certainly highly career enhancing. But the resources and dedication, the one-off, one-
time-only nature of the attacks are not suitable for most threat agents (even nation-states).
The attack might be too detectable and might require too much physical presence to be
carried off without discovery. Consequently, stunt hacks are not really about the security
of products so much as they are about the technical skills of the researcher. Irrespective
of the usefulness as an attack—a stunt hack against a well-known product, especially a
computer security product—has profound effects on the career of the researcher.
Essentially, researchers need to find vulnerabilities. They need to find tricky vulner-
abilities and then must publish the research. It doesn’t really matter if the vulnerability
can result in significant loss for the organization owning the problem. The actuality
of an impact does not play a big part of the research equation. Any organization that
has had to deal with a well-publicized stunt hack against one of the organization’s sys-
tems will know full well that the media attention of the research did not benefit the
public image of the organization, especially if that organization’s customers expect a
certain level of security from the organization. That is, security vulnerability research
can and does cause significant detrimental impact to an organization’s public image, in
some cases. If you follow this logic, hopefully you will see that, from the organization’s
perspective, security researchers can be considered a threat agent, despite the fact that
researchers do not steal or otherwise intend to compromise production systems and ser-
vices. And this consideration is in spite of the fact that the researcher is helping to find
and remove vulnerabilities from the organization’s systems.
Most researchers that the author knows personally maintain a very high caliber
of professional integrity. Despite this personal integrity, a fair amount of the time,
there is some negative impact to an organization from receiving a researcher’s report,*
especially if a significant vulnerability has leaked past whatever software (or hardware)
security practices are in use by the organization. Security research remains a double-
edged sword. Vulnerabilities are found and removed. At the same time, an organiza-
tion’s operational eco-system may be impacted through discovery and publication of
the issue in an unplanned manner. Here, we deal with security researchers as a threat
agent, despite the obvious fact that, in general, the researcher means to cause no harm
and typically sees him or herself as enhancing security, not detracting from it.
* Th e extent of the vulnerability research impact varies. Impact might only be an interjection
of unplanned work into already committed schedules. Or, impact might be as severe as a loss
of customer confi dence.

Prepare for Assessment 149
If Web-Sock-A-Rama achieves enough success to garner a significant customer base,
researchers are likely to take notice by probing any publicly available system or software
for vulnerabilities. Assuming that Web-Sock-A-Rama is one of the most often visited
sock purveyors on the Internet and has an international clientele, let’s set out the threat
attributes for security research (see Table 5.2):
Table 5.2 Threat Attributes of Security Researchers
Threat Agent Goals Risk Tolerance Work Factor Methods
Security researchers Technically
Very low Very high Unusual, one-time,
unique, complex,
Determining which types of attacks are possible is only the first step. The true value
derives from knowing which attacks are most likely to occur.2
Web-Sock-A-Rama, as defined, above, appears to have two probable threat agents:
cyber criminals and security researchers. In order to more fully understand the attacks
that are likely to come from each of these threats, we can base our prognostications
upon what we assume the threat agents are ultimately trying to accomplish. If we don’t
understand what the threats are seeking, what their objectives and goals are, it is more
difficult to prioritize some attacks as important while deprioritizing attacks that, even
if they were to succeed, would not further the threat agent’s goals. Assuming that we do
not have the resources to handle every possible attack vector equally, we want to spend
our limited resources defending against that which is probable and that which is likely to
do the most harm to our organization. We do not want to spin our wheels chasing attack
vectors that are not credible because these vectors will not advance the threats towards
their goals.
Hopefully, it is clear that cyber criminals are financially motivated. In the end,
attacks are not promulgated for fun. Whatever the strategy, cyber criminals are earning
a living through some form of theft, be it a confidence game, stealing services and sell-
ing these at a profit, outright theft, fraud, or what have you. Although the intermediate
steps, including system objectives (which we will examine shortly), may be many, the
ultimate reward is intended to be monetary.
Although it might be said that security researchers are also interested in financial
reward, that conclusion may be a bit overly simplistic. Most people want to be well
compensated in their careers. Narrowing “reward” to simple financial gain also might
be perceived as insulting by the many hard-working researchers who see themselves as
a positive force for better and increased security in the digital world? Remember please,
we are only considering security researchers as a threat from the organization’s perspec-
tive. There is a world of difference between a cyber criminal and security researcher,
even if some of the attack methods are similar.

150 Securing Systems
Hopefully, it is safe to say that career enhancement, technical mastery, and improv-
ing security are three typical driving motivations for security researchers as a class of
cyber entity that may attack a system. Obviously, individual cyber criminals or security
researchers may not fit our stereotype.
Cyber criminals will be satisfied if they can extract financial award from the orga-
nization, its customers, or its systems. Security researchers are seeking recognition for
their technical mastery and the betterment of systems’ security postures. Although
financial reward is one likely outcome from industry recognition, there are other, softer
rewards that are just as valid. Assuming these two characterizations are more or less
working stereotypes, each of these classes of threat agents is motivated by a radically
different objective.
5.3.2 List the Typical Attack Methods of the Threat Agents
Like many legitimate business owners, cyber criminals seek to maximize profit for
effort expended. In other words, an ultimate goal of financial reward, as much money
as possible, leads directly to a corollary of limiting effort expended towards the goal:
This is basic capitalism. As a result, cyber criminals try to capitalize as much as possible
on the tools and techniques that are available.
Spending months or years creating a new method to execute a series of randomly
placed instructions in a program to create an attack simply isn’t worth the effort: The
research and development costs are too high. At the time of the writing of this book, so-
called “gadget” programs take too much time. Running gadgets culled by finding bits of
code within widely distributed applications is much more likely to fall into the category of
“stunt hack.” Cyber criminals are probably not going to waste their time unless there was
a readily available tool (even if on the black market) that would make this attack trivial.
Instead, the cyber criminal who is going to attack an organization’s system will
attempt to use as much pre-existing and proven technology, whether attack methods or
attacking tools, as possible. There is a burgeoning black market for cyber attack tools,
tools similar to the attack and penetration tool, Metasploit. Generally, the people who
make the attack tools aren’t usually the people doing the attacking. The toolmakers get
paid for the tools, whether legitimately or on the black market.
Indeed, even more telling, the person actually performing the attack may be at a
lower rung of the organization, even a technical novice recruited such that if the novice
is caught, those higher up are hidden from the authorities. The actual attackers often
have quite limited technical skills. Therefore, the attacks have to be well-known, effec-
tive, and well packaged so that they can be run like other user-friendly applications.
There is very little incentive for cyber criminals, in general, to invent a lot of technol-
ogy. Such research would entail an unpaid period of development. The need to accom-
modate a lower technical sophistication of the actual attackers, and the fact that there
is so much vulnerable software available to attack leads to the conclusion: Why waste
time on unprofitable activities?

Prepare for Assessment 151
The list of cyber criminals’ attack methods might be found within the suite of
known attack variations in any of the available security-testing tools. If the tool can
test the type of system under analysis, its suite of tests makes a good starting point for
answering the question, “What are the cyber criminal’s attack methods?”
Almost directly opposite the needs of the cyber criminal is a security researcher whose
reward can be directly tied to the technical difficulty and complexity of the attack. The
reward comes from industry recognition for the technical capabilities and acumen of the
researcher. If there are any financial rewards, these will be less directly tied to the attack
method, but rather a product of a stronger curriculum vitae and the approbation of col-
leagues. The more difficult it is to execute the attack, the larger the reward.
Since many (if not most?) security researchers are employed, a typical security
researcher has the luxury of an income. Although some security researchers’ jobs entail
finding vulnerabilities, others perform their research “on the side,” outside of work
hours, as an extracurricular activity. Hence, there isn’t much imperative for speed. The
investigation will take as long as is necessary. This means that the researcher can take
the time needed to achieve success. This spaciousness with time directly influences the
technical complexity of the methodology. Unlike the cyber criminal, looking for the
fastest payoff, vulnerability research might be seen as an investment in the skills of the
researcher, like time spent studying for an additional, higher education degree. The
“final exam” in this case will be the researcher’s proof that a vulnerability is exploitable.
The proof is often delivered as a short example program demonstrating the successful
exploitation of the vulnerability on the system under investigation.
Security researchers can and do run readily available, open source and commercial
vulnerability scanners. The analyst cannot discount the well-known attack methods.
Discovery of known classes of vulnerabilities may not have the prestige of a complex
stunt hack; nevertheless, finding these proves the security testing capabilities of the
researcher and also helps to get vulnerabilities removed from deployed software. And it
is generally better for an organization to have a relatively friendly and probably honest
researcher find a vulnerability rather than a cyber criminal who will exploit it for gain
for as long as the vulnerability continues to be exposed to the criminal’s use of it.
The set of Web-Sock-A-Rama attack methods includes that suite of standard attacks
and variations that are understood sufficiently to have been programmed into vulner-
ability scanning software. But there is also the possibility of extended research probing
for more complex issues of a one-of-a-kind nature within the web store software.
5.3.3 List the System-Level Objectives of Threat Agents Using
Their Attack Methods
In order to properly defend the attack surfaces, one must understand the intermediate,
cyber, digital, or system objectives of the attack methods. For instance, looking once
more at XSS attacks, we know that the ultimate objective of a cyber criminal is to extract
money from the victim. Of course, there are a multitude of ways that an XSS can be

152 Securing Systems
used to do that, from fake product sales (illegal or pirated pharmaceuticals) to theft of
personal information like payment card numbers, account information, or an entire
identity. But how does the attacker get from the XSS vulnerability to the objective?
For many exploits there is a system-level objective that must be attained in order to
prosecute the attack to its goal. For security researchers, the system-level objective—
that is, getting a piece of scripting code (javascript) to execute in the user’s browser, or
gaining system-level privileges—will be sufficient to prove vulnerability. No more need
be accomplished.
But for other attackers, the system objective is the steppingstone to an ultimate goal,
whatever that goal may be. System-level privileges allow the attacker to “own,” that is,
completely control the attacked machine. From there, all information on the machine
can be stolen. With superuser privileges, the attacker can install software that listens
to and records every entry on the machine. If a spy, the attacker could turn on the
machine’s video camera and microphone, thus eaves dropping on conversations had
within the vicinity of the machine. And, of course, an owned machine can be used to
stage further attacks against other machines or send spam email. Essentially, a com-
pletely compromised machine can be used for the malicious and abusive purposes of
the attacker. Hence, the term “owned.”
System-level objectives are tied closely to attack methods. Table 5.3 is not intended
to be exhaustive. There are plenty of more extensive lists elsewhere, the most complete
probably being CAPEC™* at or the lists of attack methods at
Nevertheless, we are studying the AR A/threat modeling process in this chapter. The
following is offered as an example of a technique for understanding how the prioritized
threats are most likely to misuse attack surfaces.
The first three entries in Table 5.3 are purposely difficult enough to execute that
these would not be a consideration for most well-managed websites. Unless there is
a serious, unauthenticated, remotely executable vulnerability available via an HTTP
request or message into a vulnerable application server or Web server, all the other inter-
faces should be protected in such a way that getting sufficient privileges to perform one
of these first three attacks should be extremely difficult. In order to highlight security
researcher “stunt hacks,” the first three entries specifically require high privileges or
wide access, or both.
The subsequent entries in Table 5.3 are drawn from Top 10.† In order to
gain a place in the list, an attack method has to be one of the most popularly executed as
* Common Attack Pattern Enumeration and Classifi cation. CAPEC is a registered trademark
of the Mitre Corporation.
† Not all of the top 10 attacks are listed. Table 5.3 is just an example to demonstrate the process
of understanding the system-level objectives of the likely threat agents. Some of the Top 10
List are vulnerabilities, thus not detailing an attack method. Table 5.3 is limited to technical
attack methods only. CAPEC, maintained by the Mitre Corporation, details most known
attack methods.

Prepare for Assessment 153
Table 5.3 System-Level Attack Objectives
Specifi c Attack System Objective(s) Threat Agent
String code “gadgets” together
into a meaningful sequence that
escalates privileges
• User installed (and accepted) application
code running attacker’s program without
having to install an executable on the
attacked machine
• Call vulnerable system code from within an
installed application and exploit to escalate
privilege to system or superuser
Security researchers
Bypass the no-execute page
protection policy to execute code
Execute code of the attacker’s choosing within
the context of the currently logged user and a
running application
Security researchers
Use a system debugger to exploit a
buried overfl ow condition
Prove that an overfl ow condition not reachable
through inputs can execute code of the
attacker’s choosing
Security researchers
SQL and LDAP injection • execute unintended commands
• access data without proper authorization
Cyber criminals
Cross-Site Scripting (XSS) • execute scripts in the victim’s browser
• hijack user sessions
• deface web sites
• redirect the user to malicious sites.
Cyber criminals
(exposed) Direct Object References manipulate . . . references to access
unauthorized data
Cyber criminals
Cross-Site Request Forgery CSRF) • force a logged-on victim’s browser to send
a forged HTTP request
• generate requests . . . [that appear to be]
. . . legitimate requests from the victim
Cyber criminals
Unvalidated Redirects and Forwards • redirect and forward users to other pages
and websites
• use untrusted data to determine the
destination pages
• redirect victims to phishing or malware sites
• use forwards to access unauthorized pages
Cyber criminals

SQL = Structured Query Language; LDAP = Lightweight Directory Access Protocol.
Source: Data set in italics is from the Open Web Application Security Project (OWASP) (2013). OWASP Top 10 List.3
well as used on a regular and continuing basis. When we analyzed cyber criminals, we
noted their predilection for well-known and proven attack methods. The OWASP Top
10 list is representative of the most often used attacks on the Internet. Since the Web-
Sock-A-Rama site is a typical web store, it will be subjected to attacks drawn from the
OWASP Top 10 list, at the very least. Security researchers will also attempt well-known
attack methods in order to find and report vulnerabilities.
5.4 Attack Surfaces
The attack methods and their system-level objectives listed above are targeted against
“something.” That set of “somethings” are the attack surfaces of the architecture. In

154 Securing Systems
most systems—that is, examining most architectures—some attack surfaces will be
more exposed than others. Ultimately, the AR A and threat modeling process should
prioritize the exposed attack surfaces.
In the ATASM process, we try to enumerate all the attack surfaces before we catego-
rize the importance of each surface. It’s useful to avoid the prioritization process during
enumeration. Think of it as an attack surface brainstorm: Any discussion about the
legitimacy or fitness of an attack surface tends to bog down the enumeration into too
many details. In this way, the natural flow resulting from nonjudgmental observation
is broken. It’s easy to miss an attack surface, especially when it seems inconsequential,
or perhaps that function just seems to be “part of the running program.” For that very
reason, getting a good flow of nonjudgmental inspection can help to uncover all the
attack surfaces. Prioritization can come later, once everything has been uncovered. In
the process being described in this chapter, we simply enumerate all the attack surfaces
and try to avoid discussion of their importance and exposure until a later step.
In order to find the attack surfaces, we first must break down the architecture suf-
ficiently to expose them. If you’re not comfortable with the architecture decomposition
and factoring process, review Chapter 3, in which these processes are explained in detail.
5.4.1 Decompose (factor) the Architecture to a Level That
Exposes Every Possible Attack Surface
When hunting for attack surfaces, it’s important to understand the trust boundaries that
exist between components in the architecture. Figure 5.5 adds three dashed-line divisions
to the AppMaker-based, Web-Sock-A-Rama online store. The three lines repre sent one
possible set of boundaries, based upon industry-standard, three-tier web architecture.
However, many web sites do not employ all of these divisions. Some might only be
concerned with the boundary between the Internet and the web server. In this case,
the web server, application server, and database server would all exist in the same trust
segment. Another often-used web architecture separates Internet and then database into
separate spaces, leaving web server and application server at the same trust level. Indeed,
many of the leading application servers include a web server that may be used instead
of a separate and distinct web server instance. An integrated web server collapses the
web tier and application server tier into a two-tiered architecture. Following the secu-
rity principles, defense-in-depth and fail securely, the three-tier architecture is usually
considered a more defensible approach. Web-Sock-A-Rama, as depicted in Figure 5.5,
demonstrates the more standard three-tier web architecture.
However the fictitious designers of our web site might have considered trust within
this AppMaker implementation for Web-Sock-A-Rama, a strong case may be made that
the Internet is inherently dangerous, supporting the boundary between the web server
and the Internet. Further, since the web server is a “terminus” and is open to all traf-
fic (in order to sell as many socks as possible to the widest possible Internet populace),

Prepare for Assessment 155
the web server should be considered a “bastion” or resource that can be lost without
losing the entire site. It is on a demilitarized zone (DMZ) to which known bad traffic
will have access. Placing a boundary between the web server and the application logic
(application server) will offer an opportunity to protect the application code from at
least some types of attack that will be stopped at the bastion web server, and not passed
onto subsequent tiers.
If the data in the databases were lost or compromised, Web-Sock-A-Rama would
not be able to function, revenue could not be generated (or lost through fraud), and
the loss of customer financial data might engender legal consequences. In short, the
loss of data obtained through the database server would impact the functioning of this
business, and might even cause the organization to fail. Further, this data and the infra-
structure that serves it must be trustworthy. The operation of the web site assumes that
the data are correct. If you follow this logic, you may see that the data layer will be the
highest trust area in the architecture.
Some web architectures might consider both the application code and the data to be
high trust. If compromised, the application code could have profound consequences for
the web store. The reason for putting the application server in a different trust segment
Figure 5.5 Web-Sock-A-Rama trust boundaries.

156 Securing Systems
is based upon the fact that it must handle messages from untrustworthy sources. Even
though web traffic (HTTP) is terminated at the web server, the dynamic requests must
be processed by code running in the application server. Messages that possibly contain
attacks will get passed through the web server and get processed by web application
code running in the application server. For this reason, I would consider the probability
that the application server is under attack, thus placing another trust boundary between
code and data.
Take a look at Figure 5.5 again. Has the architecture been factored into all the func-
tions and components necessary to uncover all the attack surfaces?
Figure 5.6 adds an arrow to Figure 5.5 that points at the most obvious attack sur-
face: the Web server’s Internet facing HTTP receiver. The Web server is open to all
Internet traffic, which means that any attacker can probe it. The constant “doorknob
rattling” sweeps of the Internet will surely find and investigate whatever interfaces are
open and available to unrestricted traffic.
Once an interested attacker finds and catalogs the open HTTP port, then the fun
really begins. Like web vulnerability scanners, the attacker will probe every reachable
Figure 5.6 An obvious attack surface.

Prepare for Assessment 157
page with every attack variation possible. These probes (and possibly downloads of por-
tions of the site) will be unrelenting. To prevent restriction of the attacker’s address, she
or he may use multiple addresses or even shift attacking addresses on-the-fly (e.g., fast
flux DNS).
But the attacker won’t just try attacks at the network or HTTP protocol level. Probes
will attempt to catalog every vulnerability of any interest to the attacker. Since cyber
criminals sometimes specialize in particular attack methods, one attacker will search for
XSS errors, to chase Web-Sock-A-Rama’s customers, whereas another attacker might
target customer data and payment cards. A third attacker might probe for hosts that
can be compromised to add to a botnet or from which to stage attacks either inbound
into Web-Sock-A-Rama’s internal networks or outbound at other organizations. The
possibilities are many.
For every one of the above cases, and for security researchers as well, the web server
that is reachable from the Internet will be the first attack surface to examine.
Please study Figure 5.4. What other attack surfaces can you find? We will complete
this investigation in the next chapter.
Table 5.4 adds another column to Table 5.3. To which attack surface can each attack
that we’ve uncovered be applied? Assuming that there is only one HTTP/S interface
presented to the public Internet, there are only a very limited number of vectors through
which the first three, security researcher–promulgated attack methods might possibly
succeed. A failure would have to connect a number of fairly unlikely concurrent vulner-
abilities: a hole in the HTTP request processing of the Java application server allowing
the exercise of a vulnerability in the Java runtime environment (JRE) followed up by
exercisable vulnerability in system services or the operating system itself. This combi-
nation of events is not impossible; it has been seen a number of times even within the
last year or two of this writing. This is what a properly managed website administrative
crew must attend to. Streams of announcements of new vulnerabilities for the technol-
ogy that’s been deployed have to be watched on a continual basis. A mature operations
practice includes regular patch cycles to plug new vulnerabilities and an emergency
process for serious vulnerabilities that need immediate attention.
Just for the sake of this investigation, let’s assume that our fictitious Web sock com-
pany has a mature web master administration practice. The Java application server is
patched for the most serious Java and application server vulnerabilities; its security pos-
ture is up-to-date. If this is true, and all other interfaces to the Web server and applica-
tion server are sufficiently protected (we’ll get to this in the next chapter), then I think
we can discount the security researcher stunt hacks because they do not have access.
There is no exposure, and perhaps no vulnerability?
This leaves the remaining five attack methods from the list in Table 5.4 to apply
against our single exposed interface, the attack surface. If you scan through Table 5.4,
column 3, you will see that the entries are not the same. Not every attack can be
applied in the same way against the same targets, even in this limited set of threat
agents and attacks.

158 Securing Systems
The injections for downstream destinations, such as LDAP and databases, are aimed
at the application server code because that’s where the capability to pass on these attacks
is typically found. These are then attacks against the application code.
Contrast these injections with a Cross-Site Request Forgery (CSRF) attack. These
attacks can be mitigated at several different layers, depending upon how session
Table 5.4 Attack Methods Applied to Attack Surfaces
Specifi c Attack System Objective(s) Attack Surface Threat Agent
String code “gadgets”
together into a meaningful
sequence that escalates
• User installed (and accepted)
application code running
attacker’s program without having
to install an executable on the
attacked machine
• Call vulnerable system code from
within an installed application and
exploit to escalate privilege to
system or superuser
None available Security
Bypass the CPU’s no-execute
page protection policy to
execute code
Execute code of the attacker’s
choosing within the context of the
currently logged user and a running
None available Security
Use a system debugger to
exploit a buried overfl ow
Prove that an overfl ow condition not
reachable through inputs can execute
code of the attacker’s choosing
None available Security
SQL and LDAP injection • execute unintended commands
• access data without proper
Applications via
Cyber criminals
Cross-Site Scripting (XSS) • execute scripts in the victim’s
• hijack user sessions
• deface web sites
• redirect the user to malicious sites
Web Server and
applications via
Cyber criminals
(exposed) Direct Object
manipulate . . . references to access
unauthorized data
HTTP responses
from applications
and application
Cyber criminals
Cross-Site Request Forgery
• force a logged-on victim’s browser
to send a forged HTTP request
• generate requests . . . [that appear
to be] . . . legitimate requests from
the victim
Application server
via HTTP (in this
case, AppMaker)
Cyber criminals
Unvalidated Redirects and
• redirect and forward users to
other pages and websites
• use untrusted data to determine
the destination pages
• redirect victims to phishing or
malware sites
• use forwards to access
unauthorized pages
Web server and
applications via
HTTP. Possibly
also the
application server
Cyber criminals
SQL = Structured Query Language; LDAP = Lightweight Directory Access Protocol.
Source: Data set in italics is from the Open Web Application Security Project (OWASP) (2013). OWASP Top 10

Prepare for Assessment 159
management is implemented in the website. Although many would consider this a
vulnerability of the custom application code, depending upon how the infrastructure
is set up, it may be that the application server handles user and transaction states. If so,
then a CSRF vulnerability would lie in front of the application code. Another possibil-
ity might be the Web server handling session management, or even a specialized piece
of hardware running in front of the Web server? Without these additional details, and
without knowing the specifics of authentication, authorization, and user state man-
agement (against which CSRF attacks), it’s hard to put a finger on the precise attack
surface in the technology stack. For the moment, let’s place this attack against the appli-
cation server via the HTTP requests that are passed from the Web server. A common
CSRF treatment is to add a nonce to every request so as to tie the user and the user’s
current expected state. One way to generate the nonce is to implement an outbound
filter at the application server that lies in front of all applications, rather than having
every application developer code a nonce into each individual responder. Alternatively,
a common library might be distributed to every developer such that including a nonce
is a trivial set of calls to the library. Since this application was built with the AppMaker
software, let’s assume that the creation of a nonce is the responsibility of AppMaker. In
this instance, the CSRF attack will then will be aimed at AppMaker’s code.
The exposure of inappropriate data and objects will generally be the fault of the
application code. That’s assuming a well-debugged Java application server. However,
the application server might expose its objects and references, as well. All the code that
is involved in building a response—from special, local logic that is programmed in the
application, to the code in AppMaker (which builds the application for the developer),
through the application server—can cause this mistake. The attacker almost gets this
compromise for free, since not much of an attack, beyond taking a look at the HTTP
responses, is required. Have you ever gone to a website, caused a website error, and
received as a response a page full of what appears to be code? That’s what this attack is; if
the website makes this error, the attack is trivial. The attacker need do nothing but reap
the information gained. It should be noted that static content and a web server can also
have this fault, delivering coding information to an attacker unintentionally. This attack
applies to every part of the front end and middle tier of the website we are examining.
To complete this step of the process, we find that every type of attack, at least at
this gross level, should be applied to every attack surface that is relevant. As attacks are
applied it should be obvious which are irrelevant for any particular attack surface. In
addition, some attacks will start to fall off, whereas others will gain in importance.
5.4.2 Filter Out Threat Agents Who Have No Attack Surfaces
Exposed to Their Typical Methods
Through the process of filling out Table 5.4, we have already filtered out a series of
attacks that do not have an appropriate attack surface exposed to them. In fact, by
attempting to apply attack methods to attack surfaces, we have even eliminated, or

160 Securing Systems
significantly reduced, a threat agent: security researchers. Security researchers’ stunt
level hacks are unlikely since the attack surfaces required to execute these attacks are
not exposed. To simplify the process in order to understand it, we didn’t list security
researchers in any of the subsequent attack methods in Table 5.4. Nonetheless, security
researchers can and will attempt all the other attack methods we have listed. Unless
we do something very insecure on this website, stunt hacks on the website are not very
likely to occur.
We will continue the filtering process in the next step.
5.4.3 List All Existing Security Controls for Each Attack Surface
The reason that the ATASM process encourages delaying the assessment of existing
controls is to ferret out all of the attack surfaces. Discussions start to get bogged down
in rating the sufficiency of existing controls. Because of this tendency, this particular
approach to the threat modeling process urges us to wait until we have all the attack
surfaces and all the attack methods catalogued before applying existing controls in
order to prioritize. The essential problem is that we can never be 100% sure that we
are sufficiently defended. Furthermore, we have to assume that some of our defenses
may fail under sophisticated attack. It comes down to risk: There are no sureties. Risk
decisions have to be made in order to account for limited resources to apply to the
defense. Those discussions are best had at a later stage of analysis, once discovery has
been completed.
In Table 5.4, the existing security controls are not shown. We have made a couple
of assumptions for the sake of understanding the process. Namely, we’ve assumed that
Web-Sock-A-Rama has a mature website management and administrative function. In
previous chapters, we’ve listed what those functions typically are. This assumption pro-
vides the usual set of controls to the backend of our website. In the real world, I would
never make that assumption without investigating first. An AR A has a due diligence
responsibility to tie every loose end down, to dot every “i” and cross every “t.” The
upside is that one usually only has to investigate an infrastructure once to understand
its strengths and weaknesses. After that investigation, the infrastructure knowledge
can be applied to each system that will be deployed to the infrastructure, instance
after instance. The foregoing, of course, echoes and underlines the importance of pre-
assessment knowledge, such as understanding the intended deployment infrastructure.
We do not know if some AppMaker applications or some Web-Sock-A-Rama pages
require authentication. We don’t know if some transactions require authorization. We
don’t know where the firewalls are, if any. We don’t know what actions are monitored,
or even if there’s a security team who can monitor them (although a mature administra-
tive function would imply security monitoring for attacks).
Because we don’t know what security controls exist, we could assume that there are
none. If that were so, then we’d be ready to start writing security requirements. We
would skip the next step in our process.

Prepare for Assessment 161
However, this chapter is devoted to learning the process. So, just for the sake of
completing the process fully in every one of its steps, let’s assume that there is a single
firewall between the public Internet and HTTP termination. Let’s also assume that
there’s an authentication mechanism for certain operations, like changing a customer’s
profile. Assume that each individual customer is authorized to change only her or his
record and none others. Let us assume, as well, that there are no other controls existing.
5.4.4 Filter Out All Attack Surfaces for Which There Is
Suffi cient Existing Protection
The difficulty in this step is the word “sufficient.” That will be a risk decision, as
described in Chapter 4. There aren’t easy answers to the question of “sufficiency,”
because the security needs of organizations and security postures vary. For one sys-
tem at a high-security-needs organization, “sufficient” may mean layers of protections:
network firewall, web application firewall, vetted input validation library, object-level
authorization model, inter-tier encryption of communications, inter-tier bidirectional
authentication at the function level, DB rights management, and so forth. All the
defenses taken together sum to “sufficiency.” The foregoing list would be an “industry
standard” web security list. However, this list would probably be too expensive for a
small shop to implement. In addition, if that small shop’s crown jewels are all public,
the industry standard list is overkill. Probably good patching and hardening practices,
network restriction, and strict input validation are sufficient in this case? The required
security posture tends to be somewhat unique, at least to classes of systems and organi-
zations, if not to individual organizations.
Still, let’s use industry standards as our basic guide in the absence of organizational
policy and standards.
We know that our infrastructure practices are mature. We also have restrictions in
place to prevent any but intended HTTP/S traffic into Web-Sock-A-Rama. And we
have specified that there is an up-to-date firewall in place in front of our exposed inter-
face. Which of the attacks indicated in Table 5.4 are eliminated, given these controls?
If you said “none that we haven’t already eliminated in previous steps” you would be
correct. No control we have named will stop any of the five listed attack methods. In
fact, authentication (and possibly authorization) increases the value of CSRF attacks.
The attacker may masquerade as an authenticated user via a successful CSRF.
Authentication is an interesting security control because, although it does reduce
exposure to only those who’ve been authenticated, there are plenty of techniques an
attacker can use to get an account, depending upon the site: steal or crack weak creden-
tials, or perhaps simply sign up for the free version. In addition, once a site or sys-
tem implements authentication, it now has another system that will need protection!
Authentication systems are often considered critical, thus requiring tight security con-
trol: a high security posture. Implementing authentication adds significant complexity
to the security requirements of the system.

162 Securing Systems






























Prepare for Assessment 163
No attack surface that we have listed has been reduced significantly or defended
“sufficiently” from the known existing controls. Therefore, our list of attacks and attack
surfaces remains the same after this step.
We now have our set of credible attack vectors (CAV). Table 5.5 removes the attacks
and attack surfaces that have been deprioritized. Remaining is a CAV list for the website.
5.5 Data Sensitivity
Please consider Table 5.3 once again. Do any of the general data types named in the
diagram indicate the data’s sensitivity rating?
• Customer financial data
• Customer records and profile
• Product catalog
• AppMaker application metadata
• AppMaker configuration
• Application server configuration
• Web server configuration
Which of the foregoing seems the most sensitive to Web-Sock-A-Rama’s continu-
ing success?
I would pick “customer financial data.” Keeping financial data is particularly dif-
ficult if the site retains payment card information.* Loss of payment cards typically
involves impact to the public image, loss of customer confidence, expensive customer
relationship remediations, breach notifications, and, perhaps, lawsuits.
But I wouldn’t stop with financial data. What if an attacker can change the con-
figuration of the underlying web components? How about fiddling with AppMaker’s
metadata to create new, attacker-controlled applications?
The product catalog is public. But the company probably doesn’t want anyone to
change the sock prices to the attacker’s advantage, nor mess with the catalog informa-
tion, thereby hurting the ability to sell socks, or possibly embarrassing the company.
Using a three-step rating system—high sensitivity, medium sensitivity, or public—
where would you place each of the above data types?
It should be noted that a real web store would likely have a number of other data
types not shown. We’ll ignore these for our process example.
To reiterate why data classification is important, higher-sensitivity data poses a higher
attack value target. In order to assess the risk of each attack, we have to understand the
* I would try to design the site in such a way as to remove the need to store payment card
information. Payment cards are very hard to secure correctly. Note the huge breaches that
continue to take place on a regular basis.

164 Securing Systems
impact. And in order to prioritize the credible attack vectors, we have to know where
the important data lies.
Data classification is a key component of risk, both for assessing impact and for
understanding likely targets in a system. As has been explained earlier, attackers often
string more than one attack method together in order to achieve their objectives. An
attacker might execute an SQL injection against the payment card database, thereby
gaining financial information, if applications accepting user input were poorly written
such that SQL injection to databases were allowed. But accepting user SQL statements
(or partial statements) to the financial database would be an exceedingly poor design
choice. Assuming that there is no direct SQL path from customer to the Web-Sock-
A-Rama databases, a potential attacker would have to first gain access to the database
server or the inner network before being able to attack the data. That poses at least a two-
step set of successful compromises. Even so, we know that payment card data is highly
sought after by Internet attackers. Knowing this gives the defender a better understand-
ing of the CAVs that might get employed and, thus, the defenses that will prevent such
a combination of attack scenarios. Data classification assists in gauging the impact term
and CAVs. That is, it is useful to both risk terms—probability (CAV) and loss (impact).
Data sensitivity musn’t become the sole measure of CAV or impact. Each of these
risk terms are multidimensional; terms shouldn’t be reduced in a simplistic way or
assessments will suffer by possibly excluding other key factors. As we’ve seen, there are
system-level objectives, such as executing a script or code, escalating privileges, or caus-
ing data to dump, which must be factored into risk, as well. Calculation of information
security risk doesn’t reduce to a single variable and shouldn’t be reduced to one. The
approach explained in the chapter on risk reduces calculation to a few terms, some of
which can be considered in a more or less binary fashion. That amount of reduction
is as much as I think practical. My approximating approach already incorporates con-
siderable condensation. I do not recommend attempting to simplify even further, say,
down to data sensitivity alone.
5.6 A Few Additional Thoughts on Risk
Throughout the foregoing process tour and explanation, I’ve used the work “prioritize”
instead of risk. But risk is the principle underlying prioritization as this term has been
used in the explanations given above. The goal of risk calculation during ATASM is
to identify those CAVs whose lack of mitigation will prevent reaching the intended
security posture. Concomitantly, there is an imperative to limit the number of risk
treatments to that which is affordable, that which is doable. Few organizations can “do
it all.” Hence, there has to be a process to eliminate unlikely attacks while prioritizing
the most dangerous ones for mitigation.
Risk calculation is taking place throughout ATASM. Assessing risk is built into the
steps of the process. An assessor doesn’t have to stop and perform an extra calculation,

Prepare for Assessment 165
so long as the filtering that is built into the steps takes place. Once the list of CAVs has
been enumerated, it may be a good time to then prioritize these, if necessary. Depending
upon the organization and the system, it may make sense to treat every CAV in order
to build a strong defense. Or it may make business or resource sense to further rate each
CAV to produce implementation priorities. The answer to this question is dependent
upon the situation.
5.7 Possible Controls
The final steps in the ATASM process build the set of security requirements to achieve
the desired security posture. This is the “requirements” phase spoken of earlier. We
applied high-level attack methods to each attack surface. It is precisely these attack
methods that will suggest the security controls. As has been mentioned several times
above, we cannot expect a 1-to-1 mapping of control, of mitigation to attack, though of
course, some mitigations will be 1-to-1.
For instance, we’ve already applied the existing authentication to reduce the attack
surface at the application layer. As we explored, adding authentication does provide
some control, but it also adds a raft of additional attack surfaces. It’s not a panacea;
authentication is a piece of a puzzle. Further and importantly, there is the context of
this particular authentication; what does it actually add to the defense for Web-Sock-
A-Rama? If the store expects to have a large customer base, as big as possible, similar
to services such as Facebook™ or Myspace™, identity has to be very inclusive. In these
cases, identity is based solely upon possession of a viable email address, and free email
addresses are as readily available to attackers as to you, the reader, (not to mention
stolen and hacked email accounts). That is, possessing an email address that can send
and receive email proves nothing. Think of all the pets who have Facebook accounts.
I personally do not know a single dog or cat who actually reads or posts to a Facebook
account. Facebook accounts, despite what I believe to be some “best efforts” on the part
of the Facebook security team, are easily opened by attackers, just as well as pets, that
is, fictitious identities.
In the case of a web store such as we’re using for our example, at purchase time, a
credit card or other payment option is required. However, several years ago, in a United
States Federal Bureau of Investigation (FBI) presentation, it was mentioned that stolen
credit card numbers were 25 cents each on the black market. Apparently, one buys the
numbers in lots of hundreds or thousands.
Due to the realities of email impersonation and payment card theft, an authentica-
tion based upon these factors isn’t really worth much, which leaves our Web-Sock-A-
Rama website without much attack mitigation based upon authentication. Obviously,
the organization’s security team will have their hands full with fraud attempts, oper-
ationally removing fraudulent and fake accounts as soon as malicious behavior is
observed. Still, at account initialization, there isn’t much that can be done. One email

166 Securing Systems
address looks an awful lot like another; as long as the email server opens a connection,
the address is going to verify as active. The foregoing should bring the security person
to the conclusion that attackers will have accounts and will bypass authentication.
For a web store, the authentication is as much about business as it is about security.
This is true because a store wants to give each customer as personalized an experience as
possible. Please consider the following data in Figure 5.4: “customer profiles and pref-
erences.” The store retains product browsing history, purchases, interests, and perhaps
even reviews and other ancillary data. With this data, the store can offer each customer
items of possible interest, thus increasing sales and customer retention. In addition,
publishing customer reviews gives customers a sense of participation, not only with the
store but also with other customers. Allowing one customer to change another’s prefer-
ences or profile would be a breach of customer trust. Once authenticated, customers
must be only authorized to access their own data, not the data of any other customer.
To be clear, the authentication in front of the store provides little security control.
Additional controls are going to be required.
5.7.1 Apply New Security Controls to the Set of Attack Services
for Which There Isn’t Suffi cient Mitigation
We’ve already examined cross-site request forgery protection: An unpredictable nonce
will have to be sent with each authenticated and authorized page. That is the recom-
mended treatment; in this case, attack to mitigation is 1-to-1.
Table 5.6 adds defenses to each of the attack methods that we enumerated in previ-
ous ATASM steps. These are the five attacks and their associated attack surfaces (the
CAVs) that we believe are applicable to this system, in this organizational context.
Table 5.5 summarized attack surfaces with applied attack methods.
(Continued on following page)
Table 5.6 Attacks and Controls
Specifi c Attack Attack Surface System Objective(s) Control
Applications via
• execute unintended
• access data without
proper authorization
• Design application code such that
dynamic requests to LDAP and to
databases are built within the
application and not received from
• Dynamic input from users, such as
user name or item numbers, must be
validated to contain only expected
• Input not matching precisely
constrained values must return an
error to the user

Prepare for Assessment 167
Specifi c Attack Attack Surface System Objective(s) Control
Scripting (XSS)
Web server and
applications via
• execute scripts in
the victim’s browser
• hijack user sessions
• deface web sites
• redirect the user to
malicious sites
• Dynamic input from users, such as
user name or item numbers, must
be validated to contain only
expected characters
• Input not matching precisely
constrained values must return an
error to the user
• Response generation must clear all
scripting tags found within stored
(exposed) Direct
Object References
HTTP responses
from applications
and application
manipulate . . .
references to access
unauthorized data
• Do not use object revealing
protocols similar to Java Remote
Method Invocation (RMI) in
communications with the user’s
• Remove all debugging and coding
information from errors and other
user content. Errors shown to user
must be user-centric, in plain
(nontechnical) language
• Do not expose any debugging,
confi guration, or administrative
interfaces over customer/public
• Use per user or session indirect
object references
• Authorize all direct object accesses
Cross-Site Request
Forgery (CSRF)
Application server
via HTTP (in this
case, AppMaker)
• force a logged-on
victim’s browser to
send a forged HTTP
• generate requests . .
. [that appear to be]
. . . legitimate
requests from the
Include a nonpredictable nonce in the
response to a successful user
authentication. Return the nonce for
the session with every authenticated
response in a hidden fi eld. Before
processing an authenticated request,
validate the nonce from the user’s
Redirects and
Web server and
applications via
Possibly also the
application server
• redirect and forward
users to other pages
and websites
• use untrusted data
to determine the
destination pages
• redirect victims to
phishing or malware
• use forwards to
access unauthorized
• Simply avoid using redirects and
• If used, don’t involve user
parameters in calculating the
destination. This can usually be
• If destination parameters can’t be
avoided, ensure that the supplied
value is valid, and authorized for the
• Employ an indirect reference to
URLs between client and server
rather than sending the actual value
to the user
SQL = Structured Query Language; LDAP = Lightweight Directory Access Protocol.
Source: Data set in italics is from the Open Web Application Security Project (OWASP) (2013). OWASP Top 10 List.6
Table 5.6 Attacks and Controls (Continued)

168 Securing Systems
Some of the defenses are programming requirements. For instance, rewriting errors
that will be returned to users in plain language easily understood by users and not con-
taining any programming information is something that will have to be programmed
into the applications. Likewise, input validation, which is required to prevent the two
injection errors, must be done within code.
Contrast the coding defenses with not using an object referencing or serialization
protocol (RMI) to prevent direct code references from escaping to the user. This, like
building nonces into responses or using no redirects or forwards, is a design issue. The
applications making up the Web-Sock-A-Rama store will have to be designed such that
redirects are unnecessary, such that appropriate protocols are used for any code that
runs in the user’s browser.
Importantly, if you scan through the defenses listed in Table 5.6, you may notice
that the usual panoply of security controls don’t appear as treatments. Firewalls aren’t
listed. Intrusion prevention systems are not listed. The treatments given are specific to
the attacks. That doesn’t mean that firewalls shouldn’t be deployed. They should be! In
this step, we are hunting for those security controls or defenses that will interrupt each
CAV such that we can significantly lower the probability of success for attackers (or
eliminate it, if possible). For this reason, defenses tend to be specific and to the point.
In the next step, we will build the entire security posture as a defense-in-depth.
5.7.2 Build a Defense-in-Depth
In reality, there are many more attacks that will be received from the public Internet.
Because we are exploring the ATASM process, the list of attacks has been truncated to
keep the discussion manageable. Network-based attacks, such as distributed denial of
service (DDoS), have not been discussed. However, for a real site, these and many other
attacks must be defended, as well.
For example, the networking equipment that routes traffic into the site and between
its various layers would all be subject to attack. We have not examined the physical,
network architecture. But in a real analysis, we would examine everything from top to
bottom. Such an examination would, of course, include the networking equipment,
the operating systems of the servers running AppMaker, the database servers, the Java
application server, and so on.
Instead, we’ve assumed, for the sake of instruction, that the site is already supported
by a trusted and capable administrative and networking staff. That’s a big assumption.
As was noted above, due diligence requires that we make no assumptions and always
actually confirm or deny practices and technologies.
In order to build a defense-in-depth, an assessment would take into account all of
the defenses, including firewalls, intrusion prevention systems, anti-malware software
on the servers, network black hole route capabilities (for network and DDoS attacks),
and so forth.

Prepare for Assessment 169
As mentioned above, I would divide the architecture into three tiers. I would require
network restrictions between each layer, allowing only intended traffic from known
hosts/subnets over specific protocols to the intended recipients, and no other traffic,
especially inbound from the Internet. I would probably also specify an authentication of
the web server(s) to the application server to be validated at communication initializa-
tion, so that if the bastion network were lost, an attacker would also have to gain access
to the web server in order to send traffic to the application server.
Because the system handles sensitive data from users originating from the untrusted
Internet, the Transport Layer Security (TLS), most often implemented as HTTPS,
must be used between the user’s browser and the web server, at least for sensitive data
transmission. Some architectures would also require TLS (or more properly, communi-
cation encryption of some sort) between the web server and the application server,
and perhaps even between the application server and the database server? We assumed
that the networks in this system are tightly controlled through a mature administra-
tive practice. Therefore, the networks can be considered a trusted infrastructure. Since
we’ve separated out three tiers, and there is only a single piece of trusted networking
equipment separating each tier (a new assumption and requirement), the store doesn’t
feel the need to encrypt the data as it moves between tiers. There are existing protec-
tions, and the data are not moving between multiple network segments while pass-
ing between tiers.* Without this assumption of trust, communications-level encryption
would, very likely, be a requirement between tiers, as well.
Furthermore, because secure coding is an art as much as engineering, I would try
to get a web application firewall (WAF) placed between the Internet facing network
equipment and the web server (or it could go between the web server and the applica-
tion server, as well). A WAF will detect known and typical attack patterns and pre-
vent these from being passed to the application server and the application server code.
Programmers make mistakes; vulnerabilities do escape even the most rigorous secure
development lifecycles. It’s a defense-in-depth to anticipate this by removing known
attacks, thus protecting those mistakes from being exercised. In addition, most WAF
products allow custom rules, so that if a vulnerability is discovered, attacks directed at
that specific vulnerability can be removed at the WAF before they can exploit the error
during the period of remediation, which can often take some time.
I would assume that at least those controls that have been placed at the Internet-
facing edge can and might get compromised. I assume that programming errors will
escape into production. Then I design requirements to supplant and enhance the
expected controls such that the failure of any single control does not mean the compro-
mise of the entire system, and certainly protect crown jewel data and systems such as
identity and financial data with additional, overlapping defenses.
* Th e PCI standard insists on the encryption of payment information in transit. So, while I
believe the architecture as described suffi ciently protects the data, compliance with PCI may
require encryption between layers. Compliance and security are sometimes not the same thing.

170 Securing Systems
5.8 Summary
In this chapter we walked through a process of architecture risk assessment (AR A) and
threat modeling that begins with architecture, uses the concept of a credible attack
vector (CAV) to identify attack types and attack surfaces, and then applies security
controls, or “mitigations,” to build a defense-in-depth. As a mnemonic, we call this
ATASM: “architecture, threat, attack surface, mitigation.” Each of these steps contains
a series of sub-steps that when executed produce:
• A thorough understanding of the architecture from a security perspective
• A list of credible threats
• The set of likely attack methods
• The list of attack surfaces
• A set of security requirements that is specific to this system and its organization’s
The attack surfaces and CAV can be considered the “threat model” of the system.
However, as we found going through the process, we must start with the architecture
and the results of a set of investigations that we bring to the analysis.
If possible, an AR A benefits from understanding the “3 S’s”: the strategy for the
system, the structures that will support it, and the specifications of the underlying
• Threat landscape
• Intended risk posture
• Existing and possible security controls
• Any existing security and infrastructure limitations
• Data-sensitivity classification
• Runtime and execution environments
• Deployment models
With this knowledge set in mind, the architecture is decomposed into attackable
components and factored to reveal the defensible boundaries. Architecture decomposi-
tion and factoring have been discussed at some length in this chapter and in Chapter 3.
The unit to use for atomicity, the granularity at which to decompose, is highly context
Moving from ultimate attack objectives to the system-level goals of specific attack
methods, threats are analyzed and then the relevant ones are enumerated into a list.
Those threats’ attack methods, now qualified for the system under consideration, are
applied to the attack surfaces of the architecture to generate a set of CAVs.
Defenses are applied such that these specifically interrupt each CAV, as was dis-
cussed in the chapter on risk. Then, the entire set of defenses is considered as a set

Prepare for Assessment 171
of overlapping, interlocking, and supporting defenses to build enough redundancy to
create a defense-in-depth. The security requirements should be achievable, relevant,
and “real world.”
ATASM has been presented as a series of linear steps. However, in practice, an
assessment might proceed to requirements and uncover a previously unknown part
of the system, thus returning to the architecture stage of the process. Ultimately, the
goal of the AR A and threat model is to achieve a unity between security posture and
intended risk tolerance, to achieve balance between defenses and resource limitations.
1. Rosenquist, M. (Dec. 2009). Prioritizing Information Security Risks with Th reat Agent
Risk Assessment. IT@Intel White Paper. Intel Information Technology. Retrieved from
Prioritizing_Info_Security_Risks_with_TAR A
2. Ibid.
3. Open Web Application Security Project (OWASP). (2013). OWASP Top 10 List. Retrieved
4. Ibid.
5. Ibid.
6. Ibid.

Part I
The first five chapters of this book are meant to set a context and a basis for the security
assessment and threat modeling of any type of system. By “system” I mean not only
the implementation of software (“code”), but any sort of digital system integration and
deployment. This broader definition will, of course, include the writing of software, as
indeed, all digital systems reduce to software. However, there is a higher-order integra-
tion that can treat the implementation of blocks of functionality in an atomic way.
Not only has architecture risk assessment (AR A) been a mandate within standards
and by organizations for a very long time, but the continuing increase in sophistication
and complexity on the part of attackers means that flaws in architecture, missed secu-
rity features, and weak designs continue to put digital systems at risk of compromise.
There is no doubt that we should be analyzing our systems for the security that they
require to meet organizational and system security goals. Numerous examples have
been provided throughout Chapters 1 to 5 to demonstrate the context in which the
practices covered by this book exist.
The first chapter sets the stage on which AR A and threat modeling play out. For
many years, security architecture has had the task to ensure that architectures and
designs include the security and support the security that will be necessary for systems
as they are deployed. However, what that process is and how it is performed has not
been well understood outside a coterie of subject matter experts. Indeed, learning to
apply computer security to systems has so far been learned almost universally through
apprenticeship with a master.
In Chapter 1, AR A and threat modeling have been defined as applied security archi-
tecture. Furthermore, there lies a body of knowledge and a practice for applying secu-
rity architecture to systems of all types and sizes.

174 Securing Systems
In this chapter, I’ve tried to help the reader understand that it is not just secu-
rity architects who will benefit from understanding this heretofore obscure practice.
Everyone who must interact both with a threat model and its results, from develop-
ers and implementers through organizational decision makers, will hopefully benefit
through a clearer understanding. I intend to lift at least some of the obscurity to reveal
a science and an art that has become a necessary and integral function for system deliv-
ery. Chapter 1, then, is the gateway into the remainder of the book.
Chapter 2 defined what a system assessment for security is, and, hopefully, what it
is not. We have defined this sort of analysis as “applied information security.” Indeed,
the subtitle of this book is “Applied Security Architecture and Threat Models,” security
architecture being the method through which to apply the principles of information
An analyst may simply “jump right in at the deep end,” which, in this book, might
mean jumping to the six example analyses in Part II. Instead, through Chapter 2,
we explored the “3 S’s” of prerequisite knowledge domains that are typically gathered
before system assessment:
• Strategy
• Structures
• Specification
This is a mnemonic abstraction intended to help the practitioner gather and then
retain the body of prerequisite knowledge that will be applied to each assessment.
Among the strategies that are required is a working knowledge of relevant threat
agents, their capabilities, and their risk tolerance. Each organization has a particular,
unique, and individual risk tolerance. The experienced assessor will understand this
organizational context as a part of the organization’s strategy to meet its goals. The
organizational risk tolerance then seeds an understanding of the risk tolerance expected
for each system under analysis.
“Structures” is meant to encompass and represent the existing infrastructure, secu-
rity and otherwise. Any infrastructure will have strengths that can be employed to
fulfill security requirements. But every infrastructure also has limitations that will have
to be overcome, often through additional system-level security requirements.
Finally, “Specification” is meant to encompass the technical details that will influ-
ence not only the threat model but also, perhaps, how the architecture is handled.
A few examples of threat agents were explored in Chapter 2 to highlight how these
are to be understood in the context of the AR A and threat model. We introduced
attributes such as technical methods, risk tolerance, and the amount of effort typically
expended by the agents, which we termed the “work factor.” Of course, each of these
examinations was highly stereotypical; there are always exceptions and exceptional cir-
cumstances. The threat agent profile becomes a key ingredient for prioritization during
the threat modeling process.

Part I-Summary 175
Chapter 3 was an exploration of the art of security architecture as a practice. We
narrowly defined “security architecture” to the confines of the task at hand: AR A and
threat modeling. We underscored that this is a subset of what is generally considered
security architecture practice.
We were introduced to some of the concepts of enterprise architecture as these are
relevant to security architecture. In fact, it is my belief that without a grounding in
system architecture and at least a glancing understanding of enterprise architecture,
a security architect will be severely hampered when trying to assess systems. This is
because the architect will not understand the abstractions and the various types of
“views,” that is, orderings that architects typically employ to understand and manipu-
late systems.
With a grounding in what architecture is and what it provides to the organization
and to the understanding of digital systems, we explored the different types of perspec-
tives that can be represented in a system architecture. Security assessment generally
focuses on the logical and component views; these two views can even be combined
within the same diagram. We noted that, at some point, a component view must be
expressed as specified technologies in a physical view. A security architect may or may
not have to work with the physical view, depending upon the skills of any other teams
with whom the architect must interact.
A key security architecture skill is the decomposition of architecture views into units
that express the qualities required for threat models: attack surfaces and defensible
boundaries. Decomposition is the process by which an architecture is deconstructed
into its constituent parts such that it can express these attack surfaces and/or defensible
boundaries. These two attributes may or may not be represented at the same level of
decomposition. Factoring is the process by which individual units are split into their
understandable security boundaries and components. We defined atomicity for these
purposes as the need to no longer particularize, as an ability to treat some part of the
architecture as unitary. This is a peculiar and local definition of “atomic.”
Chapter 4 was devoted to risk as it relates to the attack, breach, or compromise of
digital systems. I avoided a larger and more encompassing discussion of risk as beyond
the scope of this work. Instead, Chapter 4 is an attempt to provide concepts and con-
structs with direct applicability to system assessment and threat models solely. Security
assessment requires a lightweight, rapid risk rating methodology due to the number
of individual risks that will need to be formulated and then combined into an overall
risk rating. Typically, the assessor will be pressed for time. Any risk rating system must
account for that dynamic and allow the rapid rating of risk.
We were introduced to the “credible attack vector” (CAV), which is a construct
for quickly understanding whether an attack surface is relevant or not. Indeed, the
individual terms of CAV can be treated as Boolean terms, thus simplifying risk rating.
Furthermore, interrupting a single term within a CAV describes a method for applying
security controls and mitigations. Consequently, a CAV becomes a useful tool, not only
for risk rating but also for threat modeling in general.

176 Securing Systems
Chapter 5 was entirely devoted to the lightweight AR A/threat modeling methodol-
ogy: ATASM. This acronym stands for Architecture, Threats, Attack Surfaces, and
Mitigations. ATASM is, of course, a radical reduction of a complex, multidimensional
approach. ATASM can hardly be described as a formal methodology. This abstraction
is intended merely to help practitioners organize their thinking as they analyze the sys-
tems; it is a pedagogy, not a system.
The remainder of Chapter 5 demonstrated how to apply ATASM to a fictional
e-commerce website, Web-Sock-A-Rama. An in-depth AR A and threat model of this
example was reserved for Part II of this book, as one of the example analyses. In Chapter
5, we explored it as an approach to assessment and analysis.
Hopefully, most, if not all, of the body of knowledge that experienced security archi-
tects bring to AR A and threat modeling has at least been introduced and outlined in
these five chapters. Reading and, perhaps, playing with the various concepts will hope-
fully enable the reader to grasp what is taking place within the analyses presented in the
next section of this book.

Part II

Part II
Practicing with Sample Assessments
We have gathered the required tools in preparation for an assessment. I’m going to
assume that we understand our organization’s risk tolerance as it applies to systems of
the type under assessment. Of all the possible threat agents, we have selected those that
may have the most important impact on our organization’s mission and whose method-
ologies can be applied to systems of the type that we are considering. In other words, in
preparation for the assessment, we’ve taken the time to screen out irrelevant threats and
attacks. We understand the infrastructure, the execution runtime, and the deployment
model for this type of system. Local variations from industry standards are understood.*
Hopefully, by this point in the book, you have a reasonably clear understanding of
the information and considerations that you will need to bring to bear in order to assess
a system? Presumably, you have a risk methodology with which you are comfortable?
Presumably, you are becoming conversant in architecture representations, diagrams,
communications flows? Hopefully, at this point, you’ve gained a feel for the art of
decomposing an architecture?
Now, it’s time to apply the knowledge that you’ve gleaned from the preceding chap-
ters to example† architecture risk assessments and threat models.
The best way to build skill in assessing architectures for security is to assess archi-
tectures with security in mind. Perhaps this is the only way to accumulate the skill and
* If necessary, each sample assessment will provide suffi cient context for that assessment, if this
information has not been given elsewhere in the book.
† Every architecture presented in this book is fi ctitious; the examples have been created for this
work and are for teaching purposes only. Although these architectures are based upon real-
world systems, any resemblance to an actual product or running system is unintentional.

180 Securing Systems
experience required to do this efficiently and do it well? Six architectures will be exam-
ined in some depth. Hopefully, each architecture will build upon the patterns that have
been examined in the previous examples. That is the intention, at any rate.
In order to keep the text flowing, I’ve refrained from repeating the analysis of those
architecture patterns, attack scenarios, mitigations, and requirements that we will
encounter successively after these have been discussed in the text. Instead, I have tried
to note in which previous analysis the information can be found. I hope, in this way, to
keep the text moving along without sounding overly pedantic. Each architecture will
introduce at least one new pattern for security analysis. Importantly, none of the analy-
ses is meant to be unequivocally thorough; technologies that would normally have to be
analyzed in each example have been intentionally excluded so as to keep the assessments
focused. Real-world systems contain even more than what is presented here, in almost
every case.
Those readers who skip to a particular analysis of interest may find that they will
also have to read one or more of the previous analyses in order to understand the entire
assessment, as later text sometimes refers to earlier examples. Unfortunately, I’ve sacri-
ficed six cohesive and independent analyses in favor of what I hope to be a series that
build upon each other and that highlight similarities between architectures, common
patterns, and common solution sets. At the same time, each analysis should present one
or more unique security problems not in common with the previous examples, or the
analysis will dig deeper into an area glossed over earlier.
As you gain experience with architectures and their analysis for security, I hope that
you see the common patterns that emerge. These tend towards similar treatments. The
recurring patterns and their treatments become your standards “book,” if you will. At
the same time, when analyzing real systems (as opposed to these fictitious examples
created explicitly for this book), you will also encounter unique problems and idiosyn-
cratic solutions, whose applicability cannot be generalized. One needs skill with both
of these situations—the common patterns and the unique ones. Furthermore, a great
security architect can differentiate fairly quickly between the local variations and those
problems that are well known. It is the locally unique and exceptional that will most
likely require the most attention.
If you, the reader, can tolerate my tired and inelegant prose by reading the entire
section, you may find that you’ve seen a sufficient diversity of architectures such that
by the end, you can understand varied architectures, and you can better see the attack
surfaces and better prescribe security solutions. That is my sincere hope in having ren-
dered these analyses.
Start with Architecture
The first step is to understand the architecture. Most likely, those presenting the archi-
tecture to you will be thinking about how the system works, what it’s intended for, and

Part II-Introduction 181
how it will be used correctly. This is very different from the way a security architect
must approach architecture. Architects interested in implementing a system focus on
“use cases.” In a security assessment, the security architect focuses on “misuse cases.”
This distinction is quite important.
Because we’re examining the system for security vulnerabilities and weaknesses,
there is a conflict of interest built into the assessment. The people on the other side
of the table are concerned with correctness, with implementing the requirements and
specifications that have been gathered and are supposed to define the system—that is,
its boundaries, goals, and intentions. The implementing team may have been living
with this system for some time. They may even have developed their own language of
acronyms and aliases that help them speak in shorthand. Implicit within the acronyms
are the assumptions that they’ve already made and the cohesive team’s trust boundary.
If the security assessor, the security architect, has been a part of the team, this will not
be a problem: Everyone will be speaking the same language. Unfortunately, that is not
the usual situation.
A Few Comments about Playing Well with Others
On the other hand, quite often the security architect is working with a number of proj-
ects, maybe more than just a few.* She or he will not have been a part of the forming
of the team and will not be party to the assumptions embedded in the acronym-speak
to which the team have become accustomed. It may seem to the team that the security
architect has been injected into a smooth and running process. The team’s collective
understanding and the jargon that represents that understanding—the acronyms, sys-
tem names, aliases, and such—represent a cognitive, shared team reality. Meanwhile,
the security architect assigned to a running project might feel like she or he is stumbling
about in an unknown country, trying to make sense of a partially understood dialect.
Architecture reviews are not about enforcement of guidelines handed down from some
overarching authority. They should not be conducted in a confrontational manner, nor
should they focus on problems outside real technical issues… The quality of technical
discussion can be harmed by sidebars into organizational politics, funding problems,
unnecessary expansions and contractions of scope during the review, and the absence
of key stakeholders. Reviews cannot be rubber-stamp procedures used more as a means
to gain a checkmark on some management milestone list. The review team and the
project team must forge a partnership with the common goals of validating technical
architecture decisions, fostering the cross-pollination of architecture experience across the
organization, learning from past real-world experiences, and forming a forum where
feedback to upper management can be formulated to state risks and opportunities based
solely on technical and business merits.1
* At one time, I was assigned to review 130 projects simultaneously.

182 Securing Systems
You, the security architect, must work within the parameters of the shared reality of
which you may not be a part and into which you’ve been thrown. You might wish to
step carefully across this boundary. At the very first meeting, there may be considerable
tension in the room. Obviously, a warm and friendly demeanor won’t hurt. But beyond
that, it may be useful to ask folks to slow down and explain the jargon terms to which
they’ve grown accustom.
By understanding the team assumptions and by unpacking the team’s unique brand
of jargon and acronyms, you can begin to enter into their mindset; they may even start
to see you as a part of their team (if you’re lucky). And being a part of the implement-
ing team is the powerful position, though this may seem counterintuitive. Workgroups
tend to defend their boundaries. They spent a lot of time building relationships and
trust. Those carefully crafted relationships and the trust that has been earned will often
be defended against all newcomers, no matter how friendly the intentions of the out-
sider. Nobody likes to have their “good thing” perturbed.
You may be entering a team “space” where you have not been given the relationship
and trust that the team members have already collectively developed? And, in fact,
depending upon what experiences team members may have had with security folk in
the past, you could well be at a disadvantage? You may feel that you have the exper-
tise, that you’ve been nominated by your organization to represent security, that you’re
empowered to drive security into systems, and that you uphold the policy and stan-
dards to which systems and development teams must conform. Although that may be
true, a human working group develops boundaries. There is already a process in place
with which team members are more or less comfortable. Basically, they have something
ongoing into which you are being interposed. The sooner that the team perceives you
as a fellow member and not an interruption, the sooner you will have constructive con-
versations about the security needs of the system. Outsiders may very well be resisted
by those who feel that they must defend team boundaries. Insiders, members of the
team, are the people who are most likely to receive the benefit of active listening and
constructive dialogue. Try to make the assessment not about winning and losing but
rather about a collaboration for the good of all.
Nevertheless, even as you try to become an “insider,” I caution assessors, especially
beginning assessors, from allowing the development team to dictate the entirety of the
conversation. It’s quite likely that system designers have given some thought to security,
especially in today’s environment, when cyber security concerns are heightened. Still,
they may not have looked at the system holistically. They may have been given require-
ments that are quite specific about authorization models, authentication, protection of
messages in transit, and other parameters at this level of specificity. Or it may also be
true that they have looked at the system security quite thoroughly. You won’t know
until you become familiar enough with the system, its requirements, and the thinking
that has gone into the current architecture.
To get the holistic picture, oftentimes, one must pull away from discussions about
a single security issue (the “presenting problem”). The conversation will need to be

Part II-Introduction 183
redirected to the architecture, the entire architecture of the system in which the prob-
lem manifests. I simply ask the team to explain, in the simplest terms, for what the sys-
tem is intended. This will usually open a discussion about the gross architecture, which
then can facilitate a holistic security discussion, rather than being caught in the weeds
of some particular aspect of security.
Always, as has been noted earlier in this work, do not limit yourself to the present-
ing problem. Whatever the concerns are that the team has raised regarding security,
you must learn the entirety of the architecture before you can threat model it. A threat
model that does not account for any single, undefended credible attack vector (CAV)
builds in a weak link from the start. Irrespective of any particular issue with which the
development team is concerned, back up and get an understanding of the entire system,
and most importantly, get an understanding about what it is to be used for.
Many of the mistakes that I personally have made have been engendered by my fail-
ure to understand the basic assumptions behind a system under analysis. I cannot stress
strongly enough that even when feeling pressured to get the assessment completed, tak-
ing the time to carefully understand what the system is intended to accomplish for the
organization and how the implementing team members are thinking about it will pay
big dividends as you begin to actually look for attack surfaces against which to apply
your prioritized threats.
Understand the Big Picture and the Context
The first step always is to understand why the organization initiated the implementa-
tion of the system and to understand the assumptions that are built into the system’s
architecture and design.
Start at the very highest level: What is the intention for this system in its entirety?
What organizational goals is the system attempting to achieve? Who will use the sys-
tem and for what purposes? Who will administer the system? Who will consume the
various reports and logs of the system and for what purposes? Get a feel at a very high
level for what the system is intended to accomplish in the context of the organization
into which it is being placed and for whom: these people’s relationship to the organiza-
tion and its goals. Catalog all the potential stakeholders who may have a vested interest
in any aspect of the system.
Is the system under assessment to be an application that will be deployed within an
ongoing eCommerce site? Or is this system an entirely new eCommerce site? Is this to
be a desktop application? Or is this a plug-in to a well-known desktop application? Is
this a mobile application that connects and edits documents that have been typically
created and changed with some well-known application? Or is this mobile application
intended to protect a user’s device in some way? Is it a payment processing system? A
learning management system? A content production system? An identity system? What
is the general purpose of the system, and what is the form of how the system will run?
Cloud? SaaS? PaaS? Endpoint? Server farm? Appliance? Hosted? Hybrid?

184 Securing Systems
Understand the basic form and purpose of the system well enough to understand
the context in which it will execute and will be used. Understand the system’s various
user types and why these will use the system, what the system will output to each of
its users. Understand what benefit users expect from the system, and what benefit the
organization expects to gain. Why is this system important enough to implement and
for whose benefit?
Once we have a very high-level “big picture” of the system, we’re ready to begin the
assessment. As I noted earlier, experienced security architects have a grid of all the steps
that will need to be followed in mind as they proceed. This grid or plan will be well
inculcated such that the actual assessment may seem more like a conversation than a
step-by-step process. Don’t be fooled by this apparent looseness, should you observe it.
Although an assessor may not be following my steps, she or he will be following a plan,
nonetheless. To make it easier as you begin, I will restate the overall process once again.
The first example will follow the ATASM outline, as we unfold the security assessment
in practice. Later analyses will collapse the process somewhat, to appear more “real,” as
it were.
You may notice a recursive quality in the later analyses? A component will be exam-
ined, which will uncover the need for further investigation, perhaps further decompo-
sition or factoring of the architecture. That is a typical assessment flow rather than a
rigid series of steps.
Part II assumes that before actually trying to analyze an architecture, the assessor will
have researched threat agents active against the type of system under consideration. This
research must then uncover the typical attack methods that the relevant threat agents
stereotypically employ and the usual effort that the threats will expend towards success.
Finally, some understanding of threat agent risk tolerance will be a factor in determining
just how deep and extensive the defense-in-depth will need to be to foil attack success.
With threat information well in hand (as much as one can know about intelligent
adversaries), the assessment then proceeds along the following steps (as presented in
Chapter 2):
1. Diagram the logical architecture of the system.
2. Decompose (factor) the architecture to a level that exposes every possible attack sur-
face (or defensible components). Pay particular attention to changes in trust levels.
3. Apply attack methods for expected goals to the attack surfaces.
4. Filter out threat agents who have no attack surfaces exposed to their typical
5. List all existing security controls for each attack surface.
6. Filter out all attack surfaces for which there is sufficient existing protection.
Remember that interrupting a CAV at any single term invalidates that CAV
* Th ough interrupting a CAV term will not usually constitute a defense-in-depth.

Part II-Introduction 185
7. Apply new security controls to the set of attack services for which there isn’t
sufficient mitigation. Remember to build a defense-in-depth. Security controls
can fail.
8. The security controls that are not yet implemented become the set of security
requirements for the system.
Steps 3 through 7 will comprise a formal threat model of the system. It is possible to
draw a diagram strictly for the threat model that highlights attack surfaces and controls
(both existing and yet to be implemented). You will notice that the threat model exists
in the context of the architecture of the system (as described earlier). Failure to keep the
architectural context (and the prioritized threat information) may cause the assessment
to chase irrelevant or deprioritized attacks.
Getting Back to Basics
Understand the architecture in its entirety. Understand at the architecture’s most gross
level, including any and all interactive components and each end-to-end in which the
system participates.
Diagram the grossest lines of communication, noting origination and destina-
tion (usually a single-pointed arrow pointing from origination to destination). Decide
which flows and which subdivisions of the architecture will need to be decomposed
further. Depending on system complexity, these may be on the same diagram or sub-
diagrams. When working with complex systems, you will likely need to decompose
the architecture into its component parts. Factor down to a granularity that presents
defensible boundaries.
For each defensible component, gather runtime, deployment model, and the pro-
posed infrastructure. Mark the specific lines of communication between each factored
component: origination to destination. Note the highest sensitivity of data going across
each communication flow. Note all locations where data is at rest, even temporarily.
Understand the complexity of data within each communication flow. Where there
is permanent or temporary storage of data, note what data is being stored and the high-
est sensitivity of the data in that storage. Catalog every system input—that is, every user
type and role; every use pattern at a gross level; every line of communication, especially
every point where digital data comes into the system (whether from a human or an
automated source). Catalog every transit of data (lines of communication) and every
data storage point throughout the system. If data transits more than one network, you
will need to understand the trust level and restriction of each network in relation to its
adjoining networks. A network boundary is obviously any point at which the network
addressing changes. But take note of boundaries where there are any type of network
restrictions, as well, and note sensitive data transits and storage. Note levels of trust
from inputs.

186 Securing Systems
If the system is simple enough, everything may fit onto a single diagram. If the
system is highly complex and interactive, don’t hesitate to build layers of representation
and sub-diagrams, to break out detailed representations.
The point of this investigation is to thoroughly understand the architecture as it ful-
fills the intentions of the owners and designers. Enumerating all of the attack surfaces
of the architecture cannot be done without a complete understanding. We have dis-
cussed at length the task of decomposition and the factoring of the architecture. If you
feel uncomfortable with what this means as a practice, please reread Chapter 3. One
way to understand the level of detail required is to decompose the architecture down to
the level of a defensible component. As we said in Chapter 3, a defensible component
is the level at which the security controls and their implementation in and around that
particular component can be understood easily. Since different security controls oper-
ate at different levels of granularity, you will likely need different views, from gross to
granular, of the architecture.
For instance, authentication typically happens at some sort of entry point: the entire
Web infrastructure, the application server containing components or applications, the
operating system, and so on. One might choose to place an additional authentication at
a particular function or application. But it’s useful to start at the usual and typical place
for each control. Perhaps the expected placement of the control will be sufficient to
achieve the intended protection? Do it this way if for no other reason than it’s generally
cheapest and easiest to implement in an expected manner. (Or the control may already
have been implemented, as in the login in order to get access to the operating system.
This login is typically built into the operating system from the ground up.)
Authorization is typically applied at a more granular level—the object; a database
field, row, or table; a set of Web URLs or pages; a part of a user interface (UI); a par-
ticular functionality within an application; permissions for a directory tree (read, write,
execute); and so forth. Authorization is often placed in the flow just before the resource
is accessed.
Network restrictions and firewalls tend to be applied at gross entry and exit points
within a network architecture (which may or may not have been designed with security
in mind). People who manage networks want to make these sorts of restrictions as gross
and easy to manage as possible. Long, complex lists of network access rules (ACLs) are
very difficult to get right; a single additional rule inserted in the wrong place can break
the entire rule set.
Network designers and maintainers don’t tend to be enthusiastic about placing an
individual application within its own highly restricted sub-network or virtual private
network (VPN). For ease of maintenance, the people tasked with maintaining network
boundaries are going to want to treat every subunit in a unified way. They may argue
strongly for “least common denominator.” Unfortunately, in security we tend to be con-
cerned with the exceptional case, the critical case, the high-profile or most sensitive and
restricted case. In many organizations, there is a built-in tension between the economies
of scale achieved through designing for the typical case and the requirement to account

Part II-Introduction 187
for the exceptional. Infrastructure design benefits greatly from consideration of both
of these needs: large, easily understood, and gross gradations, while also being able to
account for corner cases and exceptions.
Consider these three different cases: authentication, authorization, and network
access controls. Each may require a different level of granularity during examination.
Authentication may take place at an entry point (web front end, application server, oper-
ating system, jump server, or inter-module communications input). Authorization will
take place close to the resource that needs to be authorized (file, data object, webpage,
etc.) Network access control will take place at a network boundary, segment, or subnet.
Each of these is a different view, at a different granularity of the system under analysis.
You will likely have to factor a complex system to granularities that are familiar and
understood for each stakeholder within the implementation teams. Still, the assessment
will need to be decomposed such that every attack surface that is relevant to the priori-
tized threat list is uncovered. This implies that for any modestly complex system, you
will have multiple views. It may turn out that a view will have to be generated specifi-
cally for each type of stakeholder. Each of these views will be unique, highlighting one
portion of the holistic information that the security assessment requires at the expense
of representing other information.
For instance, the network view might hide information about application logical
functions, treating the application runtime (operating system, application server, etc.)
as atomic so that the gross network placement of types of systems can be understood:
database “zones,” Internet “zones,” application “zones,” and similar distinctions. The
networking team won’t be concerned about host security controls or application secu-
rity measures. They will be focused upon what architecture can best support whatever
network accesses and restrictions are being enforced by the network, and with what
types of networking equipment: routers, switches, and firewalls. They will need to
know how many network addresses will be consumed when the system goes into pro-
duction and what the expected growth will be over the life of the system.
On the other hand, the application architects will want to hide routers and switches
in favor of gross groupings of application functions. There may be no physical architec-
ture exposed at all within a strictly application view. The application view will highlight
business functions (or some other application-level grouping, such as common layer
function—i.e., databases, application servers, user presentation, backend transaction
processing, etc.). The network view would then be largely lost in order to construct a
logical view of functionality or logical view of components or both.
Due to the necessity for each stakeholder group, from organizational decision makers
through the various architects and on to physical implementers, the security architect
may have to piece together a puzzle of information from each individual view. Complex
systems, whose implementation spans many teams, each with its own perspective, will
typically never represent the entire security picture in a single diagram. Attempting to
do so may be an energy sink not worth pursuing. The assessing security architect will
have to ask his or herself whether this energy is well spent or not.

188 Securing Systems
If I cannot “get,” that is, understand, a system, I may attempt to diagram that which
I’m failing to comprehend. That is a more focused activity that I find always bears fruit.
If I haven’t understood, the other architects can then correct the diagram. Working
iteratively (usually on a white board), we collectively share what we know until that
which was poorly understood comes into clear focus.
At this point, I take generous notes rather than attempting to get everything into
a single representation. This has worked for me, over literally hundreds* of security
assessments. I offer my approach as one way to tame complex systems. Certainly, your
Part II-Figure 1 Recursive ATASM process.
* After I had exceeded 500 cumulative system reviews, I stopped counting.

Part II-Introduction 189
solution will depend greatly on your learning and memory style and the documentation
that your organization expects to be produced from your security reviews.
I caution the reader to develop some way of documenting architectures, especially
if you will assess many systems, and perhaps even having to undertake multiple assess-
ments running concurrently. Taking good notes alongside reasonable (if incomplete)
architecture diagrams has allowed me to assess any number of systems, one after the
other and concurrently, and still be able return quickly to where I was in an assessment
and what the issues were when I had last looked.
Let’s return to a couple of the architectures that we examined previously and start
decomposing them into useful parts. Remember, though, as we explore this process,
that it is not an exact science as much as an investigative process. We seek to enumerate
all the attack surfaces and to understand at which level of granularity controls will have
to be applied in order to protect the attack surfaces. Almost by definition then, security
assessment tends towards recursion.
Part II-Figure 1 attempts to graphically describe the recursive, investigative nature
of ATASM. At any point in the process, as one moves from architecture to threats, then
to attack surfaces, and then mitigations, a new element may need to be investigated.
The current thread can either complete and then move on to the new items, or the pro-
cess may be interrupted in favor of the discovery. Each new investigative thread returns
to the architecture to begin the process anew. There is both an organic quality to the
investigation paths as well as the recursion back to the beginning for each new element
that has been discovered.
In Part II, we examine several architectures sequentially, from enterprise level to
endpoint, hopefully giving you an opportunity to apply an AR A/threat modeling pro-
cess to a collection of disparate architectures in order to gain the experience necessary
for analyzing real-world systems.
1. Ramachandran, J. (2002). Designing Security Architecture Solutions, p. 12. John Wiley
& Sons.

Chapter 6
eCommerce Website
In this chapter, we will pick up where we left off with the AppMaker Web-Sock-A-
Rama web store architecture in Chapter 5. This architecture is reprised in Figure 6.1.
We will factor the architecture further into its constituent parts. And we will exam-
ine the architecture from different views. We will also complete the ATASM process
(Architecture, Threats, Attack Surfaces, Mitigations).
6.1 Decompose the System
Ultimately, the point of the architecture factoring exercise isn’t to document a perfect
architecture view, but rather, to find those points in the system that are susceptible to
likely attack. An appropriate set of defenses can be built from the attack surfaces. In
addition, residual risk can be raised for appropriate decision making. As noted above, I
will often have multiple views of the same architecture that I consult in order to threat
model particular aspects of the system. Security is built at many levels, top-to-bottom,
side-to-side, and front-to-back. Security is the architecture domain that interacts with
every other domain; it is the “matrix” domain* and must permeate a system in order to
build a defense-in-depth.
Looking at Figure 6.1, do you see anything missing? In the chapter about the
ATASM process, there were several views of Web-Sock-A-Rama, adding data types,
trust boundaries, and an attack surface. Still, in that series of discussions, it was inti-
mated that there must be more; the discussion was purposely kept simple in order to
concentrate on the process, in order not to get bogged down in attack details.
* I believe that I fi rst heard the phrase, “security is a matrix domain,” stated by my former
workmate, Richard Puckett (Executive CTO, Enterprise & Security Architecture, at GE).

192 Securing Systems
Over the next series of figures, we will not proceed through the process stepwise,
as we did in Chapter 5. Instead, we will proceed in a manner suggestive of an actual
assessment. We will analyze what we see and then move on to greater detail. Always,
we are looking for ATASM: architecture understanding, followed by credible attack
vectors: those qualified threats whose methods have vulnerabilities exposed to them.
When an assessment proceeds stepwise, as in Chapter 5, we try to map a sufficient
level of detail during the architecture phase. However, for complex systems found in
the real world, a stepwise, strictly linear approach causes diagrams to be overly busy
and difficult to understand. As you may see through the following discussions, it often
makes more sense to perform a series of mini ATASM assessments, each focused on the
level of detail in a particular architecture view showing some workable portion of the
system. The mini assessments will often be recursive, as consideration of an attack sur-
face then exposes more components through flows originating at the component under
analysis. The summation of risks and requirements then pulls the results from the series
of ATASM assessments together into a whole and complete analysis.
Figure 6.1 AppMaker web architecture.

eCommerce Website 193
6.1.1 The Right Level of Decomposition
I remind the reader, reprising Chapter 5, that the analyst brings the “3 S’s”—Strategy,
Structures, and Specifications—to each architecture analysis. “Strategy” encompasses
the desired risk posture and the threat landscape. “Structures” is about the set of secu-
rity controls that can be implemented and any existing security limitations that must
be taken into account. To understand “Specifications,” we have to figure out the data
sensitivity, what the runtime and execution environments are, and which deployment
model we must analyze for.
As we began to look at the system depicted in Figure 6.1, we discovered that even
the obvious attack surface at the web server actually allowed attacks buried in messages
and transactions to be passed through to other interfaces in the system. In fact, cross-
site scripting (XSS) errors were likely to crop up in the code running in the application
server. Structured Query Language (SQL) injections would, if not protected, exploit
the database server and data stores. Figure 6.2 adds these two attack surfaces for a more
complete picture of the message flow from the Internet and on through to the data tier.
Figure 6.2 Attack surfaces touched by requests from the user’s browser.

194 Securing Systems
That is, at arrow 2b, the dynamically handled messages are passed from the web
server to the Java application server. The application server calls AppMaker, which in
turn passes the message to one of the appropriate applications that were generated by
AppMaker. In order to find the appropriate application, AppMaker will have to call the
database server (arrow 4) to query the application metadata: data about which applica-
tion handles which type of message. Then, while handling the request, “other web app”
must also query various databases, perhaps customer profiles and the product catalog,
in order to build the response for the user. The database server must continue to fetch
data (“data fetch to fulfill 4”) as requests come in and responses are built. These systems
handle multiple requests in parallel.
So, obviously, as called out previously, attacks within messages can be targeted at
the web server, at the application server itself (or its configuration), at AppMaker, at one
of the generated custom applications that make up the store, or at the database server.
In this view, we have uncovered three attack surfaces. That was the conclusion of the
analysis in Chapter 5.
6.2 Finding Attack Surfaces to Build the Threat Model
In this architecture view, have we uncovered all the attack surfaces? Can we treat the
application server, AppMaker, and the generated applications as a single attack surface?
The answer to that question will depend upon how AppMaker works. For the attacks
that we’ve listed previously that target dynamic application code, like the two injection
attacks, the bug allowing the injection might lie in the application generation code. Or,
it might be fixed within the generated application. Without more information about
how AppMaker generates applications, we don’t have enough information. There are
too many possibilities.
Since our web store has purchased AppMaker, let’s assume that wherever the injec-
tion lies, it would be AppMaker’s maker (the vendor) who would be responsible for
the fix. So, in light of this relationship, it probably doesn’t matter whether an XSS lies
within AppMaker itself or in a generated application, as long as the vendor is responsive
to vulnerability reports and practices rigorous software security. Ultimately, it is the
vendor’s code that must protect the store, not Web-Sock-A-Rama. AppMaker must pro-
tect against injection attacks through rigorous input-validation techniques,* as outlined
* Th e kind of generalized application platform that AppMaker represents presents the devel-
oper with a particularly diffi cult and specialized input validation problem. Since a priori, the
designers won’t know what the inputs for any particular fi eld are going to be, they have to
design for data-driven validation. Th at is, AppMaker’s design must allow its users to specify
the type and allowable sets and ranges for fi elds at run time. Th is data will then become a part
of AppMaker’s application metadata. Input validation routines will need to fetch the relevant
metadata that defi nes a valid input for each fi eld and then validate based upon this metadata.
Hard coding data types and ranges isn’t possible in this case.

eCommerce Website 195
in the tables in Chapter 5. With these types of complex, combinatorial systems, this
case often arises, where an organization’s security is dependent upon one or more ven-
dors’ security practices. In this system, the application server presents an attack surface,
as do AppMaker and the AppMaker-generated applications. But responsibility for pre-
vention of application-level code attacks rests firmly with the AppMaker vendor.
We assumed that this web store has an authentication system. Figure 6.2 has added
web authentication. We will investigate that component and its flows in a later figure.
Still, because authentication systems are often prized targets, the authentication system
has been placed within its own separate subnet. Traffic from various components is
allowed into the subnet to perform authentications, but traffic from the Internet is not
allowed; the authentication system is “invisible” from the Internet; it is not reachable by
Web-Sock-A-Rama’s customers directly.
The architecture with which we’ve been working has not been factored into all of its
constituent parts. How can an assessor know this with certainty?
Data fetches are not generalized. You will notice in Figure 6.3 that the database
server must fetch data from every data store. In previous illustrations, we generalized
this for simplicity, showing only a single arrow to represent all data fetches collectively.
Figure 6.3 Data fetch and management interfaces.

196 Securing Systems
The flow in Figure 6.3 is more coherent with the description given. AppMaker loads
one of the generated applications, which in turn must generate a query for various types
of data. In this web store, every data store will be used. For each dynamically generated
HTTP response, AppMaker must first find (and perhaps in part, build) the appro priate
application through its metadata. Then, the application must itself consult the web
store and customer data. Hence, the database server is an intermediary between the
various data stores and applications (AppMaker and the generated web applications).
These flows are shown in Figure 6.3.
Figure 6.3 also shows that each type of functionality must be configured and
administered. All web servers, authentication systems, application servers, and database
servers have an administrative interface (“admin”) that uses a separate network interface
and typically have a special user interface (UI) that is presented only to administrators.
Administrators have exceptional powers, that is, “superuser”; they are allowed to
start, stop, change, and control the equipment. Therefore, these interfaces are always
among the most protected areas of any system. At most of the companies at which I’ve
worked, only “web masters”—highly trusted, proven individuals—are given access to
production administration. New hires must demonstrate both skill and trustworthiness
over an extended period of time before they are allowed to work on the systems that
generate revenue or interact with the public, which are exposed to the dangers of the
public Internet. Wise organizations understand that the administrative function has the
power to do great harm. In addition, the administrator is likely to be a valuable attack
target through which attackers will try to gain access to juicy administrative functions.
What has not been shown is that every separate host among the servers on which
this system runs also contains an administrative or management interface. For the
operating system, this will be a highly privileged user (that is, the Administrator, super-
user, or root account of the operating system). For storage devices and similar, it will
be yet another management interface or even a separate management console. A real
assessment would factor the architecture down to each of these systems (perhaps in
detailed views?). For this example, we will assume that each of the logical functions
shown in the diagrams runs redundantly on two or more hosts, and that the hosts are
horizontally scaled (additional hosts are added for more processing power). Data are
kept on storage devices, which themselves present an integral administrative function.
Let the “admin” rectangles in Figure 6.3 represent all of the management interfaces for
This brings us to Figure 6.4, which diagrams the attack surfaces presented by the
manage ment functions. Large arrows are used to indicate the management attack surfaces.
It must be noted that all local configuration files would also be targets and thus
should be protected at the same level as the administrative interfaces. That is, if an
attacker can add an administrator with the attacker’s password to the configuration,
or remove all authentication on the interface, or even, perhaps, cause the administra-
tive interface to appear on the Internet, this would significantly impact the system as a
whole and the organization fielding the system.

eCommerce Website 197
Because each system has a set of configuration files and, also, must store some run-
ning metadata, attack surface pointers (the large, darker arrows) have not been added
to the diagram for these; Figure 6.4 has already become visually busy enough. Still, in
the real world, every configuration file and any local, running data set is and must be
considered an attack surface. In this case, we have already stipulated that, through the
infrastructure practices “investigation” (really, an assumption in this fictitious example
system), there are a series of existing controls protecting the servers and their local hard
disks. Thus, we concentrate on just a portion of the attack surfaces in order to keep the
example focused. In the real world, I would absolutely find out what management prac-
tices and protections are implemented to defend the configuration files and metadata
sets. Figure 6.7 adds a management access control layer that requires an authentication
before administrators can access the management interfaces and configuration data.
Usually, these “jump” servers require dual-factor authentication, often in the form
of a one-time token along with the user’s password. Once authenticated into the jump
server, the administrator then is given access to the management interfaces and consoles
Figure 6.4 Management interface attack surfaces.

198 Securing Systems
only from the jump server protected sub-network. The production system management
interfaces only appear on the highly restricted sub-network, which can only be accessed
through the jump server. All other access is denied at the network level.
In this manner, an attacker who gains any one of the segments within Web-Sock-
A-Rama’s web store still cannot access any management interface. The only attack pos-
sible is to compromise one of the administrators: The attacker must not only learn the
administrator’s password, but must also somehow trick or force the administrator to
enter the one-time token. Such management networks are very difficult to breach and
thus are often employed for production Internet systems.
(We will return to Figure 6.7, below, in the discussion about payment processing
flows, attack surfaces, and requirements. A more complete picture of a management
sub-network that is restricted through a jump server is pictured in Figure 6.5. Please
refer to this figure for more clarity about the architecture and components used to build
this type of management interface protection.)
Web authentication can be handled entirely by the web server. The HTTP proto-
col contains primitives for authentication, which then are implemented in most open
Figure 6.5 Authentication and identity fl ows.

eCommerce Website 199
source and commercial web servers. Many implementations on the web make use of the
web server’s local or integral authentication services, using the native HTTP authenti-
cation messages.
The upside of performing authentication entirely at the web server is simplicity, per-
haps even technical elegance (if the integral authentication service is elegantly conceived
and carried off ). No other components need be put into place beyond the web server.
The sensitive authentication doesn’t have to traverse a network, most especially the
demilitarized zone (DMZ)/bastion network, which is the most exposed zone. Separate
protections don’t have to be created to protect sensitive authentication systems and data,
particularly, credentials. Further, the communications to perform the authentication
are likely to be faster when there isn’t a network between services. An integral service
that has been built with a particular web server in mind is likely to run well as a part of
the web server system. Using an included service avoids integration and compatibility
problems. Making use of the authentication service included with a web server (which-
ever web server) has its advantages.
But using the included authentication service contained with or as a component of
the web server has some serious disadvantages, as well. First, performing the authenti-
cation on the same processor (host) as the web server may slow the web server down.
That’s not a security concern. But if the web presence must scale to thousands or mil-
lions, this factor should definitely be considered. Sometimes, the services included with
web servers are not as robust or scaleable as dedicated systems (this is a terrible over-
generalization that may not apply in all cases). Hard evidence should always be sought
before making such a decision.
From a security standpoint, if the authentication service is on the web server, where
do the credentials reside? Either retrieval calls must be made from the untrusted web
server to some other, more trusted layer, or, more typically, the credentials are kept on
the server itself, or in storage mounted by the web server. Do you see the security prob-
lem in that? With that approach, the credentials are sitting on the least-trusted, most-
exposed component in the architecture. That, of course, makes them harder to protect.
If the credentials are kept on the web server or mounted thereon, then all the secu-
rity controls to protect the web server had better be very nearly bullet proof. Placing
that much pressure on the defense of the web server breaks the entire purpose of a bas-
tion system: If the authentication credentials are kept on the web server, it can no longer
function as a unit that can be lost in order to protect other systems. The authentication
service must not fail—authentication services are security services, namely, security
control, which any defense is hoping will not fail or will at least slow attackers down
considerably. In other words, the authentication service requires enough defense to pro-
vide significant assurance, even in the face of other systems’ failures. One does not want
to lose customers’ credentials! Having lived through one of these scenarios, I can assure
you that it is something that any responsible organization will want to avoid.
If you look at Figure 6.5, you will see an alternate architecture. The authentication
service lives in a highly restricted network. The only component that can make an

200 Securing Systems
authentication request is the web server. Authentication services are not reachable from
the Internet, or from any other component in any layer.
In Figure 6.5, you will see an arrow from Customer Profiles to the Directory in
the authentication sub-network. That’s because user profiles must be attached to
user IDs. There should be only one source of truth for user IDs. In this case, it’s the
Customer Profile. The Directory synchronizes user IDs from the Customer Profiles.
As a customer profile is created or deleted, the user ID is populated into the Directory.
Passwords are not handled by any of the customer data-handling applications. Instead,
control is passed to the authentication service, which then gathers a password from the
user (or resets the password, whatever operations must be performed). Only user IDs
move from Customer Profile to Directory. The customer onboarding application only
has write and delete privileges to the user ID (and any other attributes that authentica-
tion requires). In order to protect credentials, no other application may handle or touch
credentials in Directory.*
The Web-Sock-A-Rama authentication server and Directory will naturally be valu-
able targets containing useful functions and data for many attackers. Control of authen-
tication means that an attacker has access to all services protected by authentication.
Figure 6.6 has arrows added to some attack surfaces presented by Web Authenticator.
Certainly, gaining the authentication host provides complete control of all services run-
ning thereon. Less obvious may be the configuration files. If an attacker can recon-
figure the authenticator, she or he can control access, as well.
The middle arrow of the three on the left in Figure 6.6 points to the communication
flow that goes from Web Authenticator to Directory. If an attacker can view the com-
munications from the service to the directory, then the user’s credentials will be exposed.
A further level of network attack success might allow an attacker to control authentica-
tions: refusing service to some, granting to others (the attacker him or herself?).
Finally, there is an arrow pointing towards Directory, itself, in Figure 6.6. Gaining
the directory gives the attacker all the credentials, which are tied to all the user IDs.
That very thing happened to a company for which I worked. The attackers compro-
mised every account’s password. In other words, attackers had access to every customer
account and the services and equipment for which the highest privileged accounts were
entitled. Ouch!
But losing customer passwords is worse than that. Many people reuse the same pass-
word for multiple web sites. And they reuse their easily remembered user IDs, as well.
Consequently, obtaining access to a couple of million passwords probably also gives the
attacker access to other web sites. For the successful attacker, acquiring a large password
set constitutes considerable success!
* Th ese synchronization operations can be implemented in any number of ways: application
code, database links, synchronization routines, etc. Th ese implementation details do have
security implications. Still we will assume, for the sake of brevity and simplicity, that syn-
chronization is an application function.

eCommerce Website 201
If the user ID is the user’s email address, the attack also acquires another valuable
asset. Active customers’ email addresses are a commodity on the black market. The
addresses constitute “qualified” spam email targets. A large proportion of the addresses
are likely to be in active use, as well as tied to an actual person who can be targeted for
phishing attacks, phony drug sales, con games, the gamut of attacks that are promul-
gated through email. A customer base must include people who buy things with real
currency. From the attacker’s perspective, “Eureka!” Web-Sock-A-Rama could provide
qualified spamming leads in the event of successful compromise of the directory used
by the authentication system.
For the foregoing reasons, Directory is a very tempting target. For Web-Sock-
A-Rama, the loss impact would include costs of stopping the attack, working with
customers over lost information and breached accounts, perhaps socks ordered in a
customer’s name but sent to an attacker? Then there is the loss of customer confidence
Figure 6.6 Authentication attack surfaces.

202 Securing Systems
in Web-Sock-A-Rama’s ability to protect the customer’s interests. Future sales may be
significantly and negatively impacted?
The Poneman Institute continues to study the direct and indirect costs incurred
through the loss of user information. The results of their studies can be sobering.
Breaches typically cost companies hundreds of dollars per user. Loss of Directory by
itself might well endanger Web-Sock-A-Rama’s business, which, of course, means that
Directory should be well defended.
The network restrictions surrounding the web authentication service is one layer of
defense. Is that sufficient? I would not presume so. As was noted, this component is too
valuable to trust to a single defense. Furthermore, authentication requests are tendered
by the least-trusted component in the architecture. That component, HTTP termina-
tion, resides on the least-trusted network. What additional steps can be taken?
I would reduce the attack surface by having communications from HTTP termina-
tion authenticated before processing user authentication requests. That is, the authenti-
cation system must authenticate the HTTP component, itself. This will protect against
attacks from a compromised DMZ network. And it will protect attempts to spoof
HTTP termination. Authenticating the front-end web server does not prevent attacks
originating from the web server, however.
We once again encounter the need for rigorous input validation of each authentica-
tion request. Although this may seem like overkill to some, I would insist that the maker
of the authentication service prove a robust ability to resist ill-formed requests. After all,
we must build in an ability to resist the failure of components, particularly those that
are highly exposed. It’s important to remember that the construction of authentication
requests depends upon user (attacker)-supplied data: user ID and password (at least).
Because some data must originate from hostile networks (user IDs and passwords),
the authentication services are considered to be “exposed” components. As such, every
component that runs as a part of the authentication service must meet the hardening
standards for Internet exposed systems. This is the same requirement for every exposed
host and application that is a part of Web-Sock-A-Rama’s online store. The authentica-
tion service, despite its network protections, is no exception. Everything that’s exposed
must be configured to resist and thwart common web attacks, in case the network
protections fail or are compromised.
Naturally, as a high-sensitivity system that is key to maintaining the security pos-
ture of the site, administration of the authentication service must only be from the
restricted access “jump” server (as described above). The configuration file controls
how the authentication service works, so it would be a prime attack target. Therefore,
the configuration file must only be accessed by the administration application from the
authentication host. In this manner, attackers must first compromise the authentication
service or its supporting host before gaining control over the configuration files. And in
that situation, the attacker has no further need for exploitation of the configuration file.
Configuration file protection is thus dependent upon the defenses of the authentication
services and hosts, as previously described.

eCommerce Website 203
Any online store will need to handle payments in order to complete sales. Often, the
payments are accomplished through payment cards.* Accepting payment cards online
immediately subjects the web store to Payment Card Industry (PCI) requirements.
These are quite stringent. Some of the requirements specify details about network
restrictions and administraative functions. There are encryption requirements, and so
on. We will not explore PCI in depth. However, we must briefly touch upon PCI in
order to understand how online entities integrate with third-party services.
A merchant does not process the payments for payment card transactions. In most if
not all architectures, the supplier of the payment card does not process the payment for
the merchant, either. Although there may be situations where a bank or other issuing
financial institution will also process payments, these have become rare. Instead, third-
party payment processors are employed to actually get funds transfered based upon
the credit card or other type of card. From the merchants’s perspective, it doesn’t really
matter who the processor is, as much as the security requirements for the integration.
Whoever the payment processor is, there will be security requirements that the mer-
chant must meet. Usually, the payment requirements are based upon PCI. In addition,
there may be contractual obligations to fully meet PCI requirements. In the event of a
breach involving payment cards, failure to meet PCI typically involves a penalty against
the merchant. I have seen these penalties be quite severe. For our purposes, it is enough
to know that significant revenue-impacting penalties are typical when a merchant fails
to protect payment cards and/or the payment processor. Adherence to the PCI standard
is a critical legal protection, regardless of any actual security value in the standard.
Because our online sock company takes online payments using payment cards, the
company will be subject to the PCI standard as well as any additional intricate integra-
tion requirements that the third-party payment processor imposes.
PCI requires that every system involved in the chain of handling a payment card
must be in a restricted network. The standard also requires that every system on the
PCI network meet PCI requirements. Without delving into PCI requirements in any
detail, because Web-Sock-A-Rama has isolated its exposed networks and limited the
applications and functions allowed on those networks, the company has assumed that
the entire online store must meet PCI standards. For our purposes, everything depicted
in Figure 6.7 that lies below the Internet, except the isolated authentication service,
would be subject to PCI requirements.
Since the authentication service lies within its own restricted network, from a PCI
standpoint the authentication service has been separated. The authentication service
never handles payment cards and is not involved in financial transactions. Therefore,
it lies outside the PCI boundary. Customer’s user ID are populated from the customer
* Th ere are other types of online payment services. “Wallet” and other, similar services proxy
banking and payment card payments for retailers. Th ese services require approximately the
same type of integration that we will explore in this example.

204 Securing Systems
database. That is a “push” function; the directory that lies within the authentication
service does not call out to the customer data. Therefore, to reiterate, the authentication
service is not subject to the PCI requirements.
Even though many of the systems and applications integrated into the online store
are not involved in financial transactions, PCI is clear that if systems share a network,
then they are subject to PCI requirements. PCI considers the host and the network.
It does not discrimiate between applications sharing a host. If a host has not been
restricted at the network level from communicating with payment card handling hosts,
then it is to be considered within the PCI scope.
You might want to give the PCI proximity requirement some consideration on your
own. Does that requirement provide additional security for financial transactions?
Is the PCI standard too focused on network proximity versus actual participation in
Figure 6.7 Web-Sock-A-Rama payment processing.

eCommerce Website 205
handling the payment card? I’ll leave you to consider these questions on your own. The
answers tend to be rather controversial.
Regardless of whether the PCI provides an appropriate boundary for security, Web-
Sock-A-Rama is subject to the requirements as delineated in the standard. What this
means in practice is that the entire application server and all the applications running
on it are subject to PCI, since the payment application runs on the application server
alongside other applications. Indeed, AppMaker, the online application creation soft-
ware underlying the web site, is also subject to the PCI requirements. In addition,
the database server is subject, as are all the databases, irrespective of whether any
particular database contains payment card information. The databases all share the
network with the database server. In this way, as stated above, every application lying
within the boundaries of the three-tier DMZ architecture is subject to PCI standards.
Luckily for the company, the DMZ may be considered carefully restricted (please see
the previous analysis).
Because the company is very concerned about protecting its customer financial data
and protecting itself against a breach of payment card data, they have chosen to place
customer financial data in an additional, isolated, restricted network segment. You will
see that Customer Financial Data is isolated within a gray box demarking an isolated
network. The isolated network is not required by PCI. It is an additional measure put
in place due to the criticality to the business of protecting customer financial data. All
of the database network would be considered a PCI segment so long as the database
server must move financial transactions. However, as an additional measure, the online
retailer has chosen to implement additional protections around the storage of the data.
This extra restriction is implemented through a dedicated network for customer
financial data storage.* Only the payment application is allowed to communicate to the
financial data store. Naturally, before the payment application may interact with the
financial data store, it must authenticate to the financial store (i.e., authentication on
behalf of the data store by the database server). The defense of the customer financial
data consists of a network isolation, network access control rules (ACL), and authen-
tication of the application before any transactions may proceed through the database
server. These security measures are, of course, added on top of management, admin-
istrative, and other network and application controls that have already been reviewed.
Key will be rigorous input validation by the payment application to protect itself and
the data layer from attacks vectored through payment messages.
Although the online retailer handles and then stores customer payment card infor-
mation, the retailer cannot complete the transaction without the assistance of a third-
party payment transaction processor. This is the most common approach to handling
* A dedicated customer fi nance storage network is not mandated by PCI. Th e entire database
zone is subject to PCI due to shared database server(s). Th e sub-network has been imple-
mented as a part of the defense-in-depth of the online retail website.

206 Securing Systems
payment cards: use of a payment processor vendor. Let’s examine the payment process-
ing integration for a moment.
From the perspective of Web-Sock-A-Rama, the payment processor breaches the
protective perimeter established around the website architecture. Receipt of messages
from the processor add a new attack surface. There have been successful attacks against
payment processors. The assumption that the payment processor security is sufficient
to protect the company’s systems would be foolish. If the payment processor is com-
promised, the successful attacker has gained an avenue of attack against every retailer
making use of the processing service. An attack vectored through the processing service
has an entirely different probability than an attack from the Internet. An attack from
the payment processor would have far greater access since it would go directly to the
payment application. Internet attack is certain; compromise of payment processors is
rare, but deadly; the impact of a successful attack coming from a trusted service such
as a payment processing service will have critical impact, touching the heart of the
relationship between retailer and customer. Therefore, our online retailer must consider
what measures should be taken against the rare but serious compromise of the trusted
payment service.
Interestingly, the payment service should not trust any retailer. Successful compro-
mise of the retailers’ systems would allow attacks to be encapsulated within payment
processing requests. Sometimes, a retailer integrates with only a single payment process-
ing service, or perhaps a select few. A successful payment service must integrate with
thousands, perhaps hundreds of thousands of retailers. The likelihood that one or more
of those retailers will experience a compromise is actually quite high. Again, the payment
service would be foolish to assume that every retailer is capable of protecting its systems.
In this example, we have an integration that should be based upon mutual distrust.
The two entities must distrust each other at the digital security level, even though there
is a fairly high degree of contractual trust. That may seem paradoxical, but remains the
better security position for the reasons stated.
Sometimes, the contracts from payment processors can be quite specific and even
demanding about the security measures that must be taken to protect the payment
processing service. The security architect can easily be pulled away from self-defensive
thinking, forgetting about mutual distrust, the fact that payment processors have been
breached and might be a vector of attack, and might open an attack surface through the
integration. Attacks can go both ways. A holistic approach will not presume adequacy
of any entity or system’s security. What measures should Web-Sock-A-Rama take to
protect itself against a compromise of its payment processor’s systems? Are there other
defenses that should be taken for the integration?
As is usual for these types of integration, is cheapest to use the Internet for com-
munications between the two parties: Web-Sock-A-Rama and the payment processor.
In a later example, we’ll explore the use of leased, point-to-point lines to build a pri-
vate network. That could be a solution in this case, as well. However, the parties have
chosen the lower-cost option, the public Internet. But using a known, hostile network

eCommerce Website 207
obviously presents a security problem. What would be a solution set that would protect
all the parties to these transactions? Obviously, customers need to be protected. But, in
addition, we have noted that both parties need to be protected from each other as well
as from the omnipresent level of attack on the Internet.
Since each party mistrusts the other, each will have to attempt to ensure that attacks
are not vectored within the valid and expected messages between them. The actual
message stream will be the first attack surface requiring protection. The standard secu-
rity pattern when one cannot trust the source of input will be to thoroughly validate the
input before accepting it: input validation.
The traffic going between the two parties will be the retailer’s customers’ financial
data. When traffic crosses the Internet, one of the features of the TCP/IP protocol is
that the routing of any particular packet does not have a guaranteed itinerary. The
packet may pass over any router that can handle the path to the next destination router.
Although most of the backbone of the Internet passes through major telecom com-
panies’ equipment, which implies a probable level of security practice for those routers,
such routing is not, as noted, guaranteed. Whatever router is currently available for a
path at the moment that a packet arrives will be the router that gets used. A malicious
worker, organization, or government might have control of one or more of the routers
over which any particular packet must traverse, which, of course, means that Internet
routing shouldn’t entirely be trusted. Protection of the messages, protection of sensitive
customer financial information, is essential.
Consequently, in order to protect their sensitive traffic over the Internet, it must
be encrypted in transit. There are two common approaches to this problem: TLS or a
Virtual Private Network (VPN).* The application of each of these technologies depends
upon the probable length of the connection. And since VPN equipment tends to be
more expensive to purchase and to maintain, choice may depend upon the expected
return on investment over the connection.
Either of the foregoing transmission encryption choices provides authentication
capabilities. And either approach can be kept “alive,” that is, active for an extended
length of time.† Since our online retailer requires a robust solution and expects a strong
return on investment, they have chosen to establish a VPN tunnel for protecting cus-
tomer payment transactions between its payment application and the payment process-
ing service.
In order to prevent an attacker from establishing a VPN over which to promulgate
attacks, the VPN between the two entities must be bidirectionally authenticated. VPN
software and hardware is readily available to organizations, individuals, and, of course,
attackers. Without the authentication, a wily attacker could very well establish a VPN
* A VPN can be implemented with TLS. Or a VPN can be implemented with the Internet
Protocol Security (IPsec) protocol. For this example, we will assume that VPN makes use of
† We won’t compare the relative merits of IPsec and TLS tunnels.

208 Securing Systems
with either the payment application at the retailer, or the payment processing service.
Indeed, a perhaps more interesting attack would be to establish a VPN with each party
in order to manipulate the transactions flowing between them. This would be a classic
“man in the middle” attack scenario. That is, the VPN originating and termination
equipment adds an attack surface. Although neither party can protect itself from an
attack originating from within the other party, each must do its utmost to protect their
end and the other side from a malicious third party. Mutual authentication is the obvi-
ous choice. VPN implementations almost always provide robust authentication choices.
The one caveat to that statement is that the authentication must traverse the tunnel
after encryption has been established unless some other form of credential protection
is used, such as an X509 certificate that can be validated. To reiterate, the vast major-
ity of VPN products supply appropriate credential protection, usually in several forms.
The specific details of protected credential exchange don’t change the basic security
One additional protection at the network layer is often employed in these types of
situations. At the network layer, only the IP addresses of the third-party will be allowed
to establish the VPN. In this way, attempts to attack the VPN from other addresses will
be eliminated. This “noise” will simply be thrown away at the network level, saving log
files for any true, anomalous activity. Furthermore, should there be a vulnerability in
the VPN implementation or equipment, there will be some protection at the network
level against exploit. Network restriction, then, becomes a part of the defense-in-depth.
Customer service is a function that has not been included in the architecture exam-
ple. But a customer service function would be a necessity for any successful retailer.
Although some portions of customer service can be automated, it’s also true that some
customers prefer to speak with an understanding human who will take charge and solve
the customer’s problem. And not every problem is amenable to automation; this is par-
ticularly true of complex or multidimensional problems. Sometimes, the service person
must act for the customer, viewing and perhaps changing sensitive data, such as pay-
ment information and other financial details. Customer service automation integration
and customer service representative access are two security problems that will have to
be thought through sufficiently. Though we do not take these up in any example archi-
tecture, I invite you to take some time to consider issues, attack surfaces, and solutions.
How can appropriate access be given to the customer service representative, while at
the same time, making it difficult to steal, either from the customer or the company?
The systems and processes for the security of customer service functions could easily
fill an entire chapter. Perhaps the material could become a book? We won’t take up this
subject here, though it surely would become part of the architecture risk assessment
(AR A) and threat model of a real web store.
Take a moment to consider the types of access to data that customer service would
require to support Web-Sock-A-Rama. Where is the best place to put Support’s auto-
mated systems? If the Web-Sock-A-Rama company hosts the systems, should these
be internal or on one of the external segments? How would the architecture change

eCommerce Website 209
if customer service were provided by a third party? What other security requirements
might support a business decision to outsource this function? Are any of the solutions
already presented applicable to these problems?
As an exercise for you to build additional skill, you might consider the customer
service security issues listed above as security architecture problems.
6.3 Requirements
Table 6.1 outlines the requirements that we uncovered throughout this chapter’s analy-
sis. To provide an organizing principle to the requirements, I’ve separated them into
* A more complete list of typical information security controls and, especially, administrative
and management controls may be found in the NIST standard, 800-53.
Type Requirement
Administrative Strong authentication for administrative roles.
Careful protection of authentication credentials.
Authorization for sensitive operations.
Access on a need-to-know basis.
Access granted only upon proof of requirement for access.
Access granted upon proof of trust (highly trustworthy individuals
Separation of duties between different layers and duty task sets.
Logging and monitoring of sensitive operations .
Monitoring of administrative and management activities must be
performed by non-administrative personnel [typically, the Security
Operations Center (“SOC”)].
Patch management procedures and service level agreement (SLA) on
patch application, depending upon perceived risk of unpatched
Restricted and verifi ed executable deployment procedures.
Hardening against attack of externally facing systems.
Monitor all fi rewall alerts and other signifi cant events for signs of
Access control of authentication and authorization systems.
Network • Implement a three-tier architecture separating HTTP traffic
termination (web server) from application server, and application
server from the data layer. Allow only required communications
between each layer. No other traffic may be allowed to flow
between the layers.
Table 6.1 Web-Sock-A-Rama Security Requirements*
(Continued on following page)

210 Securing Systems
(Continued on following page )
Table 6.1 Web-Sock-A-Rama Security Requirements (Continued )
Type Requirement
Network (Continued ) • Deploy a stateful firewall in front of the HTTP termination.
• Restricted addressability of administrative access (network or
other restrictions).
• Administration and management interfaces and configuration
files must not be reachable from untrusted networks. Only allow
access to administrative assets from the management network.
• Customer financial data stores must be placed on a restricted,
dedicated network.
• Networks will meet PCI standards
• PCI networks must not be visible to untrusted networks.
• PCI networks must have restricted access, need-to-access basis.
• Web authentication systems to be placed upon a restricted
• Network must not be visible to untrusted networks. Only the
HTTP termination may make authentication requests into the
web authentication system.
• The Directory may not be accessed outside the dedicated
authentication network, except for Customer profiles and
• The authentication service network must not allow any outbound
communications to other networks beyond responses to
communications that are to be allowed.
• Use a single piece of trusted networking equipment to separate
each tier of the three-tier web processing architecture.
• Deploy a Web Application Firewall (WAF) between the Internet
and the web server. Alternatively, the WAF may be deployed
between web layer and application server.
• Customer financial data between the payment application
and the third-party payment processing service will traverse a
bidirectionally authenticated VPN.
• Allow only IP address(es) provided by the payment processing
service access to the VPN used to establish an encrypted tunnel
between the payment processing service and the payment
application. Disallow all other traffic to the VPN.
Application • Design application code such that dynamic requests to LDAP and
to databases are built within the application and not received
from users.
• Dynamic input from users, such as user name or item numbers,
must be validated to contain only expected characters.
• Input not matching precisely constrained values must return an
error to the user.
• Response generation must clear all scripting tags found within
stored content.

eCommerce Website 211
Table 6.1 Web-Sock-A-Rama Security Requirements (Continued )
Type Requirement
Application (Continued ) • Do not use object revealing protocols similar to Java Remote
Method Invocation (RMI) in communications with the user’s
• Remove all debugging and coding information from errors and
other user content. Errors shown to user must be user-centric, in
plain (nontechnical) language
• Do not expose any debugging, configuration, or administrative
interface over customer/public interfaces.
• Use per user or session indirect object references.
• Authorize all direct object accesses.
• Include a nonpredictable nonce in the response to a successful
user authentication. Return the nonce for the session with every
authenticated response in a hidden field. Before processing an
authenticated request, validate the nonce from the user’s session.
• Simply avoid using redirects and forwards.
• If used, don’t involve user parameters in calculating the
destination. This can usually be done.
• If destination parameters can’t be avoided, ensure that the
supplied value is valid, and authorized for the user.
• Employ an indirect reference to URLs between client and server
rather than sending the actual value to the user.
• Customer profiles and preferences must have privileges to
insert new user records and to delete records in Directory. No
other privileges may be granted to Customer Profiles. No other
application not a part of the web authentication service is to be
granted privileges in Directory.
• The authentication service vendor must offer substantive proof
of robust fuzz or similar testing on the authentication request
processing chain.
• The authentication service and its hosts must be hardened
against attack, as per corporate or industry standards.
• The authentication service configuration file access must be
restricted solely to administrative applications or modules of
the authentication service. The configuration file and directories
must not be accessible from outside the authentication service
• Applications must pass authentication by the database server
before gaining access to databases.
• Each application must have a unique database authentication
• The payment application must validate and verify the correctness
of every message received from the payment processing service.
VPN = Virtual Private Network; LDAP = LDAP = Lightweight Directory Access Protocol.
Source: Data set in italics is from the Open Web Application Security Project (OWASP) (2013).
OWASP Top 10 List. Retrieved from

212 Securing Systems
three areas: administrative, network, and application. These are necessarily somewhat
arbitrary; the boundaries between these abstractions are relatively “soft,” and perhaps
indistinct. Still, I hope this ordering provides at least some organization to the security
requirements that we’ve uncovered from the analysis so that these are more easily
As we’ve assumed that there is a mature administrative function for this business,
the administrative requirements section is relatively brief. I hope that you, the reader,
can make use of any number of available exhaustive explanations or standards describ-
ing the secure management and administration of computer systems. Repeating those
here, I believe, won’t provide much benefit. Most information security professionals are
already familiar with this body of knowledge.
For exposed Internet sites, networking restrictions and boundaries can become criti-
cally important. Though there are other ways to create trust boundaries, a common way
is to use network segments and then control flows between the segments and between
the systems in the segments. That’s the essential gist of the networking requirements
given in Table 6.1.
Each of the requirements listed in the application section should be familiar from
the analysis. Although there’s always the possibility that I’ve introduced a requirement
into the list that wasn’t discussed or that I’ve missed one that was given in the analy-
sis, I have tried to be thorough and consistent. Hence, if you’ve read the analysis, you
should already be familiar with each of these requirements. As you work through the
list, should you find one or more of the requirements as stated to be ambiguous, simply
go back into the text and refer to the more complete explanation.
As this is the first analysis in our series of six, I caution the reader to understand
these requirements fairly well. In order not to be repetitive, the subsequent analyses will
refer back to these wherever they arise. Remember, architectural patterns repeat, and
I’ve attempted to use the same solutions consistently throughout the examples. This
is not to imply that any of these is the only solution. Hopefully, I’ve made that clear?
Still, one of the points of this book is that there are patterns and their solution sets that
repeat. So I’ve taken some pains to reuse the same solution set rather than introducing
new security defense approaches for the same recurring problem.

Chapter 7
Enterprise Architecture
When a security architect interacts with an enterprise architecture, the work is at a very
strategic level. The ATASM process only loosely applies. There isn’t sufficient specific-
ity in an enterprise architecture to develop a threat model. Once the architecture begins
to be factored into components, it becomes an alternate, logical, and/or component
view. Furthermore, even given a component view of the enterprise, we cannot deal with
components at a sufficiently granular level to be able to discover actual, technical attack
surfaces, or to specify implementable security controls.
Instead, the object of analysis at such a gross level is to uncover the general security
requirements that will enable the enterprise architecture to succeed. Instead of repre-
senting security as a magic box along the side or underneath as some sort of transfor-
mative function that lies within the infrastructure, our task is rather to help our fellow
enterprise architects understand the sort of imperatives that security functions will need
to fulfill. Indeed, the security requirements that are delivered at the enterprise level will
help shape the security strategy for the enterprise. And these enterprise security require-
ments will then seed the security analyses for each solution (set of systems providing
a functionality) that takes its place within the enterprise architecture. Consequently,
architecture analysis of the enterprise architecture is formative and informative rather
than detailed and specific.
For the security architect concerned with building security systems, there is typically
a need for an enterprise security architecture view. Or perhaps like the Open Group’s
Reference Security Architecture, the strategic vision may be expressed as an enterprise
reference security architecture. In this work, we are consuming security systems, not
building them.* Therefore, we will skip over this form of “enterprise security architecture”
* Th ough, of course, a security system should be analyzed and threat modeled just like any
other system. Accordingly, every security system can be analyzed for its security requirements
using ATASM.

214 Securing Systems
in favor of continuing the exploration of securing systems through architecture analysis
of the enterprise architecture, while understanding the constraints by which analysis at
this level abides.
Instead, at the enterprise level one can concentrate on the security features for major
groups of users. Is there a need to keep identities? Identity must be kept for each of the
different groups of users. For instance,
• Customers
• Internal analysts
• Customer service and support
• Administrative staff
• Executives
An enumeration such as the above role list suggests an enterprise-grade identity
system with sub-systems for external and internal authentication. Or perhaps a single
authentication system should be used that can support multiple trust zones securely?
Authorization might also be accomplished through the same system, perhaps through
group membership (a common, simple approach for implementing authorization).
Naturally, there will need to be perimeter controls, such as firewall systems. These
systems suggest a need to gather and analyze the events from low-trust zones, perimeter
controls, and perhaps other activities that should be monitored. This implies a Security
Information and Event Management (SIEM) system. Using a similar reason ing pro-
cess, the search would be continued for all the major types and placements of the secu-
rity systems that will be needed.
Above is an example of the sort of strategic, system-oriented thinking that can be
accomplished when analyzing the enterprise architecture. This strategic thinking breaks
down that large “Security Services” box in Figure 7.1 into the appropriate functions and
components that will be needed to support the enterprise. The enterprise architecture is
a panorama that provides an opportunity to think big about security services as features
of the organization. Attack surfaces are viewed as the business inputs to the enterprise,
rather than trying to provide strategy based upon the surfaces presented at a technology
Figure 7.1 reprises the enterprise architecture that was introduced in Chapter 3.
Study it for a moment and consider the implications of each of the functions rep-
resented. Do presentation layers add an attack surface to the enterprise? How about
an eCommerce presence? The supply chain will interact with an entire business eco-
system of many other organizations. Interactions will probably include both people and
automated flows. Are these third parties to be trusted at the same level as the internal
systems, such as content management or data analysis? Going a step further, are there
threat agents whose goals include the business data of the organization? If so, does that
make the business analysis function or the content management systems targets of pos-
sible interest? Why?

Enterprise Architecture 215
Considering the “why attack” question for the very gross attack surfaces represented
in Figure 7.1 helps to focus the cyber security strategy for the enterprise. Remember,
we’re trying to see what components will be needed.
Or, more likely, the enterprise architecture practice will follow a period of sustained
growth, perhaps years of growth, with little or no enterprise-level architectural order.
Certainly, discreet systems will have been architected. But few organizations begin with
an enterprise view. We work towards such a view once the number of interacting sys-
tems becomes too complex to hold in one person’s mind. Due to this natural matura-
tion of architecture practice, the enterprise view must accommodate pre-existing and
legacy sub-systems. Naturally, some of those systems will be security systems and infra-
structure that have been built to protect what already exists.
Once an organization grows to a complexity that requires an enterprise view, this
view usually includes existing systems while at the same time expressing a vision for
the future architecture. There will be a mix of existing systems and functions, based
upon an existing infrastructure while, at the same time, articulating how the goals of
the organization can be accomplished in a hopefully cleaner and more elegant manner.
Thus, the enterprise security model must also account for existing controls while,
at the same time, it must consider how the architecture can be protected on into the
Figure 7.1 Enterprise conceptual architecture.

216 Securing Systems
future. This consideration includes expected changes in the threat landscape—not just
the threats of today but how these threats appear to be shifting over time. A three- to
five-year horizon is typical, despite the fact that none of us has a line on exactly what
changes will actually occur. Security architects are not seers or prophets, any more than
any other engineering discipline. Still, the virtual “tea leaves”—signs, indications, and
existing trends—can be read to offer a “feel” for likely changes, for likely increases and
decreases in activity.
Enterprise architecture, whether concerned with security or not, is as much about
vision and strategy as it is about documenting what should exist today. As you consider
the questions posed above about the architecture presented in Figure 7.1, think not just
about what might be needed today, but about how this architecture will need to be
protected on into the future, as it grows and matures. What systems will empower the
present while also sustaining anticipated growth?
For instance, what if the existing external identity store has been built for customers
only, and internal resources have to be jerry-rigged into a design not meant to include
them? Does it make sense to collapse internal and external identities into a single system
of truth, into a single all-encompassing design?
I had a situation precisely like this in which a multi-year effort did, in fact, collapse
the three distinct systems of identity into a single system. Much design needed to be
done in order to fulfill the needs of customers, partners, and the work force (which
itself was made up of three distinct sub-groups). Then, a system that could support
all the different legacy environments, while at the same time empowering an entirely
new infrastructure, had to be architected and then built. Ultimately, the collapse to a
single source of truth saved much effort that had been expended to keep all the differ-
ent systems synchronized. Money was eventually recouped and then saved. The design
was much simpler to maintain. As an enterprise-level example, a decision was made to
replace functioning systems, including a number of synchronizing applications, all at
cost to the enterprise in order to support an emerging enterprise architecture on into the
future. These are the types of strategic decisions that are made with enterprise views.
Thinking about the ATASM process, we do not know anything about the purpose
of this enterprise architecture, or the organization that fields it. Although we can cer-
tainly make some guesses that help, the first step, as previously laid out, is to research
the purpose of an architecture in the context of the organization’s objectives. Obviously,
this architecture is intended to sell something, perhaps something tangible?
Let’s find out what kind of enterprise we have diagrammed. As ATASM suggests,
”start at the beginning.” Even though we will be working at quite a high level, we still
must know what it is with which we’re working. Of course, if this were an enterprise
architecture representing an existing enterprise, a security architect would probably
not be encountering the diagram in isolation. Even if brand new to the organization,
presumably during the hiring process the architect would’ve found out what the major
goals of the organization are. In other words, encountering an enterprise architec-
ture completely in isolation from the organization itself is a fairly artificial situation. I

Enterprise Architecture 217
daresay that a security architect is rarely presented with a conceptual enterprise archi-
tecture diagram in the absence of knowledge about the organization supposedly repre-
sented by the diagram.
Even though analyzing an enterprise architecture in isolation from the organization
is a relatively artificial situation, as a methodology for learning and practicing, let’s pre-
tend that we, the security architects, have just encountered an enterprise architecture
about which we know nothing. Given our lack of understanding, the starting questions
must be, “What does the organization do?” And, “What does the organization expect
to get from its digital systems?”
7.1 Enterprise Architecture Pre-work: Digital Diskus
This enterprise is called Digital Diskus. They design, manufacture, and sell network-
ing routing equipment. Digital Diskus’ customers are medium and large organizations
that must maintain extensive networking infrastructure. The company has a sales force,
as well as channel partners—companies that provide networking equipment and net-
working expertise to their customers. These partners install, configure, and, perhaps,
also run large and complex networks. Digital Diskus’ vision statement is, “Design and
build the most dependable and the easiest to configure networking equipment.”
The company offers a wide range of sizes and feature sets to accommodate all por-
tions of the modern, interconnected enterprise. Digital Diskus’ products can be used
in core networks, throughout the corporate campus, and beyond to satellite offices.
Hence, they offer switches, routers, and a whole variety of network-related equipment
that would be needed to set up a modern, enterprise-grade network that spans many
localities. Although the company does offer security features built into routers and
switches, they do not consider themselves a “security” company.
Digital Diskus’ sales are placed through the company’s Internet facing eCommerce
site. Sales can be made directly by a customer via an online store front, through one
of the partners, or through the direct sales force. The company tries to automate their
supply chain as much as possible, so there is a need for automated interchange between
the parties within the supply chain and throughout the purchasing ecosystem, just as
there is within the sales process.
Digital Diskus’ goal is to provide highly dependable solutions in which customers
can have great confidence. Quality is much more important than price. A prolonged
mean time before failure (MTBF) is considered a competitive advantage of the com-
pany’s networking products. The company brand depends on customers who will pay
more for the higher confidence and expectation of the company’s vaunted dependabil-
ity. In addition the company’s highly trained customer support staff is an aspect of the
company’s sales pitch.
Nevertheless, the company seeks to maximize profits through careful management
of components and inventory when manufacturing its products. Since the company does

218 Securing Systems
not compete on price, it cannot afford flaky, poor quality components. Components of
the products don’t have to be boutique or specially created; components don’t have to be
of the absolute highest quality. Networking equipment manufactured by the company
must perform as described and continue to perform with as few failures as possible.
However, high performance is not one of the company’s competitive advantages.
The company has not expanded far afield from its core business. Acquisitions and
mergers have been strategic to bolster the core networking line of products. Senior
manage ment have been reluctant to stray far from a business model and set of technolo-
gies with which they are familiar and with which they feel comfortable. Their motto is,
“Networks you can depend upon.”
Digital Diskus’ executive staff are very protective of the brand and the goodwill of
the company’s customers. The company’s customers have been willing to pay a pre-
mium for an assurance that they are also purchasing access to significant expertise in
networking. A major security incident involving one or more products is viewed as an
unmitigated disaster. Consequently, the organization is rather risk averse, especially
with respect to cyber security.
Furthermore, the company’s designs and manufacturing secrets are considered
highly proprietary. Indeed, code that implements encryption algorithms is held very
closely; encryption implementations are seen not only as trade secrets but also as a trust
that protects the company’s customers.
At this point, considering the foregoing, do you have a feel for the company, Digital
Diskus? Do you know enough about its mission, its organizational objectives, and its
risk appetite to place Figure 7.1 in an organizational context? If not, what more do you
need to understand? If something is unclear, take a look at the conceptual architecture
diagrammed in Figure 7.1 while considering the introductory paragraphs describing
Digital Diskus.
7.2 Digital Diskus’ Threat Landscape
Since Digital Diskus’ products include encryption implementations, might one or more
entities be interested in the cryptography implementations? What if the company’s
products are deployed by governments, some of whom are hostile to each other? Might
one or more of these nation-states be interested in manipulating or compromising cryp-
tography in use within the networks of one of its enemies?
The questions posed in the last paragraph should be asked by those responsible for
threat analysis for the company. Whether or not the executive team of our fictitious
enterprise have actually considered these questions, we are going to assume that the
company believes that it may be the target of industrial espionage. The security team
has uncovered a number of incidents that may indicate at least some interest by indus-
trial espionage threat agents. As far as anyone in the company knows, as yet, no major
breaches have occurred.

Enterprise Architecture 219
Obviously, as we have previously analyzed, any company that has an Internet pres-
ence must be concerned with cyber crime. Beyond the public facing portions of the
company’s websites, an extensive ecosystem of partners and suppliers is primarily main-
tained over the public Internet.*
The company is very careful with the brand, allowing only limited use of the logo
and name beyond Digital Diskus’ marketing materials. Financial data are made avail-
able to qualified individuals only. Nevertheless, salesman, sales partners, suppliers, con-
sultants, contractors, and others will access company systems, some of which handle
sensitive data. Each organizational member of the ecosystem must guarantee that only
qualified and trustworthy individuals have access to the company’s systems. In real-
ity, the company have no guarantee of trust, given the extent of the ecosystem and
the number of individuals who must have access. Furthermore, given the thousands
of people who must interact with the company’s systems, it would be foolish to believe
that every individual who has access is honest and reliable. It is perhaps also fool-
ish to believe that every connected organization can meet rigorous security practices.
Consider the following quote:
The attackers reportedly first gained access to Target’s system by stealing credentials from
an HVAC and refrigeration company, Fazio Mechanical Services, based in Sharpsburg,
Pennsylvania. This company specializes as a refrigeration contractor for supermarkets
in the mid-Atlantic region and had remote access to Target’s network for electronic
billing, contract submission, and project management purposes.1
When I worked at a company that had 70,000 employees and also had another
50,000 contractors with access to the network, I often heard people say to me, “but this
application is internal only.” This statement meant, of course, that the system wasn’t
exposed since only “trustworthy” people had access. People’s faith in the ability of the
company to hire and contract only high-integrity individuals was certainly laudable
but, perhaps, not entirely well founded? I often responded to these comments by saying,
“In any city of 120,000 people, will there be at least a few criminals?” In fact, in any
city of 10,000 people, there are likely to be at least a few individuals who are seeking a
special advantage, sometimes even a criminal advantage.
Now it is true that enterprises tend to be somewhat picky about who they hire.
Indeed, it’s common to check criminal records and other similar barometers of integrity
before hiring. At least it’s common in the United States, where it’s legal to do this kind
of research. Other countries forbid background checks because they see it as an invasion
* At the risk of a complicated fi ctitious example, several of Digital Diskus’ business partners
maintain point-to-point connections. Th ese are implemented either with leased lines over
common carriers or point-to-point Virtual Private Network (VPN) tunnels that cross the
Internet. All of these connections are managed through the company’s extranet.

220 Securing Systems
of personal privacy. And in still other countries, the records simply are not good enough
to be trusted.
One could be led to believe that through performing some form of pre-employment
due diligence, the choice of employees and contractors who’ve passed muster would
eliminate at least some criminal activity. Still, I have personally been party to a number
of investigations involving fraud and theft by insiders. Although the percentage of those
who might be more criminally inclined is probably less within any global enterprise’s
employees then the per capita general average, insider attack cannot be eliminated.
Certainly, insider attack will not be eliminated when considering populations in the
tens of thousands or even hundreds of thousands of individuals, no matter how much
prescreening and selectivity has been performed.
The foregoing suggests some possibility for dishonest behavior. In addition, the pro-
pensity for humans to make honest mistakes must be added to the integrity angle. The
larger the range of experience and sophistication, the more the likelihood of error based
upon misunderstanding or just plain inattention. With populations running into the
thousands, the probability of error by commission or omission amplifies significantly.
People make mistakes; sometimes even careful people make dumb mistakes.*
Taking the above demographics into account, any architecture that includes auto-
mation, stretching from post sales back through the sales cycle and on into the manu-
facturing and supply chain networks and business relationships, will have to account
for possible insider threats. In short, those who have been granted access for legitimate
business reasons cannot be considered entirely trustworthy.
Because the company provides networking equipment to many companies and
organizations, including diverse governments, the company’s name has appeared occa-
sionally among the lists of companies related to one controversial issue or another, even
being listed as a party to competing controversies. Although the company attempts to
maintain a neutral stance politically, its customers do not. Due to this exposure, digi-
tal activists have occasionally indicated some interest in the company. Indeed, “hack-
tivists” have instigated direct cyber actions against lists of companies that have included
Digital Diskus. To date, there’ve been no incidences from digital activism. Still, com-
pany threat analysts believe that, in the future, the company may become an unwilling
activist target despite its best efforts to take no stance on political or divisive issues.
Beyond the possibility of someone with malicious intent gaining access to the com-
pany’s systems, the company is also concerned about a disgruntled or rogue insider who
must be given privileged access in order to carry out duties. The company employs a
fairly extensive technical administrative staff in order to run the various systems that
make up the company’s digital ecosystem. Should one of these highly privileged indi-
viduals, for whatever reason, decide to use her or his access to hurt the company, a
great deal of damage might ensue. Thus, technical staff are considered to be among the
company’s significant threat agents.
* Despite my best eff orts, I’ve been known to make very dumb mistakes.

Enterprise Architecture 221
Digital Diskus staff are concerned with four major classes of threat agents:
• Industrial spies
• Cyber criminals
• Cyber activists
• Privileged insiders
7.3 Conceptual Security Architecture
Every single application that interacts with users must have a user interface (UI). This
is, of course, obvious. From the diagram shown in Figure 7.1, it might be construed that
some economy of scale is sought through building a single enterprise presentation layer
through which every application, no matter its purpose or trust, interacts. That would
be a great question to ask the enterprise architect who drew the diagram.
Typically, a conceptual architecture is trying to diagram gross functions and pro-
cesses in relationship to each other in as simple a manner as possible. Simplicity and
abstraction help to create a representation that can be quickly and easily grasped—the
essence of the enterprise is more important than detail. An enterprise architecture tends
toward gross oversimplification.
Although it is possible to build one single presentation layer through which all inter-
actions flow, if legacy applications exist, attaining a single presentation layer is highly
unlikely. Instead, the diagram seeks to represent the enterprise as a series of interrelated
processes, functions, and systems. A great deal of abstraction is employed; much detail
is purposely obscured.
It is rather unlikely that a single presentation layer is actually in use at Digital
Diskus, much less planned. Instead, the diagram suggests that software designers think
strategically about the sorts of architectures that will enable presentations to be logi-
cally separated from business processing. And that is actually the point: This archi-
tecture is intended to underline that business processing must not make its way into
the presentation layers of the architecture. Presentations of digital systems should be
distinct from the processing; systems should be designed such that they adhere to this
architectural requirement.
In the same way that this canonical presentation layer is conceived as a part of
an enterprise architecture, security is represented by a unitary function that can be
abstracted and set to one side. As we have seen several times previously, this notion is
patently erroneous. Instead, for modern information security practices at an enterprise
level, computer security might be summarized through the architecture security princi-
ples that have been adopted by the organization. The organization’s architects expect the
security principles to emerge from the systems that make up the enterprise architecture.
The following set of security principles from the Open Web Application Security
Project (OWASP) are reprised from Chapter 3:

222 Securing Systems
– Apply defense in depth (complete mediation).
– Use a positive security model (fail-safe defaults, minimize attack surface).
– Fail securely.
– Run with least privilege.
– Avoid security by obscurity (open design).
– Keep security simple (verifiable, economy of mechanism).
– Detect intrusions (compromise recording).
– Don’t trust infrastructure.
– Don’t trust services.
– Establish secure defaults2
Of course, parroting typical security principles as a “security architecture” isn’t going
to be all that useful when building out the security for the enterprise. Very quickly,
astute architects and implementers are going to ask, “What do we build in order not to
trust the infrastructure? And why shouldn’t we, anyway?” Those seem to be perfectly
reasonable questions based upon a need to “make the conceptual architecture real.”
The trick is to translate principles into high-level imperatives that conceptually out-
line the security requirements that the enterprise will need. Once we have a handle on
the threat agents that are relevant for Digital Diskus, we will attempt to outline the
imperatives that will guide the enterprise architecture implementation. It is perhaps
useful to reiterate that we must work at a very high, nonspecific level. The “how” part
of implementation remains unknown.
7.4 Enterprise Security Architecture Imperatives
and Requirements
At a strategic, enterprise level, what other architectural elements, that is, security
requirements for the enterprise, might the company consider employing?
As we explored earlier, industrial espionage actors may employ sophisticated attack
methods, some of which may have never been seen before.* And, espionage threat
agents’ attacks can span multiple years. They will take the time necessary to know their
quarry and to find weak points in the systems and people who constitute the target.
Therefore, at the enterprise level, decision makers will have to be prepared to expend
enough resources to identify “low and slow” intrusions.
Furthermore, the company’s executive staff should be prepared to respond to a par-
tial breach. An attack may be partially successful before it is identified and can be
stopped. With such determined and sophisticated attackers, part of the security strat-
egy must be to frustrate and to slow ongoing, sophisticated attacks. This strategy lies
in contrast to belief that defenses are so well constructed that no attack can be at all
* So-called “zero day” attacks.

Enterprise Architecture 223
successful. Hence, security alert and event monitoring will be a critical part of the
defense in order to catch and respond to events before they become disastrous.
Because of the probability of partially successful attacks, compromise containment
can be built into the architecture such that successful attacks cannot be used as a beach-
head to move from a higher zone of exposure to less exposed areas. Architectures must
emphasize strong boundaries. Crossing a boundary must require higher privileges and
hopefully an entirely different attack methodology. These defensive tactics will subvert
and contain a successful exploit sufficiently so that the security operations team has
time to respond and eradicate. This suggests an architecture in which discrete units of
functionality, let’s say, applications, have highly limited access to resources beyond their
own area.
In Figure 7.1 you will see that almost every function is connected to the integration
systems. Whereas all applications, or least most of them, are integrated through tech-
nologies such as a message bus, one of the architectural imperatives will be application-
to-application and application-to-message bus access control. That is, each contained
set of functionalities is allowed only to integrate through the controlled integration
system (the message bus) on an as-needed and as-granted basis. No application should
have unfettered access to everything that’s connected to the integration system (here,
the message bus and other integration mechanisms).
Another architectural imperative might be to insist that messages passed to the mes-
sage bus are generated within the application code. No message destined to cross the
message bus is allowed to come from outside the application that sends the message.
What this means in practice is that whenever onward-bound data arrive in applica-
tions, each application’s code must extract the bits that will be passed through the
message bus and re-form these into a different message package, a different message
construction, before passing along the data. Although such an approach might seem
burdensome, it is an effective protocol-level barrier to getting attack payloads through
an application to some following target. The attacker must understand the new mes-
sage form and anticipate the subsequent message processing while still getting the
attack through the initial packaging and format. Somehow, an attack destined to be
passed through the application has to get through any message validation while also
anticipating an alternate encapsulation. While not perfect, this method of protection
can slow the attacker down as an attack attempts to go beyond the initial, processing
application. Again, containment is not about a perfect defense, but rather about frus-
tration and befuddlement.
Anything engineered by humans can be reverse engineered and understood by
humans. This is one of the fundamental laws of computer security. Nevertheless, it
takes time and effort to understand something, especially if the “something” is complex.
Part of containment can be the art of confusing by introducing unexpected shifts in
protocol, message form, transaction state, and encapsulation. It’s not that the defender
expects the attacker never to figure these shifts out. They will. Rather, the art is to give
the defenders’ reactive security capability sufficient time to discover and then respond.

224 Securing Systems
One additional imperative for the message bus should be protection of all traffic in
transit, perhaps provided by encryption. The encryption would have to be kept appro-
priately performant. One of the downsides of encryption is that it adds time to overall
message delivery. That’s a non-security topic that we won’t take up here. However, in
the real world, experienced architects are going to ask the hard performance questions.
A security architect will need to answer these such that encryption gets built that does
not degrade overall performance and related service level agreements (SLAs) around
message delivery that must be met. Otherwise, the encryption will never get used, per-
haps never built.
Ignoring the performance question, we have already stated that the message bus
appears to connect to almost every system. That is not a reason to encrypt or otherwise
provide message protection. Encryption does not protect the connecting systems from
each other; that is not the purpose of encryption. The key determiner for transmission
protection is the fact that Digital Diskus networks are highly cross-connected with
other entities. Even with strong due diligence activities about the security of the third
parties, their security is out of the control of Digital Diskus. Because so many enti-
ties are involved, the sheer complexity is a breeding ground for mistakes. Additionally,
there are many people who have access to the networks who must have access to various
systems, all of which connect to the bus. This level of exposure strongly suggests mes-
sage protection, if for no other reason than inadvertent disclosure. Add on that Digital
Diskus staff are concerned about industrial espionage, and a strong case is made for
message protection. The obvious solution to that is to encrypt every message that tran-
sits the bus. Alternate protection would be to highly restrict the network equipment,
restrict visibility to message bus transmissions, a segmented network, switching rather
than hubs, and so forth. As a networking company, these capabilities shouldn’t be too
difficult. However protection is implemented (encryption or network controls), protect-
ing message bus communications must be an imperative.
Obviously, if our enterprise’s risk posture is incident adverse, and the company also
intends to be fairly mature in its computer security practices, the company’s adminis-
trative functions will follow typical, industry-standard practices when maintaining the
company’s systems. Previously, we surveyed some of the activities that might constitute
such a practice. There are numerous works devoted to precisely this subject. Standards
such as NIST 800–53 delineate the practices and tasks that are required. Hopefully,
there’s no need to reiterate these here? Please assume that our digital enterprise is doing
its best to meet or exceed access control, configuration and change control, patching,
and the panoply of tasks that make up a mature administrative practice.
Even so, as an enterprise security architect, I would underscore the importance of a
mature practice. It might be useful to review and assess the current body of practice to
identify weak links. This review will be performed on a periodic basis. Such assessment
could probably be considered a part of the strategic enterprise security architecture.
Given the importance of a mature and solidified practice in this arena when the threat
agents and their methods are considered, no matter how comfortable the organization

Enterprise Architecture 225
is with what it has done, everyone should remember that sophisticated adversaries will
be studying current practices for weaknesses, continually poking at these on a regular
basis. In other words, attackers are intelligent and adaptive.
Speaking of the administrative staff, our enterprise is also concerned about a rogue
insider who might launch malicious action out of emotional distress. Part of the enter-
prise security strategy would be maintaining the good graces of those individuals who
must have privileged access. It would behoove Digital Diskus’ executive staff to try
to ensure that technical staff aren’t given obvious and compelling reasons to hurt the
company. It may seem odd to speak about human resources policies as a part of a secu-
rity strategy. But indeed, people do get angry. And angry people can do malicious and
destructive things.
Beyond the need to try and keep employees happy so that they will perform their
functions with integrity, part of the security architecture for the enterprise must be an
attempt to limit the damage that any single individual can do. This leads to standard
controls (again, NIST 800-53 describes these), such as strict access control. No single
individual can be allowed to have control over an entire subsystem and all its func-
tioning parts or infrastructure. For instance, database administrators should only have
access to the database administrative functions, not to the administrative functions
of the systems upon which databases are run. Strict separation of duties needs to be
employed so that only system administrators have access to the system but don’t have
the credentials to databases. For severe functions, such as changing a critical system cre-
dential or bringing a critical system down, several individuals with distinct and separate
roles and scope must agree. No single or even pair of individuals, especially those who
have the same precise function, should be able to perform any action that could have
severe impact on the operation of the enterprise.
Alongside these typical controls, significant activities such as changing the configu-
ration of a production system or database should be performed by independent teams
who have no interest in or take any part in the applications and other functions running
on the systems. This is standard separation of duties. The concept is used to parcel out
privileges such that two or three individuals must agree (i.e., “collude”) in order to actu-
ally apply critical changes. This control is not perfect, of course.* This control should
not be solely relied upon in isolation. Rather, separation of duties compliments access
controls and privilege-granting processes. The intention is that any single person can-
not have a wide impact when acting alone.
By analyzing the conceptual enterprise architecture, taking into account Digital
Diskus’ mission and risk appetite, and in light of the relevant threat landscape, we have
uncovered the following conceptual requirements:
* Prevention of collusion between several people if these have the right mix of privileges is a
rather diffi cult problem to prevent. No doubt, spy agencies have methods of prevention, but
typically, information security controls begin to fail when more than two individuals act in
concert. Th ree well-placed people can probably get around most technical control systems.

226 Securing Systems
• Strict administrative access control
• Strict administrative privilege grant
• Mature administrative practices (cite NIST 800-53 or similar)
• Robust and rigorous monitoring and response capabilities (external and internal)
• Strict user access controls (authentication and authorization)
• Access control of automated connection to integration technology, especially the
enterprise message bus
• Policy and standards preventing unfettered send or receive on the message bus,
coupled to strict, need-to-communicate, routing on the bus
• Application message recomposition when a message is sent from external to
internal systems
• Encryption of message bus communications
Obviously, based upon the previous architecture analysis of a web application, the
web presences of Digital Diskus would have the same security needs. Since we’ve already
completed that analysis, I won’t add those requirements here. Still, you may want to
take some time to review the requirements for Web-Sock-A-Rama, turning these into
high-level, conceptual architectural imperatives. What are the web and extranet imper-
atives that should be applied to Digital Diskus’ conceptual architecture?
At the enterprise level, the security architect needs to be thinking about the strat-
egy that will ensure the security posture intended by the organization. The activities
and methods listed in the foregoing paragraphs are not intended as a complete recipe.
Rather, the activities listed above are offered as samples of strategic architectures (which
can include processes) that will become an enterprise security architecture which sup-
ports the enterprise architecture at its conceptual level. Indeed, the foregoing discus-
sions indicate particular strategies in response to anticipated threats. These imperatives
describe the security architecture as suggested by the conceptual architecture given in
Figure 7.1. At this conceptual level, the security architecture remains highly conceptual,
as well. Specific technologies are not important. Certainly, specific products are not
needed in order to form a conceptual enterprise security architecture. The architecture
is “conceptual,” that is, expressed as security concepts.
Identity is already represented in Figure 7.1. In order to conduct business within
an ecosystem that includes customers through the supply chain, identity management
will be essential. Integration is also represented in this conceptual view that every
business function will integrate with others: Sales via eCommerce must pull from the
supply chain. In order to deliver a presence that can drive commerce via the web, con-
tent will have to be created, revised, and managed. Business intelligence depends upon
the data generated from the other business functions: commerce, customer relations,
and the supply chain, as well as from the internal applications and processing neces-
sary to support these activities. Because this enterprise seeks to automate as much of
its processing as possible, the enterprise architecture includes an orchestration concep-
tual function.

Enterprise Architecture 227
7.5 Digital Diskus’ Component Architecture
Figure 7.2 begins the process of separating the conceptual architecture given in
Figure 7.1 into its constituent components. We continue to operate at the enterprise
Figure 7.2 Enterprise component architecture.

228 Securing Systems
level of granularity, that is, view the architecture at a very abstract level. Individual
technologies and implementations are ignored. This view seeks to factor the concepts
presented previously into parts that suggests systems and processes. We have taken the
liberty to also introduce a distinction in trust levels and exposure by separating the
internal from the external, web presences from business ecosystem connections (the
“extra-net” cross hatching in the upper right), and to even distinguish between cloud
services and the Internet.
Although the cloud services are likely to be accessed via the Internet (hence, they
form a distinguished “area” in the diagram), cloud services will usually be run by enti-
ties that have had their security investigated and approved. This is opposed to traffic
originating from other, uninvestigated entities on the Internet. The Public Internet is,
as we know, a commons whose entities generally must be treated as essentially hostile,
given no further information. Cloud services may be viewed with some suspicion, but
these services have to be trusted sufficiently such that they support business functions.
The Internet deserves no such trust. For this reason, the two abstract areas are viewed
as separate clouds, separate assurance levels.
You will notice that the aggregated “presentation” function from Figure 7.1 now
has two components: “Internal Presentations” and “External Presentations.” Each of
these is represented as a series of separate presentation-layer components comprising a
larger abstraction—the “internal” or “external” presentation layer. As described above,
this component architecture expresses the imperative of the conceptual view by declar-
ing that presentation services are separate functions from the underlying applications.
In order to keep a cleaner representation, separate application rectangles connected to
separate presentation layers have not been drawn.
At this level, trying to separate out each application and its particular presenta-
tion would introduce too much detail and is hopefully unnecessary to understand
what components comprise the architecture. Instead, the architect and, specifically,
the security architect can assume some mapping that makes sense between these two
components. Clearly, there are multiples of each type of application. When reviewing
actual application instances that are individual parts of this component architecture,
the detail of precisely what links to what will be required. At this level, we are interested
in gross groupings of functions that can be understood so that when an application is
deployed to this infrastructure, we can understand the application components that
must get diagrammed in detail. Further, we will want to convey to application design-
ers just how their applications must be factored in order to express the particulars of
the expected components.
Given the architecture of a third-party, off-the-shelf system, we can quickly assess
whether or not it’s going to fit into the structures that Digital Diskus employ. If you
remember Figure 3.2 (from Chapter 3), you may now understand just how little archi-
tecturally useful information the diagram conveys? Can you place that “architecture”
within the structures given by Figure 7.2? When I was presented with an architecture
diagram quite similar to the marketing architecture for a business intelligence product

Enterprise Architecture 229
that was previously shown, I was at a loss to place any part of the diagrammed system
within the logical architecture with which I was working at the time (which looked
somewhat similar to the AppMaker zones).
The component enterprise architecture provides a guide for how to separate the
functions that comprise any particular system that becomes a part or expresses the
enterprise architecture. Consider the following patterns: Business intelligence applica-
tions are internal and will likely be called out as distinct from the internal business
applications, processing, and data upon which the intelligence analysis depends. The
content management system will also be separately diagrammed, perhaps due as much
to maintaining legacy systems as to the introduction of web content as a separate and
distinct discipline. Content must be transported and stored in the externally available
zone so that it can be presented through those systems. This architecture protects the
content creation and management functions from exposure by exporting published
content to the external stores. If a new content management system is to be introduced,
the patterns listed above (and more, of course) can be applied to create an architecture
that will fit into and support the component architecture given in Figure 7.3.
Likewise, integrations and their mechanisms will be distinct systems. In the same
way, any system that provides identity services or storage must be internal, while also
supporting an external copy that can be hardened against exposure. At the same time,
the customer-supporting and business-enacting applications must be available exter-
nally. An external infrastructure supports the external applications; these external
applications have to be written to that discrete, external infrastructure.
As was previously discussed, a message bus spanning trust boundaries presents a
particular security problem. Such a bus may provide a path for attacks to get passed
within messages from less trusted, exposed systems, such as the customer or eCom-
merce applications, to any of the more sensitive or higher-trust systems located in the
internal zone. As we noted through the conceptual architecture discussion, great care
should be taken to authenticate every bus access, as well as to review the message and
transaction paths through the bus such that a compromised application can neither get
control of the message bus nor attack systems to which it has not been granted access.
In the component architecture given by Figure 7.2, the external versus the internal has
been delineated. You will note that the message bus crosses that boundary. Further, the
externally accessible portal to the bus is a part of the external infrastructure (the bus
crosses the infrastructure box, with an open end on the external side).
There will probably be access-granting policies and procedures that protect the bus
from rogue applications. Hence, the bus must pass through the Access Controls &
Validation security systems (which, at this granularity, remain largely ).
You will note that some of the security imperatives listed for the conceptual architec-
ture appear in Figure 7.2. The monitoring system has been separated as a component.
Access controls and validations are diagrammed at the entrance to externally exposed
systems and between external and internal zones. The control layer between exter-
nal and internal must function in both directions. Obviously, harmful and disallowed

230 Securing Systems
Figure 7.3 Enterprise component fl ows.

Enterprise Architecture 231
traffic must be kept out of the internal network. In the other direction, those who
have no need-to-access external systems, most especially the administrative accesses,
are prevented from doing so. Figure 6.7 of the AppMaker web system represented at
its bottom an administrative access control layer preventing inappropriate internal to
external access. Trying to separate these two layers in Digital Diskus’ enterprise view
will be too “noisy,” or too busy, adding too much detail to grasp easily. Therefore, at
this component enterprise level, the two similar functions have been abstracted into a
single control layer lying between the two trust and exposure zones.
Figure 7.3 adds data flows between the components depicted on the enterprise com-
ponents view. Not every component communicates with every other. However, func-
tions such as process orchestration will interact with many applications and many of
the databases and data repositories. Each instance of a particular orchestration will,
of course, only interact with a select few of the components. However, at this gross
level, we represent orchestration as a functional entity, representing all orchestrators as a
single component. Hence, you will see in Figure 7.3 that Process Orchestration interacts
with a wide variety of the internal systems. In addition, Orchestration has access to the
Message Bus, which pierces the trust boundary between internal and external systems,
as described above.
Orchestration, for example, is one of the components with a high degree of intercon-
nectivity. Both Data Analysis and Content—creation, deployment, and management—
similarly touch many internal systems. Likewise, the Supply Chain applications must
interact externally through the External Integrations, as well as send messages through
the Message Bus to many business applications and databases. When a complex process
must be modeled through automation along the supply chain, orchestrations will likely
be created to order transactions properly. The same would be true for eCommerce or
customer interactions. In other words, many of the systems, external and internal, must
interact through various communications means. The number of attack surfaces multi-
plies exponentially as these complex systems integrate.
Take a moment to consider which threat agents are likely to attack the various systems
in this architecture. Then consider their attack methods: Which are the juicy, valuable
targets shown here? Where might particularly proprietary trade secrets lie? Company
financial information? Customer information (subject to privacy issues, as well)?
You may observe that security monitoring takes events from both access control
layers. But there are a few other systems that also deliver events to be monitored and
perhaps acted upon. The external infrastructures and Service Oriented Architecture
(SOA) management have flows to Security Monitoring. The Message Bus would nor-
mally, as well. Any system that breaches trust boundaries has the potential to vector
attacks and, therefore, must be watched for attack-like and anomalous behavior. In a
real enterprise, the Security Operations Center function (SOC) would take events and
alerts from almost all the disparate systems with any potential for misbehavior. SIEM
could be a topic for an entire book, itself.

232 Securing Systems
After adding these gross level flows to the enterprise component architecture, as
seen in Figure 7.3, hopefully you will see that the diagram becomes unwieldy. There
are too many arrows connecting to multiple systems to easily delineate the relationships
between any particular subset. This is intentional on my part in order to highlight the
need for breakout views of subsystems. An overly crowded diagram becomes as mean-
ingless as an overly simple one.
Take as an example, the directory copy (often, an LDAP* instance) in the external
area at the upper end of Figure 7.3. Many of the external applications within each appli-
cation type, as well as presentation layers, authentications, and so forth, must interact
with the directory. In addition, the directory copy must be updated from the master
directory that exists within the internal network. Even at this gross level, where particu-
lar applications are not specified, there have to exist arrows pointing to Directory Copy
from almost every external component. Given that these components also must com-
municate outwardly, receive communications inbound, and send messages through
the message bus to internal components, just the representation of Directory Copy’s
interactions, by themselves, multiplies communication flows beyond the eye’s ability to
easily follow and beyond easy comprehension.
Figure 7.3 then becomes too “busy,” or “noisy,” to be useful, even if this figure does
represent in some manner, flows between components. At this point in an assessment,
the architecture should be broken down into subsystems for analysis. Hence, we will
not continue the assessment of this enterprise architecture any further. Even using a
gross component view at the enterprise level, an assessment focuses upon the general
security strategy for the enterprise:
• Threat landscape analysis
• Organizational risk tolerance and posture
• Security architecture principles and imperatives
• Major components of the security infrastructure (e.g., identity and security
• Hardening, system management, and administrative policies and standards
Our next architecture assessment will be of the data analysis and business intelli-
gence component of this enterprise architecture. This subsequent Digital Diskus analy-
sis will afford us an opportunity to dig into subsystem particulars in more detail.
7.6 Enterprise Architecture Requirements
At the enterprise level, security requirements are generally going to devolve to the secu-
rity infrastructure that will support the enterprise architecture. That is, the conceptual
* Lightweight Directory Access Protocol.

Enterprise Architecture 233
“security services” box in the enterprise conceptual diagram will have to be broken out
into all the various services that will comprise those security services that will form an
enterprise security infrastructure.
Because we have moved infrastructure security architecture outside the boundaries
of our scope and focus, there are no specific requirements given beyond the examples
outlined in this chapter. Certainly, each of the following architectures that has some
relationship to a larger organization will necessarily consume portions of a security
infrastructure. Therefore, we assume for the relevant subsequent assessment examples
that a security infrastructure is in place and that it includes at least the following:
• Firewalls that restrict network access between network segments, ingress, and
perhaps, egress form the enterprise architecture
• An ability to divide and segment sub-networks to trusted and untrusted areas that
define levels of access restriction
• An administrative network that is separated and protected from all other networks
and access to which is granted through an approval process
• A security operations Center (SOC) which monitors and reacts to security incidents
• An intrusion detection system (IDS) whose feeds and alerts are directed to the
SOC to be analyzed and, if necessary, reacted to
• The ability to gather and monitor logs and system events from most if not all
systems within the enterprise architecture
• An audit trail of most if not all administrative activities that is protected from
compromise by administrators
• An enterprise authentication system
• Some form of enterprise authorization
The foregoing list, while not exhaustive, will provide touch points into the enter-
prise security architecture for the example analyses in Part II.
1. U.S. Senate Committee on Commerce, Science, and Transportation. (March 26, 2014).
A “Kill Chain” Analysis of the 2013 Target Data Breach. Majority Staff Report for
Chai rman Rockefeller.
2. Open Web Application Security Project (OWASP). (2013). Some Proven Application
Security Principles. Retrieved from Category:Principle.

Chapter 8
Business Analytics
An enterprise architecture can be thought of as a map of the interactions of the individ-
ual systems that comprise it. One of the systems expressed in Figure 7.2 is the business
intelligence and analytics system. Typically, these systems handle business proprietary
and sensitive data about the performance, strengths, and weaknesses of business execu-
tion. Therefore, business intelligence systems are usually not exposed to untrusted
networks and to untrusted parties. The Digital Diskus data analysis and business intel-
ligence system exists only on the company’s internal network. No portion of the system
is exposed on either of the external zones (Internet and Extranet).
8.1 Architecture
We begin with the first “A” in ATASM: “architecture.” First, we must understand what
a business analytics/intelligence system does and a bit about how the analysis function
works. The diagram given in Figure 8.1 has had enterprise components not directly
connected with the business analytics and intelligence system removed. Figure 8.1 may
be thought of as the gross enterprise components that are involved in business data min-
ing for Digital Diskus.
[D]ata science is a set of fundamental principles that guide the extraction of knowledge
from data. Data mining is the extraction of knowledge from data via technologies that
incorporate these principles.1
Like many enterprises, Digital Diskus has many applications for the various pro-
cesses that must be executed to run its business, from finance and accounting to sales,
marketing, procurement, inventory, supply chain, and so forth. A great deal of data is

236 Securing Systems
generated across these systems. But, unfortunately, as a business grows into an enter-
prise, most of its business systems will be discreet. Getting a holistic view of the health
of the business can be stymied by the organic growth of applications and data stores. A
great deal of the business indicators will either be localized to a particular function or
difficult to retrieve from specialized solutions, such as spreadsheet calculations. A busi-
ness analytics system will then be implemented in order to gain that holistic view of the
data that has been lost due to growth. Data mining is a general term for the function
that a business analytics system provides. The data mining must reach across the silos
of data in order to correlate and then apply analysis tools.
In order to analyze the activity and performance of the enterprise functions, a busi-
ness intelligence system will pull data from many sources. Thus, there are obvious
connections to the backend databases. What may be less obvious is that the analytics
will make use of the message bus, both as a listener to capture critical data as well as
a sender to request data. Analysis has to tie transactions to entities such as customer,
partner, or employee, and messages to things like item catalogs and pricing structures.
Each of these data types may reside in a distinct and separate data store. The internal
content archive will also be used for analysis, which is, again, another completely dis-
parate storage area.
As is typical for business intelligence and analytics systems, the system will want
to make use of existing business processing functions such as the internal business
Figure 8.1 Business analytics logical data fl ow diagram (DFD).

Business Analytics 237
applications that produce statistics, for example, sales bookings and fulfillments.
Sometimes, needed processing results can be obtained from databases and other reposi-
tories. In these cases, the analysis engine will have to fetch results that may only be
available in the user interface of a particular application. As a result, the data analysis
and business intelligence system also has a “screen scraping”* capability that accesses the
presentations of the business applications. Another important analytics source will be
the metadata associated with individual business applications. Metadata may describe
what data is to be pulled and how it may be processed.
It’s fairly usual for a business intelligence and analytics system to touch pretty
much all of the backend systems and data stores. These analysis systems contain code
that understands how to crawl flat file directories, parse spreadsheets, read databases,
HTML and other content representations, and includes code that can parse various
data interchange formats. You may think of a business analysis system as being a data
octopus. It has many arms that must touch a great deal in order to collate what would
ordinarily be discontiguous, siloed data and systems. That is the point of business intel-
ligence: to uncover what is hidden in the complexity of the data being processed by the
many systems of a complex business. It is the purpose of business intelligence to con-
nect the disconnected and then provide a vehicle for analysis of the resulting correlation
and synthesis.
The system shown in Figure 8.1 comprises not only the business analytics and intel-
ligence but also the many enterprise systems with which analytics must interact. In
order to consider the entire system, we must understand not only the architecture of
the business analysis system itself, but also its communications with other systems.
The security of each system touched can affect the security of business analytics. And
conversely, the security of business analytics can impact the security posture of each
system it touches. Consequently, we must view the interactions as a whole in order to
assess the security of each of its parts. Figure 8.1 diagrams all the components within
the enterprise architecture with which business analytics must interact. Not all of these
interactions involve the analysis of data. Other interactions are also critically important.
Consider the data flow “octopus,” as shown in Figure 8.1. How can the analysis
system gather data from all these sources that, presumably, are protected themselves?
If you arrived at the conclusion that the business analysis system will have to main-
tain credentials for almost everything, much of which is highly proprietary or trade
secret, you would be correct. One of the most difficult things about a business analysis
system is that it has to have rights—powerful rights—to almost everything at the back-
end of the enterprise architecture. The security posture of the business analytics system,
therefore, can significantly affect (perhaps lower) the posture of the system to which it
connects. Since it has rights to a great deal, the system can be considered a repository,
not only of the sensitive data and results that it produces but also of access rights. If
* “Screen scraping” is an automated process that grabs data from a user interface as though it
were a human interacting with the system.

238 Securing Systems
these rights are not protected sufficiently, business analytics puts every system from
which it must gather data at greater risk of successful attack.
In the illustrated use case, business analytics also listens to the message bus to gather
information about transactions coming from the external zones. As you think about
this, does that also expose the business analytics system to any potential attacks?
We haven’t yet considered everything shown in Figure 8.1. Identity systems and
security systems are shown on the right of the diagram. We will return to these after we
consider the data analysis system itself. I purposely left all the components as they were
represented in the component enterprise architecture so that you can see how business
analytics fits into and interacts as a part of the enterprise architecture. Each component
diagrammed in Figure 8.1 is in precisely the same place and at the same size as these
components were represented in Figure 7.2.
Since the flows have become rather crowded and hard to visualize, I’ve broken
them out in Figure 8.1. I have removed those flows that represent connections between
Figure 8.2 Business analytics data interactions.

Business Analytics 239
non-business analytics components in order to highlight only the flows to and from the
business analytics system. Please refer back to Figure 7.3 if you don’t understand how
the various components interact.
Figure 8.2 is a drill down view of the data gathering interactions of the business
analytics system within the enterprise architecture. Is the visualization in Figure 8.2 per-
haps a bit easier to understand? To reiterate, we are looking at the business analysis and
intelligence system, which must touch almost every data gathering and transaction-pro-
cessing system that exists in the internal network. And, as was noted, business analytics
listens to the message bus, which includes messages that are sent from less trusted zones.
8.2 Threats
Take a moment to consider Figure 8.2 as a series of targets for an attacker. Think about
the threat agents that we outlined for Digital Diskus and what each might stand to gain
by compromising one or more of the components in Figure 8.2. This consideration is
based upon “T”—threats—as we’ve already outlined as active against Digital Diskus.
As we move to system specificity, if we have predefined the relevant threats, we can
apply the threats’ goals to the system under analysis. This application of goals leads
directly on to the “AS” of ATASM: attack surfaces. Understanding your adversaries’
targets and objectives provides insight into possible attack surfaces and perhaps which
attack surfaces are most important and should be prioritized.
Industrial spies will be interested in which products are most important to the mar-
ket, what sells best and to whom, and for what reasons? Competitors want to understand
on what particular characteristics Digital Diskus’ success rests. They will be looking to
steal Digital Diskus’ technical advantages. The technical advantages are likely to rest
within the intellectual property and trade secrets of the company. Spies also might be
interested in current sales bookings, future sales, revenues, and the like. Are there other
espionage targets?
Cyber criminals might be interested in personal data and financial details, including
any account data. This would be the most direct financial attack. They might also be
interested in areas in which inventory items, recalls of products, and information about
customer returns are stored, in case there’s an opening for equipment theft and resale.
They may also be interested in employee information, since it’s likely that at least some
of Digital Diskus may make healthy salaries.
Are there any internal targets that digital activists might attack? How about a rogue
insider seeking to disrupt or take vengeance?
In light of the “octopus” nature of this business analytics system, its connections
to other systems and communications flows will be prime targets for many attackers.
Knowing this, we can move on from threats to cataloging targets, which leads to
attack surfaces.
Much key business data will be presented to analysts and decision makers in the
reports and analysis screens of the business intelligence system. Obtaining access to

240 Securing Systems
this critical business information will most certainly be a target. The business analy-
tics system presentation of the analysis, its reporting and viewing capability, will most
certainly be a prime target for several classes of threat agents. (Please see Figure 8.3, to
which we will return shortly.)
Because the business analytics system must be given enormous rights, the system
itself should be considered a gateway to the many other juicy targets with which it
interacts and from which it pulls data. At what level of sensitivity would you place the
business analytics system? In order to provide its functionality, business analytics must
be connected to much, if not all, of the existing business relevant data, irrespective of
where that data gets generated or is stored. And this access is irrespective of whatever
security protections have been placed around the data.
Will business analytics have to handle and store credentials? Credentials to what?
The data sources and applications from which the analytics system gathers its informa-
tion. This is a fundamental property of business intelligence systems. That is, if each
source is protected through authentication, the analytics system will have to maintain
Figure 8.3 Business analytics system architecture.

Business Analytics 241
a user ID (account) and credential for each system that it must touch. Therein lies one
of the first security issues that an architecture analysis must deal with.
Not only are the user interfaces of the analytics system obvious attack surfaces,
but so will be any storage mechanisms. The security picture is more complex than
just protecting the credentials themselves. For instance, if an attacker can change
the configuration of the system, she or he might add themselves as an administrator,
thus gaining rights to manipulate or steal the source credentials. Consequently, the
running configuration and metadata of the system will also present an interesting
target opportunity.
It’s useful to understand a highly connected system like business analytics in situ,
that is, as the system fits into its larger enterprise architectural context. However, we
don’t yet have the architecture of the system itself. Figure 8.3 presents the logical com-
ponents of this business analytics system.
There are five major components of the system:
1. Data Analysis processing
2. Reporting module
3. Data gathering module
4. Agents which are co-located with target data repositories
5. A management console
There must be disk storage for the processing module to place temporary items and
to store the results of an analysis. Of course, there is also storage for configuration of the
system. The management function writes out the configuration and metadata to the con-
figuration files. When the processing module starts, it reads its run ning configuration.
Reporter must also read its configuration. The management console will place con-
figuration data onto permanent storage for each component, which is then available
during initiation.
Since Reporter must present results to users, it reads Processing Storage. It also can
initiate processing sequences, so Reporter communicates with Processor, as well as read-
ing from the temporary storage set aside for Processor.
Data Gathering gets started by Processor, which then reads its configuration data. All
the communications to the data sources are performed by Data Gathering. Processing
tenders a request that a particular set of data be collected from across a set of sources
and then delegates the task to the gathering module. Gathering supports two distinct
access methods: native accesses (such as SQL* database queries), SOAP† (SOA) calls,
and so forth.
* Structured Query Language.
† Simple Object Access protocol.

242 Securing Systems
The data gathering module also supports agent software. So-called “agents” are
small pieces of code that include a set of libraries and commands (provided in several
languages) that implement a communications stream from Data Gathering to an agent.
There is also an Application Programming Interface (API) that must be implemented
so that commands from the gathering module will get processed properly. Agents then
run on instances of a data source for which Data Gathering does not have a native
access method implemented or where such a protocol does not exist. The agent is co-
located with the data source. Each agent contains code that understands the unique
properties of a data source for which it has been implemented.
Agents for many standard data sources are supplied by business analytics.
Additionally, users may program their own agents to a standard API and library set,
allowing Data Gathering to call specialized custom code agents written by data analyt-
ics’ customers. Through this extensibility, this business analytics system can retrieve
data from virtually any source.
The system is built such that each module could be run on its own separate server,
that is, on separated hosts, using the network for the intra-system communications.
However, Digital Diskus has chosen to install the processing module, the data gatherer,
and Reporter all on the same host. Their communications consequently are made
between the modules locally on the host and do not cross the actual network (i.e., these
use “localhost” as the destination). The management console runs on a separated host,
as we will explore below.
8.3 Attack Surfaces
In this context, where several components share the same host, how would you treat
the communications between them? Should these communications be considered to
traverse a trusted or an untrusted network? If Digital Diskus applies the rigor we indi-
cated above to the management of the servers on which business analytics runs, what
additional attack surfaces should be added from among those three components and
their intercommunications when all of these share a single host?
Given that we already know that Digital Diskus requires and maintains a rigorous
set of security standards for its administrative functions, how does this “infrastructure”
influence the attack surfaces? This is one of those situations in which the architecture
of the system under analysis inherits security capabilities from the infrastructure upon
which it’s deployed. There are no standard users maintained on the hosts on which
business analytics is running. There are only high-privileged users who were charged
with keeping the host, as well as the applications running on that host, maintained,
patched, and hardened—that is, all the administrative tasks that go into running
industrial-strength enterprise systems. This situation is quite unlike a user’s endpoint
system, which will be exposed to whatever the user chooses to do.
Please remember that Digital Diskus’ administrators are supposed to be sophis-
ticated and highly trained individuals. Presumably, they have been issued separate,

Business Analytics 243
individual corporate systems on which to perform their usual, digital tasks: email, Web
research, calendaring, documents, and spreadsheets. They do not use the servers they
administer for these tasks; that would be against policy. Therefore, the only users that
business analytics servers should be exposed to are the administrative staff. Although
the administrative staff maintaining these hosts does have high privileges on the sys-
tems, it is assumed that they know how to protect these appropriately. Working within
the strictures of appropriate separation of duties, these individuals would not have any
rights to the interfaces of the business analytics system itself. That is, they shouldn’t have
access to Reporter, Management Console, the processing module, or Data Gathering.
They are not entitled to view the results of the business intelligence analysis. Still,
despite the separation of duties that prevents administrative staff from being users of
the business analytics system, they will have rights to the operating system that runs
the applications and thus, they will inherit rights to all the data that is kept locally or
mounted onto those servers. So, although the user interfaces of the system might not
pose a potential attack surface for an insider attack, the data stores of the system most
certainly do.
If I were analyzing this particular business analytics system placed within the con-
text of the enterprise in which it is supposed to be run, I would not worry about the
communications flows between the modules that are co-hosted. My reasoning is that
the only access to these communications is via the high privileges of the administra-
tion accounts. If these accounts have been lost to an attacker, a great deal more than
the intra-module communications is at stake. We have already stated that the privileges
on these servers are protected by a number of controls. These controls are inherited by
the business analytics system and will be taken into account in the threat model. The
controls, if executed properly, are likely to prevent or significantly slow an attack to
business analytics from its operating environment. However, you may note that we have
opened up another attack surface that can come from one of our threats: rogue insider
attack to the very sensitive data that business analytics produces.
Communications from the data gatherer to data sources will be of interest to many
attackers. Before analysis, some of the raw data will be sensitive and proprietary. Even
the internal network, considering the breath of the Digital Diskus business eco system,
cannot be considered entirely trusted. Therefore, communications of sensitive data
must have protection going over the network. The network communications to data
sources are an attack surface. Which brings us to the agents.
The data gathering agents are located on various data stores or within the stores’
associated data access services. An agent is required in every instance in which a native
data protocol cannot be used to pull data from the storage or application. An agent
will be written for each proprietary, nonstandard access protocol. It might be written
to parse an XML data format, or to read a proprietary file of one kind or another, for
example, a Microsoft Word document or Excel spreadsheet. The agent must have rights
to read the data source. In this instance, when a data-retrieval operation is instantiated,
the data gatherer sends the credentials down to the agent for the operation. Credentials
are not stored at agents.

244 Securing Systems
If an attacker can retrieve the API and libraries, then use these to write an agent, and
then get the attacker’s agent installed, how should Digital Diskus protect itself from
such an attack? Should the business analytics system provide a method of authentica-
tion of valid agents in order to protect against a malicious one? Is the agent a worthy
attack surface?
I would ask the business analytics vendor what protections they had placed in the
protocol or interchanges between agent and data gatherer to preclude an attacker writ-
ing a malicious agent. The agent’s software is a target, in and of itself. With a rogue
agent, an attacker can send data of the attacker’s choice to the business analytics system,
as opposed to real data. Furthermore, a badly written agent, API, or libraries could
open a vulnerability that could be misused by the attacker to take over whatever host
the agent is running on. Therefore, the data gatherer must protect itself against mali-
cious agents; there’s one attack surface. And each agent is potentially an attack surface
introduced to the system on which it’s running.
The native access interchanges will be protected by whatever native defenses are
available for that access method. These, then, would be out of the assessment scope of
the business analytics system, by itself.* However, this does not place out of scope the
credentials used for those native access methods. These, like all of the other credentials,
must be considered a prime attack surface of the business analytics system.
The management console has direct control over the configuration of all the other
components. Indeed, credentials for each access method and data source will be input
to Management Console. That makes Management Console, in its entirety, an attack
surface: every input and every output.
Why should the output of Management Console be considered an attack surface?
Previously, the point was made that all inputs should be considered attack surfaces.
However, when the outputs of the system need protection, such as the credentials
going into the business analytics configuration files and metadata, then the outputs
should be considered an attack surface. If the wily attacker has access to the outputs
of Management Console, then the attacker may gain the credentials to many systems.
Further, the attacker will know which data sources exist and, quite possibly, the access
methods for those data sources (because the protocol and/or agent for each access must
be configured). Because of the foregoing, this particular set of outputs contains critical
and sensitive information that affects the security posture not only of the business ana-
lytics but of many other systems, as well, and should be considered an attack surface.
Naturally, the configuration files are also a prime target themselves, as stated above.
What may be less obvious is that when Management Console reads the configura-
tion files, this also creates an attack surface at Management Console. If the attacker
can insert attack code to exercise a vulnerability in the configuration file processing
* Th ough, most certainly, each native protocol should be considered within the larger scope of
the organization’s security posture.

Business Analytics 245
routines, the attacker might gain Management Console, or even privileges on the host
that runs it. Remember, Management Console is running on a separate host in this
implementation. That server is, of course, a target that inherits the protections of the
administration function at Digital Diskus. But a subversion of Management Console
itself might give access to all the functions that Management Console controls, which
are, of course, sensitive.
Access controls to Management Console itself, authentication and authorization to
perform certain actions, will be key because Management Console is, by its nature,
a configurator and controller of the other functions, a target. Which brings us to
Figure 8.4.
Figure 8.4 Business analytics user interactions.

246 Securing Systems
Figure 8.4 returns to a higher level of abstraction, obscuring the details of the
business analytics modules running on the host.* Since we can treat the collection of
modules as an atomic unit for our purposes, we move up a level of granularity once
again to view the system in its logical context. Management Console has been broken
out as a separate component requiring its own defenses. The identity system has been
returned to the diagram, as has the security monitoring systems. These present possible
attack surfaces that will need examination. In addition, these will become part of the
defenses of the system, as we shall see.
Each of the two user interfaces, Analytics Management and Reporter’s interface, is
implemented as a Web server. User interactions take place over the HTTP protocol.
These web interfaces go beyond standard HTML pages. They present dynamic content.
And particularly, the reporting module (i.e., “Reporter”) must be able to dynamically
generate sophisticated data representations, charts, visualization s, tables, and similar
views of data. The reporting module needs a rich, HTML-based interface. This inter-
face makes use of scripting and other browser-based presentation technologies. Still,
while sophisticated features in the browser, such as JavaScript and XML processing,
will need to be turned on in order to make use of the interface, the representation is
entirely browser based. No special extra software is downloaded to a user’s endpoint.
The foregoing indicates that business analytics web code must resist all the usual
and well-known Web attack methods. Further, the reporting module interface must
be coded with care to disallow attacks against the data through the interface, such as
SQL injection attacks. The interfaces are attack surfaces, and, specifically, they are Web
attack surfaces.
Beyond a malicious agent, there is another attack surface in Data Gathering.† In
order to process many different kinds of data in different formats and presentations,
some part of the business analytics will have to normalize the data and put it in a form
that can be correlated. The data will have to be parsed and perhaps reformatted. Data
will certainly have to be normalized. There is a fair amount of preprocessing that any
correlation system must perform before data from different sources in different formats
can be brought together into a holistic picture. From an attack perspective, this pre-
processing means that the data will have to be read, its context and metadata (field
names, etc.) understood, and then the data will be rewritten into some form that is
reshaped (normalized) into a standard format. If attackers can exercise a vulnerability
that exists in the data processing routines, they may be able to use that vulnerability to
subvert the data preprocessing or even go beyond that to the operating system.
Depending upon the vulnerability, Data Gathering at the very least might be caused
to simply stop functioning (denial of service—“DOS”), which would halt all business
intelligence operations. Without accurate data updates, the results become stale, or
perhaps even false. But what if the attacker can cause the data to change? Then false
* “Host” in this instance means a “logical” host. Th ere will be several of these servers to scale
the processing and for redundancy, in case of failure.
† Data Gathering is shown in Figure 8.3.

Business Analytics 247
information will go to Digital Diskus decision makers. Poor or perhaps even disastrous
decisions might result. The decision makers are dependent upon the accuracy of the
business analytics.
An overflow condition as data are being processed could cause the data gathering
module to exit into the operating system and give escalated privileges to an attacker.
In order to exercise this sort of vulnerability, the attacker must present the exploit code
encapsulated within the data that’s being processed.
How might an attacker deliver such a payload? The obvious answer to this question
will be to take over a data source in some manner. This, of course, would require an
attack of the data source to be successful and becomes a “one-two punch.” However, it’s
not that difficult. If the attacker can deliver a payload through one of the many exposed
applications that Digital Diskus maintains, the attack can rest in a data store and wait
until the lucky time when it gets delivered to the business analytics system. In other
words, the attacker doesn’t have to deliver the payload directly to Data Gathering. She
or he must somehow deliver the attack into a data store where it can wait patiently to be
brought into the data gathering function.
You may remember from the architecture examination that Data Gathering is run-
ning concurrently with the business analytics processing system? This means that a
data gathering compromise might affect much of the core business analytics system
due to the manner in which business analytics has been deployed at Digital Diskus.
Each of the three modules running concurrently and in parallel on the processing server
becomes an avenue to take over not just the server, but the other two modules. In other
words, the security posture of each of the three modules running on that core server
profoundly affects the security posture of all three.
The reporting module primarily presents a user interface. To do so, Reporting reads
data that’s been processed by the business analytics system. As such, it certainly has an
attack surface in its user interface. And one could make a case that an attacker might
get through all the processing that takes place within Data Gathering and the analysis
engine, thus delivering an attack into the results store. However, there’s a great deal of
processing that happens between Data Gathering and processing. The data will have
been read, preprocessed, and continuously manipulated in order to produce results.
It is far more likely that an attack will have an effect on either the Data Gathering or
processing so long as the reporting module does not take data in any native format.
In this system, the reporting module does not handle data from any sources. Its input
is strictly limited to the processed results. In Figure 8.3, you can see that Reporting’s
arrow points towards the processing store—this indicates a read operation initiated
from Reporting.
The architecture, as described, leaves the system dependent on the defenses that are
built into Data Gathering. If it doesn’t do its job to protect the entire system from mali-
cious or bad data, an attack could slip through into the processing module. An attack
might even make it all the way through into the results, at least hypothetically.
If I were designing the business analytics system, I would place my defenses within
the data gathering module (“Data Gathering”). However, Digital Diskus has purchased

248 Securing Systems
the business analytics system and thus has little or no control over the innate protec-
tions that have been built into the system. With no further information, an assessment
would have to mark as a significant risk malicious payloads within data. We can, at the
very least, count Data Gathering as one of our attack surfaces for further investigation
into what mitigations may exist to protect the business analytics system. I would have
important questions for the programming team of the business analytics vendor.
A further investigation over which Digital Diskus does have direct control will be
into the protections that the source data systems provide against a malicious payload
aimed at business analytics. If an assessment cannot, with some certainty, mitigate an
attack surface, the assessment can move outwards to other protections that lie beyond
the system under assessment, that is, what protections exist within the connected sys-
tems (in this case, the data sources). Or, failing there, move continuously outward
through the chain of interactions and perimeters to build a set of defenses. Perhaps it is
actually rather difficult for an attacker to get a payload into any data source due to the
layers of defense surrounding the data sources?
If there is high confidence that the protections are in place around and within data
sources, then the likelihood of a successful attack through Data Gathering will lower
significantly. Since business analytics is a third-party system, the best defense will be
to find the attack surfaces leading to primary attack surfaces of the business analytics
and to secure these. These other systems are under direct control, as compared to this
third-party system, business analytics.
There still remains a possible attack surface in the processing store. If an attacker
can introduce a payload to the reporter, they may be able to compromise that module,
or even compromise the core system. Although we have noted that passing an attack
through all the processing that takes place from data source to the processed results
should be fairly difficult if business analytics is written defensively, if the permissions to
the data store are not set correctly, this would allow an attacker to drop an attack to the
Reporter “from the side,” that is, by compromising the results store directly. The results
most certainly present an attack opportunity if the permissions on the results store are
not set defensively, which, in this case means:
• Processing store is only mounted on the host that runs Processing and Reporter
• Write permission is only granted to Processing
• Read permission is only granted to Reporter
• Only a select few administers may perform maintenance functions on the processing
data store
• Every administrative action on processing store is logged and audited for abnormal
In Figure 8.3, you can see an arrow from Reporter to Processing. This is a com-
mand and control channel allowing users to initiate processing and analytics while they
are viewing results. This is, of course, an additional attack surface to Processing. If an
attacker can either gain control of processing or slip a payload through the reporting

Business Analytics 249
module’s inputs specifying processing actions, then a payload might be delivered to the
processing module. As we noted above, the communications between the two modules
are taking place entirely through the machine, that is, the localhost network that is
entirely contained within the kernel of the operating system on the host. In order to
subvert these communications, an attacker would already “own,” that is, would have
completely compromised the superuser privileges, the underlying host’s operating sys-
tem. At that point, the attacker has complete control and, thus, has no reason to prose-
cute another exploit. There is no additional attacker value to subverting the localhost
communications, as these and the process execution space of all three modules are
under attacker control.
There are two other systems whose connections with business analytics we have not
yet examined. The identity system provides authentication to the various interfaces of
the business analytics system. And you will see that there is a security event monitor-
ing system that collects events of interest from business analytics, as it does from many
systems within the enterprise architecture.
Because business analytics was written as an enterprise-grade system, not only does
it have an integral authorization mechanism, but it can also make use of either of
two typical enterprise group membership functions: Microsoft Active Directory or an
LDAP directory. Since Active Directory can also function as an LDAP, systems requir-
ing authentication and group membership (pseudo authorization) services can leverage
either or both of Active Directory’s supported protocols. It is rare that an enterprise-
grade system does not, at the very least, support the LDAP protocol. Organizations that
manage their identities through Active Directory can make use of that identity system
for the whole enterprise rather than programming authentication and authorization
information into each application. This is precisely the case with this business analytics
system at Digital Diskus. Digital Diskus maintains Active Directory as a foundational
enterprise infrastructure system.
Consolidating various identity systems that had become unmanageable through
organic accretion some years ago, Digital Diskus collapsed customer, partner, and
employee identities into its Active Directory. Therefore, at the company, there is a sin-
gle identity system that the other applications may make use of. Indeed, the company
makes broad use of Active Directory groups such that membership in a group may
be used to give rights to the various applications that make use of Active Directory.
Although not a true role-based access (RBAC) system, this approach is quite common.
Before permission for access is granted, membership in the appropriate group is con-
sulted, providing a fairly simple approach to enterprise authorization. Digital Diskus
has provided a self-service interface for group creation and maintenance. In addition,
some of the most coarse-grained groups, such as employee, partner, or customer, are
automatically populated; membership in one of these enterprise-wide groups depends
upon which relationship the identity has with Digital Diskus.
The details of identity management are beyond the scope of this book. However,
security architects typically have to have a fair sense of how identities are managed,
what systems are employed for identities, how identities get populated if groups are

250 Securing Systems
being used, and which groups get populated through automation and which groups
are populated manually. Furthermore, the system for granting permissions and group
membership should also be understood, as, obviously, if an attacker can gain member-
ship to a restricted group, they then gain those privileges inappropriately.
The business analytics management console and the business analytics reporting
console restrict users based on authentication to the active directory. In addition, rights
to the coarse-grained roles within the system, such as user, executive, and administra-
tor, are granted through group membership. Membership in the appropriate group is
granted through an approval process. Not every employee or any other class of identity
type is summarily granted access to the sensitive system. There is an approval process;
for the purposes of this example assessment, we don’t need to go into the details of that
approval process. Nevertheless, in this type of assessment in a real organization, the
assessor should understand the group membership process since it must be considered
as a part of the system’s defenses.
An interesting question poses itself when considering these two systems, both of
which are considered very sensitive by the company. Which system provides an attack
vector to the other? When the business analytics system requests an authentication, is
it possible that an attack could be promulgated from the Active Directory backwards
in the response? When the business analytics system requests a determination on group
membership, likewise might a response contain an attack?
On the other hand, can Active Directory be attacked through authentication and
authorization requests?
The answers to each of these questions I believe would be, “Yes.” Each system is at
risk of attack from the other. There are two mitigating factors that may be considered.
Active Directory is a trusted system to every other system that makes use of the authen-
tication and authorization services. If Active Directory fails, many other systems that
are dependent upon it will also fail. Identity systems such as Active Directory are typi-
cally considered to be among the “crown jewels” of any organization’s data and systems.
Consequently, an organization that doesn’t sufficiently protect its identity system has
already degraded its security posture dramatically. The subject of identity system pro-
tections, just as identity systems themselves, can and should be an entire book by itself.
Indeed, the identity system, though interacting with our business analytics system,
is out of scope. Identity systems are typically regarded as part of the infrastructure. As
we know, the strengths and the weaknesses of the infrastructure are inherited by the
systems that are deployed upon them. And any system that is foundational to customer
interaction, the business ecosystem, and the internal business systems must be regarded
as highly sensitive. Nevertheless, the identity system does not depend upon the busi-
ness analytics but rather the reverse. That will be a key factor in answering the question
about which system poses a danger to the other.
Data Gathering most certainly presents an attack surface when it reads the iden-
tity stores: to gather data about the various entities, such as employees, partners, and
customers, whose identities are maintained by Active Directory. Similar to any data

Business Analytics 251
source, the identity data might contain an attack to the data gathering function. Since
data from identity are gathered via native Active Directory protocols, business analytics
must maintain credentials that have privileges to read this data.
As we have noted above, Active Directory is highly protected by this organization.
Therefore, although we have uncovered an attack surface, attack from this data source,
as compared to the many other data sources from which Data Gathering must collect,
is probably unlikely. However, the astute assessor will note that business analytics now
becomes part of the Active Directory security posture because Data Gathering must
have privileges to the identity store in order to correlate other business data with iden-
tities and identity metrics. Although Active Directory as a system is outside of scope,
because Active Directory is at such a deep level of sensitivity, risk for the identity system
would have to be noted because of the rights that the business analytics system must
maintain in order to access identity formation from Active Directory. In other words,
business analytics significantly poses an attack vector to Active Directory, as opposed
to the other way around.
As we have underscored repeatedly, security assessment must be done holistically.
And further, if the assessment has a due diligence responsibility to uncover risks for the
organization, the dependence of Active Directory’s security posture upon the posture
of business analytics would have to be noted as a part of the threat model for Active
Directory at Digital Diskus.
Irrespective of the scope of this particular assessment, systems interconnect. When
they do, the security posture of each system can significantly affect or alter the security
posture of the other systems to which it connects. In this case, business analytics must
become a part of the tight security posture around the identity system. In addition,
the security requirements for business analytics must match those of this very sensitive
identity system, at least for the credentials.
It should be noted that, most certainly, Microsoft understands the criticality of
Active Directory as a foundational part of many organizations’ infrastructures. There’s
no doubt that they take the security posture of the system quite seriously. It’s a safe bet
that Microsoft attempts as rigorous a set of protections as they can implement, knowing
full well that entire enterprise architectures are dependent upon the security posture of
Active Directory. With that in mind, let’s assume, for the purposes of this assessment,
that Active Directory is in fact protectable, in general, and, in this instance, has been
configured to be sufficiently protected. With this assumption, we can be satisfied that
it will be very difficult to launch an attack through the Active Directory to our business
analytics system.
The foregoing analysis hopes to answer the question posed earlier: Which of the two
systems affects the security posture of the other? Theoretically, the answer is that influ-
ence is bidirectional. However, we concern ourselves with how business analytics affects
the security posture of Active Directory and not, for this analysis, the other way around.
There is another system that interacts with business analytics: the security monitor-
ing system. The security monitoring system will monitor the activities and changes

252 Securing Systems
on the hosts that support business analytics modules. This would be a normal part
of the security of a mature administration function, as we have previously described.
Importantly for this assessment, changes to the configuration file are logged; those
change logs are audited for suspicious events. In this way, if an attacker does manage to
breach the protections around the configuration files, hopefully, such anomalous activi-
ties will be noted quickly. The goal of this sort of monitoring is to stop any malicious
activity before it can do serious damage. A further goal is to let administrators know
that someone is independently paying attention to events. This type of monitoring is
typical and would be described in any number of security standards covering the secu-
rity of administrative activities.
Does security monitoring pose an attack surface for either business analytics or the
security monitoring system? Typically, the security monitoring system gathers change
logs that are output by a system and by the operating system. Usually, the logs sit on
disk until collected, at which point they may be archived or deleted. Similar to the data
gathering activities of the business analytics system, the security monitoring system will
be at some risk to attacks embedded within the logs. And it would be very difficult to
sell a security monitoring and event correlation system if it was easy to compromise the
monitoring system. Although such weaknesses have been discovered in security moni-
toring systems, vendors depend upon the rigorousness of their processing defenses so
that they can successfully sell their systems.
The security monitoring and correlation system will also be considered part of the
security infrastructure. Furthermore, a security monitoring system will operate as one
of the most sensitive systems an organization maintains. Therefore, all due care should
be taken in purchasing and implementing as bulletproof a system as humanly pos-
sible. One of the differences between a security event managing system and other sys-
tems will be that a security system expects to be attacked and should be built with
industrial-strength defenses. When implementing such a system, all due diligence
should be taken to make sure that purchased components have many integral defenses,
that these defenses are in depth, and that the implementation instance takes advantage
of all of the defenses possible.
We have yet another situation where a highly sensitive system, security monitoring,
might be profoundly affected by the security of business analytics, just as the identity sys-
tem, Active Directory, was affected by business analytics. Let’s assume, for the moment,
that the security monitoring system is a rigorous, industrial-strength security system that
protects itself very carefully from malicious data in logs and other event sources.
In this situation, the monitored outputs coming from business analytics are not
the system’s analysis results, but rather, come from the application about its activities.
At least some of this output would be the result of the programming logic steps of the
application. Items like data accesses, the size of the access, maybe even details about
the data retrieved, might get logged by Data Gathering. In this way, a retrieval of data
not consistent with the intentions of the programmers of the system might show up as
anomalous activity that would then fire a security alert for further investigation.

Business Analytics 253
However, in order to debug systems, application programmers will typically send
events about internal processing steps and similar to logs, as well. In more than one
system, I have seen user or other credentials that then get exposed in unprotected ways.
Thus, the log files themselves might be an attack surface, not to subvert the business
analytics system but rather to gain valuable information that probably shouldn’t have
been logged at all. Application logs tend to be quite uncontrolled compared to other
types of system outputs. If the vendor of the business analytics system was foolish
enough to have placed data within the logs that contains possible attack value, that
would be yet another conversation with the vendor about software security.
Nevertheless, if the log files contain juicy data about the workings of the busi-
ness analytics system, or even worse, hints about the nature of the analytics results,
then these would constitute a significant attack surface. One of the investigations that
should be conducted would be to discover exactly what goes into the logs, ascertaining
just how sensitive the information, log item by log item, or when aggregated, might
be. Thus, the very act of logging data for the security monitoring system uncovers yet
another attack surface.
The logs might be an attack surface against the security monitoring system, includ-
ing all the mitigations that are in place against log file tampering as well as protections
against attacks contained within log items. Since the logs originate solely in the busi-
ness analytics application, and can only be written by business analytics for read only
by security monitoring, this is a fairly long shot for the attacker. Presumably, the only
people with access to business analytics code are those authorized to change it. How
do they even know which security systems one of their customers may use (attacks are
usually quite specific to application and execution environment)?
But the logs themselves may constitute a target if the data they contain is useful.
In this case, these are simply flat files on disk. Technically, these are not a part of the
formal architecture of the system but rather an output that must then be managed
properly. Nevertheless, a prudent assessment would ascertain the data contained in the
log files. And a prudent assessment would consider the log files as a possible target.
At this point, I urge you to consider the diagrams of business analytics once again.
Do you see an attack surface that hasn’t been discussed? As far as I know, given the con-
straints and background information that has already been described, all the relevant
attack surfaces have been uncovered.
One of the biggest mistakes that I’ve made has been overlooking a detail during
assessment. That’s why I make liberal use of peer review in my assessments, because it’s
easy to miss something important. I ask you, the reader, to be my peer reviewer. Have
I missed anything? This is not a trick question. Pretend that I’ve performed an assess-
ment and that now I’m asking you to peer review my work.
Another advantage of a peer review is a diversity of opinion. Each of us has a
unique sense of risk, and each of us favors a unique security posture. As such, I believe
it’s important to consider alternate views when prioritizing attack scenarios. I’ve given
my views on several of the attack scenarios presented in this analysis that should be

254 Securing Systems
de-prioritized. What’s your opinion? Do you agree with me, or would you change the
prioritization to raise an attack surface or attack vector or to lower one that I’ve consid-
ered important?
I suggest that you take some time and imagine yourself as the security architect
responsible for the enterprise architecture of Digital Diskus. If you were implementing
the business analytics system for this enterprise and enterprise architecture, how would
you protect the business analytics system and the systems with which it interacts?
The following section comprises a list of the attack surfaces that have been uncov-
ered during this analysis.
8.3.1 Attack Surface Enumeration
• Data gathering credentials store
• Data source credential inputs
• Data gathered from sources
• Business analytics message bus listeners
• Analytics results store
• Analytics results presentation (“Reporter”)
• Configuration files and system metadata
• Management console inputs
• Management console configuration outputs
• Management console host
• Processing module input from Data Gathering
• Processing input from Reporter (commands)
• Processing module configuration file
• Data Gathering module
• Data Gathering module configuration file
• Data Gathering inputs, especially from data sources
• Data source agents
• Communications with data sources
• Reporting module user interface
• Reporting module configuration file
• Business analytics activity and event log files
8.4 Mitigations
As you consider the attack surfaces in the list above, what security controls have already
been listed?
I have stressed the importance of industry-standard administrative and system
management controls as they apply to the business analytics system. Because of the

Business Analytics 255
importance and sensitivity of the system’s configuration files, the temporary results and
analytics results files, these two storage areas require special attention beyond the usual
and typical that we have already stressed. As was already explained, each of these stor-
age areas represents an opportunity not only to control the business analytics system,
but perhaps to have far-reaching consequences for the entire enterprise architecture.
Due to this sensitivity and the comprehensive consequences of a successful attack to
either of these areas and data, particular attention must be paid to the execution of
access restrictions and the separation of duties.
Essentially, no matter how particular and careful Digital Diskus is about the imple-
mentation of access controls, some administrator, at least one, is going to have access
rights to each of these areas. That’s because there will need to be someone who can
execute those tasks that require superuser privileges. Unfortunately, modern operat-
ing systems require someone who, at the very least, has privileges to firefight during
a crisis.
Let’s suppose that a junior engineer makes a grand mistake by corrupting a couple
of key operating system shared libraries on several of the business analytics hosts. Let’s
suppose that it’s an honest mistake; the engineer thought that he was executing a script
designed to check the validity of packages, mistyping the command line parameters
to the script at a time when he had sufficient privileges to damage the systems. And
let’s suppose, just for the sake of an example, that the mistaken script causes several
systems to reboot in a state from which the system cannot recover. Usually, operating
systems survive such unexpected crashes without damage. But to give an example in
which firefighting capabilities might be required on a critical system, let’s suppose that
several of the redundant processing module hosts come up with damaged operating
system files that can no longer execute. This is not a far-fetched example. Such crashes
do occasionally happen; someone has to come in and get the systems up and running.
Often, in these situations, there may very well be significant management pressure
demanding that the systems are fixed as quickly as possible. This could especially be
true at a time when financial records are being closed or at some other business inf lec-
tion point.
With the kind of pressure described in the example above and the criticality of the
business analytics systems, solutions to such situations often take a very senior engineer
with much system experience and deep understanding of the application, as well. This
individual must “smoke jump” the problem by using superuser privileges to rebuild the
operating system.
The questions that then will be asked for this type of critical system that maintains
highly sensitive data will be something like, “Who should have these privileges and how
many people need them?”
Oftentimes, the easiest and most manageable solution to who and how many people
need high privileges is to give everyone who may have to alter the system the ability to
work at the highest privilege level. In a situation in which there are only three system
administrators, the tactic of giving everyone high privileges may make a good deal of

256 Securing Systems
sense? After all, on a small team, if someone makes a significant error or, worse, is dis-
honest or downright malicious, one or both of the other two people are likely to notice.
However, organizations that field large administrative staff cannot rely upon a sys-
tem based upon personal checks and balances over the actions of any particular indi-
vidual. If there are 100 administrators spread out globally in different time zones, who
knows who’s performing what actions at any particular time? Furthermore, what if
most of those administrators are responsible for every machine on the network, so that
the administrative duties can be handled around the clock, 365 days per year? Many
individuals may be given very similar duties, so that the absence of any single individual
will not be missed. This is very typical in organizations that administer hundreds or
even thousands of machines across the globe.
Generally, system administrators are among the most trusted staff at any organiza-
tion. They have to be: They can bring systems up or down, and they certainly have
access to at least some of the critical files and data of the organization. Indeed, it’s easier
on everyone if privileges and access controls are kept simple. Generally, it’s prudent to
eliminate as many special cases as possible in order to reap the rewards of economies of
scale and efficiencies of task.
Competing against simplicity and economies of scale are the differences in data
sensitivity and system criticality. In the case of business analytics, there appears to be a
clear need to protect the configuration files and the results files as carefully as possible
leaving as small an attack surface as can be managed. That is, these two sensitive loca-
tions that store critical organizational data should be restricted to a need-to-access basis,
which essentially means as few administrators as possible within the organization who
can manage the systems effectively and continuously.
A typical requirement for this kind of system would be to limit the number of
administrators who have access to it to a small number. What that number is will need
to be determined by taking the factors of global, round-the-clock support and appro-
priate levels of personnel redundancy into account. There is no hard and fast number
that will work for every organization. One of the factors influencing the decision will
essentially be the number of individuals who have the experience and proven trust to
execute the task. If there are seven of these, that may be a better decision than a smaller
number simply because the difference between six and seven poses no or little addi-
tional risk to the organization. I would avoid attempts to pull a number out of the air
since, in my experience, no such perfect number exists. And time may be wasted and
relationships hurt if the number question turns into a tug-of-war between four versus
seven, a tug-of-war between system administrators and security.
Instead, it may be more fruitful to examine individuals’ past history and expertise,
management chains, line management capabilities, and other direct factors that will
influence the ability of the individuals to carry out the duties and carry this burden of
responsibility successfully.
The requirement would read something like, “superuser privileges may only be
granted to high trust, master administrator-level personnel on a need-to-know and

Business Analytics 257
need-to-support basis.” Keep requirements like these at a high enough level that those
charged with execution can implement them without being micro-managed or blocked,
as conditions and technologies unfold and develop.*
There are tools readily available (but sometimes expensive and nontrivial to imple-
ment) that can help to control high-privilege access, especially in situations as con-
fronted by Digital Diskus’ implementation of business analytics. These tools grant
privilege levels only for the duration of the session or task. When the task is completed
or the designated period ends, the privileges are revoked and must be requested from
the system once again.
Access can be granted in a granular manner by tools of this type. That is one of the
primary functions of a server access and change management system. Implementing
a server administrative privilege control tool would allow the extensive administrative
function at Digital Diskus to provide one level of access across most of the servers, while
reserving critical server access to a limited, higher-trust set of individuals, precisely as
required for the business analytics system.
Such an access and change control tool will also log actions taken by administra-
tors on systems (remember that all actions on the systems are being monitored by the
security monitoring system). Consequently, every action taken can be audited, both
for malfeasance and for inevitable human mistakes. Typically, a tool controlling highly
privileged access can also dole out permissions in such a way that for some systems or
some operations, only the most highly trusted individuals are granted permission.
If Digital Diskus hasn’t deployed an administrative access control tool, the imple-
mentation of business analytics might be seen as an opportunity to upgrade the security
controls across the administrative functions. In other words, new complexities to an
enterprise architecture often present opportunities for building an architecture that bet-
ter meets those complexities rather than trying to simply accept additional risk due to
the way “things have always been done.” Another typical response would be to jerry-rig
current systems with high-maintainance manual processes in order to meet the chal-
lenge. Although either of these responses may be the appropriate organization decision,
an architecture inflection point at which new requirements foster new capabilities is
usually an opportunity to discuss strategy as well as any available tactical solutions.
I will point out that since the organization already has a robust implementation of
Active Directory groups, an interim solution might be to employ a group membership
as a way to restrict access to these two particular storage areas. Such a solution would
depend upon the operating system on which business analytics runs and the operating
environment for storage areas (both of which are currently ) and whether these
can make use of Active Directory groups for authorization to particular storage areas.
* Steve Acheson taught me to keep my requirements at suffi cient specifi city to be implementable
but not so specifi c such that implementers cannot adjust to changes without rewriting the

258 Securing Systems
If we were actually implementing the system, we might have to engage with the
operational server management teams to construct a workable solution for everyone.
For our purposes in this example, we can simply specify the requirement and leave the
implementation details unknown.
The actual data that is pulled into the business analytics system might contain
attacks. Some of this data originates from applications that are hosted on the exposed
network, the DMZ. Those applications, of course, will be under fairly constant prob-
ing and attack. Although Digital Diskus makes a concerted effort to build resistant and
self-protective applications, no security measure is perfect. It should be assumed that,
at least occasionally, hopefully rarely, a sophisticated attack will breach the protections,
input validations, and messaging rewrites that we’ve already noted have been built into
the exposed applications. That is, a strong defense must assume that an attack will
come through.
If an attack makes it through, it may lay in wait in data storage until the business
analytics system pulls the data that contain the attack and then attempts to process
it. At that point, any authentication or communication encryption that is used will
provide no defense whatsoever to the stored attack; the attack will be gathered up as a
part of the data that’s being processed. The business analytics system must be as self-
protective and securely programmed as external applications are expected to be. It is
important to note that one of the inputs into Data Gathering is from messages that
originate from external applications as they pass over the message bus. So an attack that
makes it through the external applications might be processed by the business analytics
processing module before the attack data are stored. Data Gathering must be robust
and rigorous in throwing away any data that is invalid or poorly formed, no matter the
source or the timing of the collection.
However, in a system that must process many different forms of data, that process-
ing must be very “plastic.” Generalized data format processing must be very data driven
so that it can accept just about any form of data imaginable, as the processing code is
going to have to process data in many forms and presentations. There is a significant
conflict between rigorous input validation and normalization and the processing of
many varied data presentations and types. This is what they call in computer design
“a nontrivial problem.” As has been stated previously, but should be noted again, the
implementation and coding of the business analytics system is not within the control of
Digital Diskus. What is the best way for Digital Diskus to mitigate the risk from overly
generalized data processing?
In Chapter 3, where we examined the art of security architecture, holistic analysis
was discussed at some length. The risk of an attack coming through one of the data
streams is a good example of how a system’s security posture shouldn’t be confined to
the system itself, but rather, enlarged to include all the systems with which it connects.
In this case, the cleanliness of the data, the protection of the data processing mod-
ule, depends upon a strong defense against malformed data in applications that were
likely built without any concern for the business analytics system. In this case, which

Business Analytics 259
is a typical timeline for adding a business intelligence to an organization, the business
analytics system has been implemented after many of the applications that produce the
data to be analyzed were built. Nevertheless, business analytics depends upon those
applications’ defenses against malicious data. Part of the defense of the business analy-
tics system will be how thoroughly the data is cleaned before it is allowed to pass
inwards and onwards to the systems that the business analytics will touch.
However, the defense of business analytics shouldn’t rest solely with the quality of
applications and other data sources. Below, in the discussion of user interface imports,
we will add another assurance step for determining the quality of the business analytics
systems’ defenses themselves.
There are several user interfaces that require restricted access. When we devel-
oped our threat model, the reporting module’s user interface and that of Management
Console were called out as a likely attack surface. Furthermore, within each of these
interfaces, certain inputs were called out as particularly sensitive: the interface provided
in order to input credentials for data sources and the interface where the analytics
results are displayed.
• Management console inputs
○ Data source credential inputs
• Reporting module user interface
○ Presentation of analytics results (“Reporter”)
The data source credential values will be input into a subset of Management
Console’s user interface. Likewise, the presentation of results is a subset of inputs and
outputs within the user interface of the reporting module. Depending upon the granu-
larity of authorization permissions, it should be possible to isolate each of these sensitive
areas from the general user interface in a granular manner. That granularity of access
and restriction must be finer than a role that is given broad access* to a number of
functions or even the entire interface. Fine-grained access in this case may mean single
pages, collections of pages, or access that has been confined to individual fields. One of
the requirements would then be a fine-grained authorization mechanism.
This business analytics system includes coarse-grained roles, such as system admin-
istrator, analyst, and executive, as shown in Figure 8.4. (These are examples and do
not comprise an exhaustive list of system roles. Such a list isn’t needed for this analy-
sis.) But the system also allows the super administrator to assign privileges in a fine-
grained manner. For instance, there can be any number of “administrators” who can
add analysts or executives to the system in order to perform typical, day-to-day user
management. But each of these administrators should not be given permission to edit
credentials that will be used by the system.
* Th e authorization mechanism could be a native business analytics role or membership within
an Active Directory group, as we explored above.

260 Securing Systems
This, however, cannot be the entire protection for these sensitive interfaces. Even
though all due care must be observed in granting access on a strictly need-to-know
basis, it has already been stated that Digital Diskus is concerned about insider attack.
The system must implement protections against the various attacks that can be
exploited through user interfaces (from web weaknesses to memory handling errors,
and so forth). But, as was noted, Digital Diskus doesn’t have control over the program-
ming of the system. Has the maker implemented the appropriate input validation and
sanitization routines in order to protect the code from user input–based attacks?
From Digital Diskus’ viewpoint, since they do not have direct control over business
analytics’ implementation, they have two (industry-standard) activities that they may
use as a risk treatment: study the vendor’s security practices, with particular emphasis
on software security, and test the system itself for vulnerabilities and attack resistance.
Let us assume that Digital Diskus’ security department has conducted a thorough
analysis of the vendor’s software security practices, with particular emphasis on how the
system is built and how the vendor finds and fixes vulnerabilities during the vendor’s
Secure Development Lifecycle (SDL). Further, a thorough attack and penetration test
(A&P or “pen test”) was performed by Digital Diskus security personnel on the system
before it went into production. Although the pen test did find a few minor issues, no
major issues were uncovered. Therefore, Digital Diskus has some confidence in the
system’s ability to resist attack to its inputs. “Inputs” for the pen test included the user
interfaces, the configuration file processing functions, and the data gathering inputs.
Because Digital Diskus’ internal network cannot entirely be trusted as a sole control
to restrict access because of the broad range of people and organizations that participate
in the Digital Diskus business ecosystem, communications from users to the business
analytics system and between data sources as data is gathered should be protected in
transit. In other words, the network cannot be completely trusted. The business analy-
tics system has the ability to present its user interfaces via HTTPS. HTTPS will meet
the requirement to protect user interactions with the system across the less than com-
pletely trusted network. And any data gathering access that supports Transport Layer
Security (TLS) should be configured to do so. This aspect of the communications
is somewhat dependent upon the capability of the data source. To facilitate secured
communications, the data agent API and libraries support TLS. Generally, native data
gathering protocols pull data from the source. When this is the case, if the data source
can act as a TLS server, Data Gathering is capable of being a TLS client. Consequently,
in all instances that support a TLS capability, TLS is being employed across Digital
Diskus’ internal network.
8.5 Administrative Controls
It can’t be stressed enough that complex, interactive systems such as the business
analytics server presented here require significant administrative protections. This is

Business Analytics 261
particularly true when the system integrates and exists within a fairly complex digital
ecosystem, as has been described about Digital Diskus. There are too many ways for
an attacker to get close enough to the business analytics system to assume that simply
because it runs within an internal network, the internal network’s perimeter protections
will be sufficient.
Adding to the general ecosystem access, privileged access must also be considered,
which points directly towards a rigorous web of interlocking administrative security
controls and practices, as has been previously described and cited. There is no way
around the necessity for the management of this system to enact virtually all the con-
trols described in the various standards.*
Several times in our exploration of Digital Diskus, we’ve encountered a mature and
rigorous set of administrative controls. Which of the attack surfaces that we’ve uncov-
ered do administrative controls protect? At the operating system level, every server that
is part of the infrastructure hosting the business analytics modules will inherit the body
of practices that Digital Diskus administrators observe.
Access will be restricted to a need-to-know basis. As we have noted, changes to the
systems are monitored and audited. At the application level, files and directories will be
given permissions such that only the applications that need to read particular files or
data are given permission to read those files. This is all in accordance with the way that
proper administrative and operating system permissions should be set up. The business
analytics systems and tools don’t require superuser rights for reading and executing
everything on the system. Therefore, the processing unit has rights to its configuration
files and data gathering module files. The reporting module reads its own configura-
tion files. None of these can write into the configuration data. Only Management
Console is given permission to write data into the configuration files. In this way, even
if any of the three processing modules is compromised, the compromised component
cannot make use of configuration files to compromise any of the other modules in the
system. This is how self-defensive software should operate. Business analytics adheres
to these basic security principles, thus allowing the system to be deployed in less trusted
environments, even less protected than what Digital Diskus provides.
8.5.1 Enterprise Identity Systems (Authentication
and Authorization)
The user interfaces of the system use the enterprise identity system for authentication
and then authorization (group membership) to its various privilege levels In addition,
* It should be noted that many of the largest and most famous breaches were a direct result
of an organization’s failure to adhere to the entire body of administrative and management
controls typically found within the relevant standards. A security architect must continuously
bear this fact in mind.

262 Securing Systems
other systems used to support business analytics also make use of the enterprise iden-
tity and access control system, Active Directory. The relationship between the identity
system and business analytics was examined in some detail above. Authentication via
the corporate directory and authorization via group membership still remain two of the
important mitigations that have been implemented.
Having reviewed the available mitigations, which attack surfaces seem to you to be
adequately protected? And, concomitantly, which attack surfaces still require an ade-
quate defense? Once again, please review the list of attack surfaces that were uncovered
during the threat modeling exercise. In the next section, we will prescribe additional
defenses necessary to build the defense-in-depth for the business analytics system at
Digital Diskus.
8.6 Requirements
The final task of the business analytics analysis consists of finding mitigation for every
attack surface that does not already contain sufficient protection to meet the security
posture laid out in the description of Digital Diskus, above.
When I do these analyses for a real system, I list every requirement, even those that
are already built. In this manner, I document the defense-in-depth through the security
requirements. However, for the sake of brevity, the mitigations listed above will not be
reiterated in this section. If you believe that an attack surface has not been sufficiently
protected through this analysis, then by all means, reread the discussions above.
Since the quality of the software security being built into the system is not under
the direct control of the organization implementing it (Digital Diskus), a key require-
ment will be repeated assurance that the vendor maintains rigorous software security
practices, including finding and eradicating vulnerabilities before release; implement-
ing input protections; and maintaining proper memory handling, proper multithread-
ing and multitasking, secure web interfaces, and the rest of the panoply of software
security design and implementation requirements* that secure software must meet.
This requirement can be fulfilled by performing an attack and penetration test on
all the inputs of the system and its software updates before the software is allowed to
go into production. Particular attention will have to be paid to Data Gathering; Data
Gathering and subsequent processing functions will require fuzzing tests in order to
achieve some assurance that attacks will be rejected properly.
In order to prevent an attacker from obscuring an attack or otherwise spoofing or
fooling the security monitoring system, the business analytics activity and event log
files should only be readable by the security monitoring systems. And the log files per-
missions should be set such that only event-producing modules of the business analytics
* For further information, please see my chapter, “Applying the SDL Framework to the Real
World,” in Core Software Security, by James Ransome and Anmol Misra (CRC Press, © 2014).

Business Analytics 263
system may write to its log file. Although it is true that a superuser on most operating
systems can read and write any file, in this way, attackers would have to gain these high
privileges before they could alter the log files that will feed into the security monitoring
The administrative requirements for well-managed servers and services have been
listed previously in several places. Table 6.1, for Web-Sock-A-Rama, contains a number
of examples. And NIST 800-53 has been cited several times. For the purposes of brev-
ity, the functions necessary to create a strong administrative defense will not be reiter-
ated for business analytics (or for any succeeding architecture analysis). Please assume
that the same requirements hold true for this system as it is analyzed in this assessment.
Table 8.1 summarizes the additional security requirements that Digital Diskus will
need to implement in order to achieve the security posture required for this sensitive
system, the business intelligence and analytics system. As we noted previously, require-
ments are broken down into three large categories: administration, network, and appli-
cation. These categories are for convenience. Any other suitable abstraction can be used.
The important point of the categories is to note that defenses are implemented at differ-
ent layers throughout an operational execution stack. And, indeed, some of the require-
ments are not technical but rather process and even personnel oriented.
Table 8.1 is not intended as a complete listing of requirements from which the secu-
rity architecture would be designed and implemented. As I explained above, when I
perform a security architecture analysis, I try to document every requirement, whether
the requirement has been met or not. In this way, I document the defense-in-depth of
the system. If something changes during implementation, or a security feature does
not fulfill its promise or cannot be built for some reason, the requirements document
provides all the stakeholders with a record of what the security posture of the system
should be. I find that risk is easier to assess in the face of change when I’ve documented
the full defense, irrespective of whether it exists or must be built.
However, this is a pedagogical work, not an analysis of a living system. These archi-
tectures are merely examples* intended to help you, the reader, become more facile at
analyzing the security of a system. As examples, I hope that you feel free to reread or
even study any portions of the analysis, as you require. Consequently, Table 8.1 only
lists those requirements that would be in addition to the mitigations that have already
been documented above, in the mitigations section. In this way, I hope to keep the
analysis more focused.
It’s always possible that I’ve missed something. If you will remember, at the end of
the attack surface section, I asked you to check my work. Perhaps you came up with
something that has not been discussed? If so, how would you protect that attack sur-
face? That analysis lies at the heart of this book. Your risk sense may not coincide with
* Every system presented in this book is based upon real-world systems that the author has
assessed. Nevertheless, these systems are entirely fi ctional. Any resemblance to actual systems
or products is purely coincidental.

264 Securing Systems
Table 8.1 Business Analytics Security Requirements
Type Requirement
Administrative Typical industry standard controls have already been specifi ed earlier. Please
refer to these.
All modules and executables of the system must be hosted by one of the
formal, corporate administrative teams.
All modules and executables of the system must be run on corporate-
maintained hosts.
Administrative access to the storage areas for system confi guration fi les and
temporary and fi nal results for the analytics must be restricted to a need-to-
know basis and only to a small number of highly trusted and time-of-service
proven administrative personnel.
Access to the permanent storage for the confi guration fi les for the system
and to the storage area for processing results will be granted only for the
period required to perform an administrative task and then privileges will be
revoked. Privileges will be granted only on a need-to-know basis.
Read and write privileges for system log and event fi les must be set in order
to protect the fi les from inadvertent or malicious alteration. Only the business
analytics module producing the event fi le may have write permissions, and
only the security monitoring system may read the event or log fi le.
Network All executables of the system must run within the corporate internal network.
Application Fine-grained authorization to Reporter and Management Console functions
will be employed. In particular, permission to add, delete, and write data
source credentials must be restricted to a need-to-know basis. In addition,
access to “executive” aggregations of business data must be restricted only
to vice president or above personnel.
Authentication must be enabled before access is granted to the system for
every user role and access. Authentication will be against the corporate,
standard identity system.
Every input of the system must be self-protective and prevent malformed and
other input injection attacks. Since coding of the system is not under Digital
Diskus control, the vendor must demonstrate a successful attack and
penetration test to include input fuzzing against all system inputs. All
high- and medium-severity fi ndings must be fi xed in each software update
before it may be installed on a corporate network.
For every data source that supports authentication, an authentication must
take place before data may be accessed by the system.
For every data source that supports encrypted communications, encryption
must be enabled to protect data as it is transmitted to/from the system.
Data gathering agents will be run only with suffi cient system privileges to
read the data to be gathered. Write/execute privileges on raw data sources
are not required and must not be granted. Care will be taken to run data
agents at a lower privilege than root, superuser, or administrator, if possible.
If the operating system allows, a data agent should be run under a unique
User ID assigned expressly to the data agent. Under no circumstances,
should the data agent be run under the User ID of the user of the system or
an administrator.

Business Analytics 265
mine. I invite you to add or subtract from my lists as you see fit. For, in that way, you
begin to make ATASM your own.
By now, I hope that you are becoming more comfortable with the ATASM process.
Assuming that you no longer need to be reminded of the steps and how each contributes
to the understanding of the whole and to each other—Architecture, Threats, Attack
Surfaces, and Mitigations—I will no longer note the process as we analyze the remain-
ing architectures in the following example assessments. Furthermore, in order to pace
the remaining analyses, I will not list out the attack surfaces that are uncovered after
the threat model phase of assessment. Instead, we will focus on the two most important
products of architecture analysis for security: security requirements and residual risk
left over after the requirements are implemented.
The requirements that result from the AR A/threat model process comprise the
security posture of the system. It is the architecture requirements (mostly, the controls
that will make up the system’s defense-in-depth) that become the security inputs into
the architecture, as its developed, which then must be expressed by the design, and
subsequently implemented in the software, hardware, infrastructure, operating system,
execution environment, processes, and practices of those who will use, administer, and
maintain the system. Without requirements, “securing” must be removed from the
title of this book; only a risk assessment has been accomplished from the analysis. This
book is about the process, tools, and craft of bringing systems of any size and type to
a desired risk posture. The requirements that specify those mitigations and controls
that exist and will be built comprise one of the essential goals of ATASM—that is, the
“mitigation” phase of the process.
In a case where the requirements that can be built do not bring the system to the
desired security posture, there is residual risk,* as described in Chapter 4. Clearly stating
any residual risk is also a key output of the ATASM process: Decisions will need to be
made. Should the organization accept the risk? If not, must the organization invest in new
capabilities in order to treat the risk? Does the system go live with the risks untreated?
Or should the project be stopped until treatment, that is, mitigation is possible?
The task of the assessor is to present the residual risks in a manner that can be
understood by each stakeholder community, each stakeholder type. In the organiza-
tions in which I’ve worked, it has not been the task of the security architect to render
risk decisions (though in some organizations, it is). Instead, the security architect is
the organizational “truth teller” who tries to convey risks in a manner that facilitates
decision making. The architect has been charged with uncovering risks and assessing
their organizational seriousness, a due diligence task for the organization, for which the
security architect has been fully empowered.
It has also been the task of the security architect to understand who the appropriate
decision makers are, depending upon the scope and seriousness of the risks that are to
* We are concerned only with negative risk. Positive risk is out of scope of the process of archi-
tecture risk assessment (AR A) and threat modeling.

266 Securing Systems
be raised for a decision. The architect (or peer reviewing architect team) must decide the
scope of the risk’s possible impact (consequences). The scope of the impact dictates at
what level of the organization risk decisions must be made. The decision maker(s) must
have sufficient organizational decision-making authority for the impacts. For instance,
if the impact is confined to a particular system, then perhaps the managers involved
in building and using that system would have sufficient decision making scope for the
risk. If the impact is to an entire collection of teams underneath a particular director,
then she or he must make that risk decision. If the risk impacts an enterprise’s brand,
then the decision might need to be escalated all the way to the Chief Operating Officer
or even the Chief Executive, perhaps even to the Board of Directors, if serious enough.
The scope of the impact is used as the escalation guide in the organizations for which
I’ve worked. Of course, your organization may use another approach.
Thus, there must be two primary outputs of the ATASM process, at the very least*: a
set of architecture, design, implementation, and testing requirements and a statement of
any system risks that the requirements do not adequately address in the presence of the
threats the system will face when it goes live. In the succeeding analyses in this section,
we will focus on these results.
In order that the remaining analyses are as brief as possible, we will proceed in a
more realistic manner. As the architecture is understood, attack surfaces are uncovered.
While fresh in the mind, requirements will be suggested at that point in the text. There
will be no more lists of threats, attack surfaces, or mitigations. Rather, we will strive to
develop as complete a set of security requirements as we can for each system analyzed.
1. Provost, F. and Fawcett, T. (2013). Data Science for Business. Sebastopol (CA): O’Reilly
* Th is does not preclude other outputs, such as a threat model or a description of what has been
analyzed, the list of threats that have been considered, and similar artifacts.

Chapter 9
Endpoint Anti-malware
Let’s now leave the world of Digital Diskus and turn to a set of architectural problems
that is different from securing enterprise architectures. You may remember the discus-
sion of endpoint antivirus software from Chapter 3. Figure 9.1 diagrams a more com-
plete set of the components that might be used within endpoint antivirus software.
Figure 9.1 Endpoint security software.

268 Securing Systems
As was explained in Chapter 3, the granularity to which level software must be
decomposed is radically different compared to the previous analysis—business analytics.
If you recall, we didn’t have to break down the executable package(s) that comprised
the three core modules of the business analytics system. That’s because we could make
assumptions about the running environment, the underlying host and its security pos-
ture. For the business analytics example, we knew something about the protections—
both technical and process—that were applied to the host and operating system upon
which the core of the business analytics system runs in that environ ment. Consequently,
we discounted the ability of an attacker to listen in on the kernel-level communica-
tions, most particularly, the “localhost,” intra-kernel network route. Additionally, if
an attacker has already gained sufficiently high privileges on the system to control
processes and perhaps even to access process memory, “game over.” That is, the attacker
no longer has any need for exercising most of the typical vulnerabilities since she or he
can do whatever is desired. The only users on these types of systems are high-privileged
users who perform maintenance tasks. Users of the software’s functionality don’t have
to also be users of the operating system.
9.1 A Deployment Model Lens
For software that must go on an endpoint to protect it, the assumptions about operating
system detections that were made in the previous analysis no longer hold true. Software
intended for a machine that will be primarily one person’s computer and which will
likely run at least some of the time on possibly untrusted networks mustn’t even assume
that the endpoint is “clean.” Oftentimes, this is especially true in the consumer market.
The user is installing antivirus software in response to the belief that the machine is
already compromised. In other words, endpoint security software must assume that,
very similar to software exposed to the public Internet, it is being installed into an
aggressively hostile environment. Any assumptions about the operating system being
free of successful compromises would cause the endpoint antivirus software, itself, to
become a juicy target. At the very least, assumptions about a safe environment might
lead security software makers to believe that they don’t have to take the care with their
security posture that, in fact, is demanded by the real-world situation.
The foregoing leads us to two axioms:
• Assume active attackers may be on the machine even at installation time.
• Assume that attackers will poke and prod at every component, every input, every
line of communication.
As you may see, this is somewhat analogous, though at a different level of granular-
ity, to the threat landscape for exposed inputs to the Internet. However, in that situation
we were able to make assumptions about protections to hosts and networks that were
under the control of sophisticated staff (repeating what’s been previously stated).

Endpoint Anti-malware 269
In this situation, the network must be assumed to be hostile. The system “adminis-
trator” may have very little understanding of computers, much less security. Even so, that
user may be running with significant privileges so they can install software. Consumers
typically give themselves greater privileges simply to make life easier as they go about
their daily business with the computer. This is a very different situation from having a
professional administrative staff providing protections to the system. And, the endpoint
situation is radically different from a situation of restricted, need-to-access privileges.
Due to the likelihood of elevated privileges, endpoints (especially consumer end-
points) make juicy targets. Compromise of the user account is equal to compromise of
the operating system, since the user is the administrator. Compromise at administrative
privileges means that the attacker controls the machine and every user action taken upon
it. All data belongs to the attacker. In addition, the attacker may use the machine for her
or his own purposes, quite likely without the owner of the machine knowing or under-
standing what is taking place. This level of control offers attacker value in and of itself.
Every threat agent that has so far been mentioned will be attacking broadly dis-
tributed security software. Although consumers probably don’t need to worry about
state-sponsored industrial espionage, if the antivirus software is aimed at businesses,
especially enterprises, and is in use by consumers, the maker of the software should
consider these threat agents among those likely to be attacking the software.
Of course, cyber criminals will want to get around any protections of the antivirus
software. They may even test the software for vulnerabilities that allow them to mas-
querade (spoof ) as the antivirus software during attacks. Indeed, I think it’s not too
much to say that every type of attacker and researcher is “gunning” for every type of
security software, including antivirus software intended for endpoints.
If the maker of antivirus software wants to be successful, the software has to be as
close to bulletproof as the maker can possibly make it. Nothing is perfect; we certainly
should understand at this point that no software can be proven bug free and that no
security posture is 100% risk-free? As an aspiration, in my experience, security software
vendors understand the level of attack that their software must resist. Failure to do so
will put the company’s future at significant risk.
9.2 Analysis
As we explored in Chapter 3, there is a natural division of privilege and access between
user and kernel execution. Even at administrative privileges, this boundary is still
important to acknowledge and protect. Gaining the kernel, the attacker has everything,
absolutely everything under attacker control.
Since it’s so dangerous, why would designers install software into the kernel at all (or
make use of kernel software)? In order to capture events and activities across every appli-
cation, as we noted in Chapter 3, security software will have to “breach” or get out of
its assigned process space. The kernel has visibility to everything—all drivers and their
associated activity flow through the kernel. In order to gain access to all this activity, and

270 Securing Systems
protect against anything abnormal or malicious, security software has to first achieve
full visibility. This is typically achieved by running software within the kernel.
The user mode application must initialize and start the kernel software (usually as a
driver). But after the kernel software has been started, the flow should be from kernel to
user. In this way, attack from user to kernel is prevented once the entire security system
has been started. That still leaves the attacker an avenue to the kernel through the ini-
tialization sequence of the kernel driver during startup. That call cannot be eliminated;
kernel drivers must get started; kernel services need to be initialized and opened. This
one opening call remains an attack surface that will need mitigation.
In Figure 9.1, there is only one kernel mode component, the module that “watches,”
that is, catches events taking place throughout the operating system and from within
The kernel driver should not allow itself to be opened from just any binary. Doing
so, allowing any user mode binary to open it, opens up the attack surface to whatever
code happens to get an opportunity to run on the operating system. Instead, the attack
surface can be reduced if the kernel driver performs some sort of validation that only
allows the one true Antivirus (AV) Engine to open it. Depending upon the operating
system’s capabilities, there are a few methods to provide this authentication: binary sig-
nature validation over a binary hash, which can then be re-calculated. What the kernel
driver must not do is simply check the name of the binary. Filenames are easily changed.
Cryptographic signatures are best, but not all operating systems can provide this
capability to kernel mode processes. The solution to this problem, then, is operating
system dependent. Hence, without knowing the operating system specifics, the require-
ment can be expressed generally. It may also be true that our endpoint security software
must run under many different operating systems. We can express the requirement at a
higher level, requiring authentication of the AV Engine binary, rather than specifying
the authentication method. Whatever executable validation method is offered by the
operating system will fulfill the requirement.
Previously, we examined three components that are shown in Figure 9.1. A user
interface is shown that allows the user to control and configure the security software
to meet the user’s needs. There is an AV Engine that performs the security analysis on
files, network traffic, and other events. The events are captured through the kernel
driver sitting across the kernel/user mode trust boundary. Obviously, the user inter-
face must be accessible by the user; it must run as an application in “user mode,” like
any other application. However, unlike a word processor or spreadsheet application,
the user interface can set security policy, that is, the user can turn off and turn on
various security functions, such as whether files are scanned as they’re opened (“real-
time scanning”), or set policy such that suspicious files are only scanned at scheduled
times at a periodicity of the user’s choosing. From a security perspective, this means
that the user interface has the power to change how security is implemented on the
machine—the power to change how well or how poorly the machine is defended by
the security software.

Endpoint Anti-malware 271
In consumer-oriented products, the user will have the ability to turn security func-
tions off and on. In corporate environments, usually only system administrators have
this power. The user interface can take control of the security of the system. That, of
course, makes the user interface component an excellent target for attack.
Likewise, the AV Engine performs the actual examination to determine whether
files and traffic are malicious. If the engine can be fooled, then the attacker can execute
an exploit without fear of discovery or prevention. Consequently, a denial of service
(DoS) attack on the AV Engine may be a very powerful first step to compromising the
endpoint. To contrast this with the user interface, if the attacker is successful in stop-
ping the user interface the security services should continue to protect, regardless. On
the other hand, if the attacker can stop the AV Engine, she or he then has access to an
unprotected system. Each of these components presents an important target; the targets
offer different advantages, however.
As was explained in Chapter 3, any component that runs in the kernel should be
considered a target. The kernel acts as the superuser, with rights to everything in the
operating environment and visibility to all events. If a kernel component contains an
exploit, the consequences of exercising the exploit are catastrophic, at least in the con-
text of the running operating system.
Testing shows the presence, not the absence of bugs.1
Despite best efforts, no software can be proved error free. With that fact in mind,
the defense should be built such that the impact of a vulnerability, perhaps a vulner-
ability that can be used to run code of the attacker’s choosing, will be minimized. If we
can’t prevent the leak into production of a dreadful vulnerability, at least we can make
the exercise, the access to that vulnerability, as difficult as possible. For security soft-
ware, we have already stated that rigorous assurance steps must be built into the testing
for the software. Still, we shouldn’t completely depend upon the success of the testing;
rather, we should make it very difficult for an attacker to access a kernel component.
If the kernel driver is written correctly to its requirements (see Requirements below),
it should only accept an incoming connection at startup, exactly once, and from the
validated engine component only. By meeting these requirements, the attacker must
compromise the engine in order to have access to the kernel mode driver. Additionally,
since the driver initialization and open happen very early in the startup sequence of
the operating system, the attacker must be ready and waiting to take advantage of any
vulnerability at that early moment during startup. That places another barrier to the
attacker. Although this additional barrier is not insurmountable, it does mean that
merely getting the attacker’s code to execute in user space does not guarantee access to
the kernel driver.
For most operating systems, there’s protection to the kernel driver by guaranteeing
that only a single open operation may occur. Further, the open call may only be made
from a validated binary. An additional restriction can be that the open driver call may

272 Securing Systems
only take place during the startup sequence. The attacker has only one avenue through
the endpoint security software to get to the kernel. And that avenue is solely through
the engine, once, at a point at which there is no logged-in user.
The foregoing implies that an attempt to get from user to the kernel driver will
have to be attached to operating system initialization (boot sequence) during normal
operation. (Usually, such a system change requires higher privileges.) Then the attacker
must either lie in wait or force a restart. A forced restart may get noticed by the user as
unusual and unexpected, in response to which the user might perform a security check.
This is not an easy or straightforward attack scenario. It has been done. But it’s far from
ideal and fraught with possible failures.
The AV engine itself will have to be written to be as self-defensive as possible. Even if
the AV engine validates the user interface before allowing itself to be configured by the
user interface, still, those input values should not be entirely trusted. The user interface
may have bugs in it that could allow attacker control. In addition, the user interface
might pass an attack through to the engine from external configuration parameters.
We’ll examine that input in a moment.
The AV engine has another input. In order to determine whether files are malicious,
they may have to be opened and examined. Most certainly, in today’s malware-ridden
world, a percentage of the files that are examined are going to contain attacks. There’s
nothing to stop the attacker from placing attacks in suspicious files that go after any
vulnerabilities that may lie within the file examination path. Thus, the AV Engine must
protect itself rigorously while, at the same time, examining all manner of attack code.
In fact, the entire path through which evil files and traffic pass must expect the worst
and most sophisticated types of attacks. The file open, parse, and examination code
must resist every imaginable type of file-based attack.
If the foregoing comes as a surprise to you, I will note that most industrial-grade,
commercial antivirus and malware engine examination code must resist attack in pre-
cisely this manner; the need for rigorous self-protection has been in place for many
years,* as of the writing of this book. Rigorous self-protection has become quite normal
in the world of security software, and, especially, malware protection software.
That this code is written to be expressively self-protective and resistant doesn’t mean
that bugs don’t get introduced to these sorts of engines from time to time. They do, and
will continue to be. But any vendor with some amount of integrity understands this
problem and will do their best to avoid getting caught out by a bit of attack code that
was in a malicious file. Still, I would count “self-protection” and “attack resistance” as
security requirements for the entire examination code path. What that comes down to
is careful memory handling, safe library functions, and rigorous input validation.
* Although the media may trumpet the occasional instances in which malware engines fail,
it may place these into context to consider that a typical instance of a malware engine will
examine tens of thousands of samples correctly and, most importantly, safely.

Endpoint Anti-malware 273
Input validation to be implemented in general purpose, data-driven code is actu-
ally not a simple problem. This has been mentioned above (the business analytics data
gathering and processing modules). For the security person to blithely declare “validate
all inputs” begs a very real and somewhat difficult problem. If the coder doesn’t know
what the inputs are, precisely, how can the input be validated?
Although a complete solution to a data driven, general purpose parsing and exami-
nation engine is beyond this book, I do want to note in passing that this remains a
nontrivial software design and implementation problem. The solution set is likely to
contain data-determined ranges and acceptable input sets based upon each file format.
In addition, in order to prove the defenses, a level of assurance may be attained through
a formal and thorough set of software fuzzers* that become a part of the parser and the
examination engine’s test plan.
Figure 9.1 introduces a few more components to endpoint security software’s archi-
tecture. Because the capability to examine many different file types is a specialized func-
tion, it’s typical to place the file-parsing capability within its own, specialized module.
That’s how our fictitious antivirus software is written, as well. The makers of the soft-
ware want to examine as broad a range of file types as possible. This is so that attackers
cannot simply package up their attack in some obscure file type, which then allows the
attack to slip past the security software. In order to offer the customer as complete a set
of protections as possible, the software needs a “can opener” that is a “jack of all trades,”
readily opening and understanding the formats of just about every file type imaginable
that may occur on each operating system that’s supported by the software. So, as is typi-
cal in this situation, the file-opening software is a separate module.
Suspicious files are passed to the file module to be opened and normalized. The nor-
malized file is then passed back for examination by the engine. The file parsing module
only accepts communication from the engine and no other component. If the filing
module requires any configuration, it is passed through from the user interface to the
engine and then passed when the file parser is started. Would you consider the delivery
of configuration information an attack surface of the file-parsing module?
Another special component is the communications module. Figure 9.1 presents a
standalone system, so why does it need a communications module? In Figure 9.2, we
can see that the system engages in communications with automated entities beyond the
endpoint itself. Even if this were not true, a communicator might be employed for inter-
module communications within the system. In this case, the simple, independently
operating case, it would be a matter of design style as to whether to abstract communi-
cations functions into a separate module. The design was chosen in this case, not only
* Fuzzing is a software testing technique that employs random input values in order to fl ush
out instabilities in software. Core Software Security: Security at the Source, by James Ransome
and Anmol Misra (CRC Press, © 2014), has a more complete description of fuzzing and its
application to security testing.

274 Securing Systems
because the system does in fact support inbound and outbound communications (as
depicted in Figure 9.2) but also for reasons of performance.
The engine must react to events in as fast a manner as possible in order to stop an
attack or, at the very least, recognize an attack as quickly as possible. Time to identi-
fication is a critical factor. The software maker wants to identify attacks in as little
processing time as possible. Communications, and especially network communica-
tions, can take quite a lot of computer time. Although 250 ms is hardly a blink of an
eye (one quarter of a second), a huge amount of processing can take place in 50 ms.
Two hundred-fifty microseconds is almost an eon in computer time. By spinning off
Figure 9.2 Endpoint security software with management.

Endpoint Anti-malware 275
network communications to a specialized module, the engine code saves processing
time for what’s important. The engine won’t have to block continued processing until a
response has been received—the communication module does this instead. In fact, the
AV engine won’t even waste any precious processing time setting up the asynchronous
response handler. All of these time-intensive functions can occur outside the security
processing chain. In this system, all communications have been extracted into their
own module in order to remove them from performance-hungry examination code.
The engine sends events and alerts to the communications module, which then
passes these along to the destination, whether that destination is local, that is, the user,
or some other destination. In this system, the communicator is also responsible for any
log and event files that exist on the machine.
The user interface passes the communications modules’s configuration to it at sys-
tem startup time. The communications module takes input from the engine in the form
of alerts, which then must be passed along to any external management software and
to the user, or placed into the log files on local storage. Communications also takes its
configuration and any user actions that need to be passed to any destination from the
user interface. Where communications are sent is data driven, dictated by the configu-
ration given to the module during its initialization or when the configuration changes.
If the communications module can be stopped by an attacker, malicious actions can
thereby be hidden from the user or any management software monitoring events on
the machine. If the module can be compromised, the attacker might have the ability to
obscure, or even change, events as they’re sent onwards. Further, inbound events, such
as the management software changing the security policies on the endpoint, could be
changed to the attacker’s benefit. For this reason, the communications module must
validate what it is given and must only allow the other modules in the system to send
it messages.
Inward-bound communications, such as software updates, will flow from
Communicator to the user interface manager. In this system, the user interface module
is also responsible for updating software, such as a new set of malware identity signa-
tures, new policies (and other configuration items), or even updates to the system’s
modules themselves. We will take up the requirements for inbound communications
below, with the introduction of management software in Figure 9.2.
As we have seen with other systems that we’ve analyzed, configuration files are
a prime target. Security policy and the functioning of the system can be changed
through the software’s configuration file. In a product that must operate whether it
has a network connection or not, such as endpoint security software, the product must
configure itself based upon files kept on local storage. That’s a thorny problem because
the configuration files are a target that can change protections. Why?
The software that reads the configuration files, as we have seen, must run in user
space and at lower privileges. Typically, this would be the logged-in user of the system.
The logged-in user has the right to open any file available for that level of privilege.
Since the software is running as the logged-in user and must have the rights to read its

276 Securing Systems
own configuration file, under many operating systems, the configuration file can be
read by any application running as the logged-in user. That’s a problem.
Furthermore, if the user decides to make changes, the user interface software has to
be able to write the configuration files back to disk, once again, as the logged-in user.
Do you begin to see the problem? If the logged in user’s configuration module (the user
interface, here) can access and change the files, so can the user. This means that any of
the user’s applications can change the files. This also implies that all an attacker has to
do is get the user to run an application whose malicious task is to change the configura-
tion files. Bingo! Security software would now be under the attacker’s control. And that
must be prevented. It’s a thorny problem.
I’m sure it’s obvious to you that the configuration files used by any security software
constitute a high-value target. This is no different from other systems that we’ve exam-
ined. And it is a standard pattern, not only for security software but for any software
that provides a critical and/or sensitive function, as well. The configuration file is usu-
ally (and typically) a valuable target.
Different operating systems provide various mechanisms for addressing this prob-
lem. Under the UNIX family of operating systems, an application can be started by
the logged-in user, but the application can switch to another user, a specialized user
that only has the capability to run that software. This non-human, application-only
user will usually only have rights to its own files. The Windows family of operating
systems has other mechanisms, such as slipping into a higher privilege for a moment
while sensitive files are read or written and then slipping back to the logged-in user for
the continuing run. For both of these mechanisms, the superuser can circumvent these
protections easily. That is the way the superuser privileges are designed: The superuser
can do whatever it wants within the operating system. However, it should be noted that
if an attacker has gained superuser privileges, there is usually no need to mess around
with configuration files, since the attacker already owns the operating system and all its
functions and can do whatever is desired. Further exploits become unnecessary.
Typically, therefore, security software protection mechanisms don’t attempt much, if
any, restriction of superuser privileges. These “super” privileges are designed into modern
operating systems and, as such, are very difficult to protect against. Instead, it is usual to
focus on restricting what the logged-in user can do. In this way, the focus is on prevent-
ing privilege escalation, rather than preventing what escalated privileges can accomplish.
I will note that some security software in today’s market employs sophisticated file
restriction mechanisms that go beyond what operating systems provide. For the sake
of this analysis, we will not delve into these more extraordinary measures. It’s enough
to understand that the configuration file, in and of itself, is a target and needs protec-
tion. And the inputs in the user interface that read and process the configuration file
also comprise an important attack surface that requires protection. For the sake of this
analysis, we will confine ourselves to protections that can be provided by the operating
system and obvious software protections that can be built into the security software.
Protecting files and secrets on local storage could fill an entire chapter, or perhaps even

Endpoint Anti-malware 277
an entire book, devoted solely to this subject. It is enough, for this analysis, to identify
the configuration files and the user interface inputs, input fields from the user, and the
inputs when the configuration file is read as attack surfaces requiring protection.
There are a few subtle conditions when writing files that may allow an attacker to
misuse the output to write files of the attacker’s choice. Indeed, there are vulnerabilities
in file output routines that can even allow an attacker execution of code. The point
is to misuse the user interface file writing routines to play tricks on the application or
the operating system to get malicious code onto the machine or to get malicious code
executed through the security software’s file output routines.
Furthermore, the user interface will have to be written such that items taken from
the user through the user interface can’t directly be output to the configuration file.
That might be a convenient way to get a configuration file attack back through the user
interface. The foregoing suggests, of course, that outputs to the configuration file will
also require security attention. The output should be considered an attack surface.
It is perhaps obvious, and not worth mentioning except for completeness, that inputs
of the user interface are, indeed, an attack surface. We’ve seen this in previous analyses.
The treatment for input attack surfaces is always the same at a high level: input valida-
tion. Injections of any sort must be prevented.
Do you see an attack vector surface that hasn’t been covered in the preceding text?
By all means, once again, check my work. Be my security architect peer review. Has
anything significant been missed?
In Figure 9.2, we introduce management of the security software. If for no other
reason than updating the antivirus signatures for new strains of viruses, this architec-
ture would not be realistic without this component. Somehow, the AV Engine has to be
easily and readily updated on a regular and continuing basis. New strains of computer
viruses occur at an alarming rate, sometimes thousands per day. Somehow, that infor-
mation has to get to our system as there won’t be timely protection until it has been
updated. This is one of the responsibilities of the communications module. The update
has to come from somewhere trustworthy.
If an attacker can control the update of the malware signatures, they may be able
to hide an attack. For instance, imagine that the update to the signature set identifies
a new strain of virus. If the attacker can prevent the update, through whatever means,
then the new virus strain will not be identified and, thus, prevented. Consequently,
signature (and code) updates are important targets requiring significant protections.
9.3 More on Deployment Model
As has been hinted during the foregoing discussion, security software does not stand
alone, by itself, with no support beyond the endpoint. The days of entirely encapsu-
lated and enclosed endpoint antivirus or anti-malware software have been gone for
some time. The most pressing reason that one doesn’t simply go to the store, purchase

278 Securing Systems
the update, and then come home and install it is that malicious programs mutate and
change too much and too often. The rate of change of malicious program types and
variations has aggressively increased to the point at which it takes security companies
major effort—large analysis systems and teams, as well as hundreds, if not thousands,
of samples per day—in order to keep pace.
The upshot of the current situation is that an antivirus program needs updates on
a continuing and repeated basis. Sometimes, these updates come more than one per
day. The installation of the update has to be seamless such that the user is not incon-
venienced. Imagine if every day on your working computer, laptop, tablet, what have
you, you had to stop what you were doing two or three times a day to install new secu-
rity software. That wouldn’t be a very good user experience. As of the writing of this
work, security software makers have solved this problem by allowing the software to
update itself whenever necessary.
The automatic update model, however, poses some significant challenges. How does
the endpoint software establish the trustworthiness and veracity of the server that’s
pushing the update? In fact, when an update event gets posted, how can the software
know that this isn’t coming from a malicious source?
To make matters more complex, the validity of the update must be established such
that it hasn’t been tampered with between manufacturer and download at the end-
point (integrity). In fact, most security vendors don’t want the threat agents to have the
details of what’s been prevented nor the details of how protections work. At the least,
these proprietary secrets are best kept such that uncovering them requires a significant
work factor. Defense is better if the attacker doesn’t have the precise details of the
strategy and tactics. The foregoing general requirement suggests that communications
should be confidential. This is particularly true when the communications must cross
untrusted networks. If updates are coming from servers across the public Internet, then
the network would not only be untrusted but also hostile.
In Figure 9.2, the management console for the security software is shown as an
“unknown” network. That’s because the deployers may place the management func-
tion wherever is convenient for them. Perhaps management traffic only crosses the
local network. But perhaps it crosses the Internet. That decision is up to the owner of
the software. In this case, let’s assume that the management console is deployed upon
an organization’s local network. And, for the moment, let’s assume that computers are
managed on that same network.*
This is only one deployment model. Generally, for consumer-oriented security soft-
ware, the management console would be a globally available cloud service. We will
take up cloud management problems in the next architecture. For this architecture, the
management console can be considered to exist on an organization’s internal network.
* In a real-world situation, laptops leave the organizational network. But they still need to be
manage d.

Endpoint Anti-malware 279
With this assumption, we can examine how a customer premise deployment model
influences security decisions that are made about a system.
A working concept, when considering the security of customer premise equipment,
is to understand that different customers require different security postures. Systems
intended for deployment across a range of security postures and approaches will do
best by placing security decisions into the hands of the deployers. Indeed, no matter
the guidance from the vendor for a particular piece of software, customers will do what
is convenient for their situation. For instance, regardless of what guidance is given
to customers about keeping the management console for our endpoint system off of
untrusted networks, customers will do as they wish. And, having exposed the manage-
ment console to greater danger, the customer may very well come back and request
hardening documentation in order to reduce the exposure. Numerous times, I have
seen deployment teams insist that a sensitive management console must be placed so
that it’s accessible from the Internet. This is generally considered poor security prac-
tice. But that statement is meaningless in the face of significant business pressure to do
things in some locally unique and idiosyncratic manner.
In a situation in which the software designers also control the infrastructure and
have fairly complete knowledge of the management and security practices (as we’ve
seen in earlier examples), a lot of assumptions can be made. For instance, if the system
is intended to be deployed to a highly restricted network, then there may be no need to
build in extra security controls, such as encrypted tunnels, from the user to the manage-
ment software. Such features become “nice to have.”
Conversely, where one cannot make any assumptions about where the software will
be deployed and what the needs of the customer will be, the security features should
include a set of protections that can be taken as a whole, or control by control, as
required by the deployers. Perhaps at one organization all management consoles are
placed on a somewhat open network? In a situation where the network is not providing
access control, the software will need to implement authentication and authorization
capabilities so that only intended personnel have access. The implementing team ought
to be able to configure the system to encrypt communications over their open net-
work. For this example security system, the communications to the endpoint software
might include policy and other configuration changes. The sensitive configuration
changes proceed across the network from the management console. Hence, deployers
may wish to encrypt these sensitive communications. Like the ability to encrypt console
to endpoint communications, each security capability needs to be under the control of
the administrators managing the system. Each security feature should be individually
adjustable for the needs of the deploying organization.
As much as possible, the software should place its security features and configuration
in the hands of the deployers. The software maker cannot possibly determine the best
settings for every deployment and every organization. Rather, the appropriate approach
is to place configuration of the security posture in the hands of the customers, who
can then make the best decisions for their instance. In this way, systems intended for

280 Securing Systems
a range of independent and locally unique deployments (“customer premise”*) requires
an approach that enables the security needs of those who will deploy the system to be
met, not those who make it.
Returning to Figure 9.2, the management console in this case is intended to be
customer premise equipment. In order to be successful, it will require protection for all
communications, both inbound and outbound. Since the management console itself
presents an attack surface, as we have seen in previous examples containing manage-
ment interfaces for various systems, both authentication and authorization must be
integral for those organizations that have no standard systems. But for those organiza-
tions that do have authentication and authorization infrastructures, the system must
also be able to consume these as appropriate. [Lightweight Directory Access Protocol
(LDAP) is the most widely supported authentication and group membership protocol.]
Implementing authentication and authorization systems is a nontrivial task. The
design must account for the protection of stored credentials, their “password strength,”
and a host of other details. We will not take these details up here, since there are any
number of works that thoroughly cover this subject.
The endpoint security software must be able to validate communications from the
management console. There must also be a mechanism to ensure that policy, configura-
tion, and update communications flowing from the management console to the end-
points has not been tampered with or interfered with, en route.
Since the management console and the endpoint software don’t run on the same
operating system instance (i.e., machine), a vendor binary digital signature that’s been
generated across the binary at creation doesn’t authenticate this particular management
instance. There is no way for the signature and hash to be sent in such a manner as to
prove that the signature belongs on this one management console. While a vendor’s sig-
nature can prove the validity of the vendor’s binary, it cannot prove the validity of this
particular management instance. Without an instance credential, an attacker might
set up a rogue management console that pushes the attacker’s security policy to all the
endpoints under management.
In this case, we need an authenticator that can be transmitted across the network
and which has been generated for this instance. There are several ways to accomplish
this. The way not to do it is to hardcode a credential in the binary of the endpoint and
the binary for the management console as a shared secret. Statically shared secrets, a
single shared secret for all instances, is one of the biggest mistakes that is made when
designing systems. Such a secret should be considered compromised as soon as there are
more than a few instances of the software that are beyond the control of the vendor.
Such secrets are fairly easily ferreted out. They get posted to security-related websites
and lists, as well as to attacker sites. When thousands of copies of the software are in use
* “Customer premise” equipment or software is intended to be used entirely on a customer’s

Endpoint Anti-malware 281
in the field, the secret is not a secret anymore. Furthermore, anyone possessing the secret
now has control over every instance of the software that uses that secret. Not very secret.
The important thing is to force whatever credential will be used to be generated at
installation and to a particular management context, such as an organization, a group,
a department, a network, whatever works for the way that the deployers divide up the
management of the software.
The details of particular types of credentials are beyond the scope of this work.
X509 certificates could be generated, used, and validated. In this case, the private key
used to sign the certificate will need protection. A password generated for only this
purpose will work. It then must be protected wherever it is stored. There are a number
of ways to achieve the desired result. Numerous and frequent discussions take place
among security practitioners about the merits of one approach versus another. I believe
that, for this purpose, we can state that the endpoint communicator must authenticate
the management console before allowing any communications to proceed. The creden-
tial will have to be protected in storage and during transmission (i.e., either encrypted
during transmission or some other scheme, such as using a public/private key pair).
The obvious solution to communications integrity is to bring up an encryption
tunnel to protect the communications across networks. Typically, this is done with
Transport Layer Security (TLS). Again, there’s more than one way to do this suc-
cessfully. The details are beyond the scope of this work. Still, an appropriate security
architecture for the management console to endpoint communications must include
this requirement.
Before deploying new software, whether the updates are to the running modules or
to the malware identification mechanisms used by the engine, the validity of the updates
must be established. However updates are obtained for dispersal by the management
console, whether by download from the software maker’s website, or through some sort
of push outwards to the management console, attackers attempting to insert malicious
code must be prevented. This validation could be done with a standard binary hash
and signature. The hash value can be checked for validity. The signature will be made
with the private key that can be checked against the public key for validity. This is the
standard approach for this problem.
For most organizations, any system having to do with the organization’s security
will be considered sensitive and critical. As such, the parts of the security system imple-
menting management of the system are typically considered need-to-know, restricted
systems and interfaces. In particular, any system, such as this management console, that
can change the security posture of other systems requires significant protection. We
explored this topic at some length and depth when examining several of the previous
architectures. All of the same reasoning and the same security requirements apply, once
again, to this management console. If you have any questions about what those require-
ments are, please feel free to read those sections once again. Like all critical and sensi-
tive management systems, this management console will also need to be well managed,
with careful consideration of access controls to the operating system and storage areas.

282 Securing Systems
9.4 Endpoint AV Software Security Requirements
• Events and data flow from kernel driver to AV Engine only (never from engine to
• Only the AV engine may open the kernel driver, and only on system startup.
“Open” is the only control operation allowed from user mode to kernel driver. No
other component may communicate with the kernel driver.
• The kernel driver startup and initialization code must validate, sanitize, and put
into canonical form inputs from AV engine.
• Kernel driver initialization and startup must occur during the operating system
startup sequence and must complete before users are allowed to log on to the
• Kernel driver initialization must complete before user logon to the system.
• Kernel driver initialization must occur as early as is possible during operating
system startup.
• Before communications are allowed to proceed between any two or more modules
in the system, validation must be performed on the identity and integrity of the
calling process/module/binary. The preferred mechanism is validation of a digital
signature over the binary. The signature must be signed by the private key of the
software manufacturer.
• Every other component except the kernel driver must run in user mode.
• Installation should confine the reading and writing of the configuration files to
the User Interface only.
• The system must have the ability to encrypt communications from/to the
management console. This must be system administrator configurable.
• The management console must contain a user authentication system.
• The management console must contain a user authorization system.
• The management console must be able to authenticate to an LDAP.
• Management console authorization must be able to be performed by LDAP group
• The administrator must be able to configure the management console to use any
combination of:
○ Local authentication
○ LDAP authentication
○ Local authorization
○ LDAP group membership
• The user interface must re-authenticate the user before allowing changes.
• The management console will be able to run in a hardened state. There will
be a customer document describing the hardened configuration. Hardening is
configurable at customer discretion.
• Input validation coding must be implemented on every system input, with particular
care to file parsing and examination path, the reading of the configuration files,
and inputs from Communicator.

Endpoint Anti-malware 283
• The file and event handling paths through the code must be rigorously fuzzed.
• All components of the system must be built using a rigorous Security Development
Lifecycle (SDL), with particular emphasis on secure coding techniques, input
validation, and rigorous proof of the effectiveness of the SDL (i.e., security
assurance testing for vulnerabilities).*
• Vulnerability testing before product release must be thorough and employ multiple,
overlapping strategies and tools.
The above list of requirements completes the security analysis upon the endpoint
security system. Because the management console is intended to be installed and run by
the customer rather than by the vendor, the management console has to have sufficient
security features so that the customer’s preferred security posture can be implemented:
customer “manageable.” This is as opposed to hosted systems, which will be man-
aged by the same organization. Manageable (by others) as opposed to managed (by the
organiza tion that created the software). This is an important distinction that was called
out earlier in this book: “the deployment model.”
As was noted, the list of requirements presented here cannot be taken as a complete
list, since some of the requirements refer to previous discussions and are not reiterated
here. For a real-world system, I would list all requirements, so as to create a complete
picture of the security needs of the system. The architectures presented in this book, I’ll
reiterate, should be taken as examples, not as recipes.
1. NATO Science Committee (1969). Software Engineering Techniques. Report on a
conference sponsored by the NATO Science Committee, p. 16. Quote from Edsger
Dijksta, Rome, Italy, 27th to 31st October 1969. Retrieved from http://homepages.cs.ncl.
* Please see Core Software Security: Security at the Source, by James Ransome and Anmol Misra
(CRC Press, © 2014), for a complete discussion of the SDL and how it can be implemented

Chapter 10
Mobile Security Software
with Cloud Management
We might almost take the discussion for endpoint security software assessed in Chapter 9
and apply it more or less completely to mobile software. Many of the security problems
are analogous. The software has to provide protections, whether it’s connected to a net-
work or not. On the other hand, configuration and management are delivered over the
network when the device is connected. In order to examine yet another architectural
pattern, this example mobile security product will make use of cloud-based manage-
ment software and a Software as a Service (SaaS) “reputation” service. Just for clarity,
for many real-world mobility protection product implementations, the customers may
deploy their own management servers, which is exactly analogous to the problems we
examined for the management console of the endpoint security system. In this example,
we will not take up issues related to management from a cloud-based service.
10.1 Basic Mobile Security Architecture
Figure 10.1 presents the architecture for a hypothetical mobile security protection sys-
tem. Many of the components are the same in the two endpoint security architectures.
Incoming and outbound communications have to be established and maintained. An
engine must process system events and examine possibly malicious data items. The
engine has to respond to these with protective actions while, at the same time, raising
alerts to the user of the device and, perhaps, outbound to the management components.
These functions will likely be relatively familiar to you, by now?*
* If you have doubts, please re-read Chapter 9’s endpoint security analysis.

286 Securing Systems
Once again, as noted previously, the processing engine must be able to examine a
gamut of different file types and formats. Everything that was stated earlier about end-
point security applies just as well in this situation.
10.2 Mobility Often Implies Client/Cloud
We’re going to extend the feature set somewhat for mobility software. As of this writ-
ing, most devices are generally not islands unto themselves. For many of these devices,
the network is “always on.” Except for the devices out of range or off-grid, the network
connection is omnipresent as a core piece of the functionality of the device.
The protections from the security software must continue when the device is taken
off the network, such as when it’s off-grid, or in airplane mode and similar. Still, much
of the time, software writers can expect the device to be online and connected, not
only to a local network but to the World Wide Web, as well. Web traffic, as we’ve seen,
has its own peculiar set of security challenges. What are the challenges for an always-
connected, but highly personalized device?
Figure 10.1 Mobile security software.

Mobile Security Software with Cloud Management 287
Most notably, attackers set up malicious websites. These can be returned through
search engines, of course. But also, there are the ever-present phishing attacks bom-
barding users. In addition, people personalize their devices with special purpose appli-
cations of all kinds. Malicious website URLs get delivered through many different
vectors. This mobility security software includes a URL reputation service running
in the cloud. When the user clicks on the URL in order to access a website, an event
gets generated from the kernel to the AV Engine, indicating that the URL needs to be
checked first before access.
A reputation service is so-called because many behavioral data points are kept about
a particular URL, file, or Internet Protocol (IP) Address. Data is collected from vari-
ous sources about the observed behavior of the object for which reputation is being
calculated. A URL (or other object) will fall somewhere in a continuum from absolutely
malicious to reputable (good), with various degrees of “gray” in between those two
poles. Based upon the reputation rating, software can prevent access, present the prob-
lem for user decision, or passively allow the access.
The details of the data used in the calculation are generally proprietary and closely
held secrets of the vendors providing the reputation service. For our purposes, it’s prob-
ably enough to know that reputation services are often implemented in a cloud, gener-
ally a global cloud. They are operated as a Software as a Service (SaaS). Reputation is
usually checked before proceeding by a piece of local security software (the AV engine,
in this case). The check is performed over the Internet via some encapsulation within
the HTTP protocol. The backend of the reputation service usually involves “big data,”
the archive and examination of billions of individual data items that form the basis of
reputation calculations.
We will take up the security of the reputation service below, when we analyze
Figure 10.2. For the moment, it’s important to note that the communicator shown in
Figure 10.1 implements all the functions listed previously for endpoint security soft-
ware. There is one additional communication feature that has been added. The com-
munications module must request reputation from the reputation service before taking
action on an individual URL that the user has attempted to access.
Although it may not be apparent when looking at mobile devices, they do have per-
manent storage. Files are kept on the device’s permanent storage. Applications create
files. Applications manipulate files. Device users view and edit files. Indeed, on the
device’s storage, a typical operating system directory structure does, in fact, exist. There
is a “tree” of directories (“folders”) that is exactly what you would find on your com-
puter or your laptop, regardless of which of the usual operating systems you use. On
many mobile operating systems, the file and directory details are hidden from the user.
Most of these operating systems assume that there is but a single user who has access
to all the files. Since mobile operating systems highlight application functionality, the
details of file access are hidden behind application user interfaces.
The details are more or less hidden from the application, as well. Each application
has a set of directories assigned within its sandbox. Visibility to other directories is
not allowed.

288 Securing Systems
Figure 10.2 Mobile security software with cloud management.

Mobile Security Software with Cloud Management 289
The presence of files implies a security need to examine files to identify those that
may be malicious. Included among the user’s downloaded applications might be mali-
cious applications. But bear in mind that a “mobile device” is nothing more than an
operating system running on a computer. The usual assortment of vulnerabilities that
are file based can also exist on a mobile device. As a result, again, just like endpoint
security software, this architecture includes a file opener and parser. The same analysis
applies as it did in the previous architecture that we examined.
“Intercept” in this architecture replaces the endpoint security software’s kernel
driver. Analogous to the kernel driver, Intercept vectors events on the device to the
engine for analysis. Depending upon the mobile operating system, the intercept module
may exist as an operating system service, which is then made available to applications,
or the intercept function must be supplied by the software vendor and installed with
the security software.
In at least a couple of mobile platforms, no application may install software into
the kernel. As was noted in Chapter 3, the “system” area exists between the appli-
cation mode and the kernel. There are three layers—application, system, and kernel
(in Figures  10.1 and 10.2, the kernel is not diagrammed, as it is not relevant to this
For our purposes, the security issues are approximately the same regardless of
whether Intercept exists as a part of the operating system or whether it was written and
installed with the application. If the intercept service is “native”—that is, a part of the
operating system—obviously its secure coding and assurance is not under the control
of the mobile software maker. Still, the design issues are analogous. The secure coding
and code assurance activities will be the same in each case. If the interceptor is written
for the operating system, the same requirements as given for the kernel driver above
apply to the interceptor, as well. If the interceptor is supplied with the operating system,
then the security software inherits whatever security strengths and weaknesses have
been built by the operating system makers.
Intercept initialization should be performed and communication flows opened early
in the operating system “boot” process, before the user is allowed to log on, in order
to stymy misuse of the startup calls. We saw this previously in the endpoint example.
As in the previous architecture, events should flow only from system to application,
never the other way, just in case an attacker gains control of the application. It is stan-
dard design practice to limit communications from lower privilege to higher privilege.
The appropriate event flow is, of course, a design requirement.
Even though the file system appears to be closed, more or less, from the normal user
of the device, every input we examined in the endpoint security case requires the same
security requirements as we uncovered previously. After all, there is the possibility that
a malicious program can transcend the sandbox and launch an attack via the device’s
file system.
Figure 10.2 introduces the cloud aspects for the mobility security software. The
diagram shows two different services interacting with the endpoint: management and
policy services, and a reputation service.

290 Securing Systems
For mobility architectures that I’ve encountered, enterprises and larger organizations
(or highly security-conscious organizations) tend to prefer to deploy management con-
soles onto the organization’s network. Through local deployment, the organization can
control this sensitive function. This situation is analogous to the management console
that we examined previously for endpoint security software.
10.3 Introducing Clouds
For the consumer and the small business markets, management is typically provided
through a multitenant, shared service run by the software manufacturer. Because the
service is shared across many “tenants,” the cost of providing the services for each cus-
tomer is minimized. Economies of scale help to lower the price so that individual con-
sumers and small businesses can purchase protection from their more limited budgets.
It is this case that we will examine in this example.
Communications from each endpoint must flow across the untrusted Internet. Any
number of networks and Internet providers may be crossed as packets are routed. Hence,
in Figure 10.2, the arrow from the communicator reaches into the Internet, whereas
disconnected arrows move to the cloud-based services. The disconnect between arrows
is to indicate the unknown nature of routing, the unknown networks across which the
traffic must pass.
With thousands of devices or perhaps millions to support, communications are insti-
gated from the device to the service. Most mobility operating systems include a service
to “push” a “notification” to the device, even to a particular application. In this case, in
order to simplify the architecture for study, let’s assume that the only push notification
issued by the management service is a command to instigate communications in order
to receive further commands or updates. Although, in actuality, notifications might
be more complex, this one notification would be sufficient to roll out updates, policy
changes, and the like. Assuming the architectural limitation of a single, “call home”
notification, allows us to state that all communications from the communicator to
the cloud services originate from the communicator. Notifications pushed through the
operating system service come through the interceptor and appear as an event passed
to the engine. Push notifications have a different flow than communications from the
application to its services.
Exactly the same problems exist from the management service to the mobile endpoint
software that exist from the management console to the endpoint. Communications
must be protected over untrusted networks. The device must authenticate to the ser-
vice, at the very least. The security software must have some method for validating the
authenticity and integrity of code and data downloaded from the site to the device. Each
of these issues was examined in some length, above. The requirements are the same.
What is different with cloud services are the challenges from multitenancy. In the
last, cloud architecture example, we will examine multitenancy at some depth. This is a

Mobile Security Software with Cloud Management 291
service problem. It’s not really germane to an examination of an endpoint security solu-
tion. We will hold off until we investigate the next architecture.
As we saw in the Web-Sock-A-Rama example, simply placing services on the Internet
invokes a raft of security necessities: server hardening, layering and trust levels, manage-
ment interfaces, and so on. If you feel unfamiliar with this coterie of requirements,
please return to that earlier analysis for an in-depth discussion of Internet exposure.
Let’s turn our attention to the additional feature: the reputation service introduced
in Figure 10.2. As we noted above, “reputation” in this context refers to how trustwor-
thy or malicious an object may be. “Object” might be a URL (web destination), a file,
an email domain or address, an IP address, and so forth. The calculation of reputation
is outside our scope. It is sufficient to acknowledge that security reputation services
exist, both free and commercial. The object is presented to the service, which returns a
reputation—some sense of how safe or dangerous the object may be to access. Usually,
reputation is delivered as a grade along a scale from known to be malicious to known
to be safe. Considering the hundreds of millions of websites on the Internet, gathering
sufficient information about websites and then calculating a score is a nontrivial, “big
data” problem. Significant resources are expended in order to make as good a guess
about reputation as possible.*
Usually, the commercial services are “cloud” grade. That is, the services are available
across the globe so that user access time is reduced. The services are always available,
with failover systems that can pick up upon an instance failure. Cloud services are usu-
ally redundant and backed up to various points of presence spread out geographically
so that a local catastrophe cannot bring the service down. (We will take a look at the
backend architecture of a cloud service in the next analysis.)
One of the big challenges with cloud-based reputation checks is performance. Users
don’t typically want to wait a few seconds while the reputation of potential URLs is
checked. Most of us have come to expect that websites are at the immediate tips of our
fingers and that access and loading of the content should take place rapidly and without
delay. This presents a tricky security problem.
Since the reputation service exists in the cloud, the challenge can be summed up as,
“How can a reputation be securely retrieved without slowing Web access down so much
as to create a poor user experience?” The details of cloud performance, points of pres-
ence, calculating the shortest distance, data caching, and associated issues are largely
beyond the domain of security. We will take a cursory look at some of these issues in
a subsequent, cloud-based SaaS service analysis. Still, it’s important that the security
components of the reputation service don’t inordinately affect overall performance.
This is one of those cases in which there has to be “just good enough” security, and
no more, because security controls often impact performance. For instance, bringing
* Business success probably depends upon accuracy. Still, mistakes do get made, and inaccurate
ratings get produced.

292 Securing Systems
up a Transport Layer Security (TLS) connection takes a fair amount of computer and
transmission time.
One key question when designing a service such as a reputation service is to decide
how secret the reputation needs to be kept. Obviously, if attackers know that a mali-
cious site has a poor reputation, they may respond to that situation by changing the
domain of the site, or they may take other evasive action. That’s an argument for only
allowing the mobility software to check reputations and to protect the reputation ser-
vice from query by any other systems. Further, if the vendor intends the service as a
commercial offering for profit, allowing open access to everyone would be a competi-
tive disadvantage.
On the other hand, owners of domains may have their web properties misclassified.
Domain owners need a method to check the reputation of a domain and to address
a poor reputation rating with the reputation service provider. I personally have had a
couple of websites misclassified once or twice over the years and have had to go to a
reputation service and complain. Supposedly, all my websites are legitimate. As far as
I know, none of my sites allow or foster malicious activities, at least not intentionally.
Still, I have been classified as malicious a couple of times. Mistakes get made. Any
reputation service that expects to stay viable must offer domain owners a way to check
their domains and URLs and to lodge a complaint if misclassified. Obviously, redress
can also be abused; no doubt, redress services are abused.
So, what is the right security posture for a cloud-hosted reputation service? The
answer to this question is hotly debated regularly. Even within organizations, there will
be a diversity of opinions about how much security is “enough.” Most security controls,
unless it is trivial to implement, are likely to draw significant discussion and generate at
least a few strong opinions. Even so, we have a system under analysis. One of the sys-
tem’s components is a cloud-based reputation service that provides proprietary features
for commercial mobile security products. Therefore, the reputation service cannot be
wide open and without access restriction.
As we have seen, security decisions must be based on the objectives that are to
be fulfilled by the security. Earlier examples have explored the issues confronting an
Internet-exposed system. Both services in this example require the same protections
from constant and unremitting attack from the Internet. This should come as no sur-
prise. These protections must include host and network hardening, as well as all the
management security capabilities that have been previously laid out.
10.3.1 Authentication Is Not a Panacea
Generally, Web systems that don’t serve public content will require authentication to
reduce traffic, and thus exposure, to only the intended population rather than anyone
who can send traffic on the Internet. As we saw previously, websites with wide-use

Mobile Security Software with Cloud Management 293
demographics may get very little value from authentication. In this case, these services
are to be consumed only by paying customers. That, of course, won’t stop an attacker
from obtaining a paying account from which attacks can be mounted. Authentication
can’t be the only control in place, for this single reason. Still, and nonetheless, the vast
majority of paying customers are looking for security protection, not for a system to
attack. Authentication is likely to reduce the attack surface by at least removing expo-
sure to all the automated attack sweeps that are ever present on the Internet.
But in this case, is it the user who should be authenticated? Well, yes and no. The
users must be old enough to manage their accounts, manage their devices, configure
their security policies, and, of course, pay for the services. For these reasons, there will
have to be a user interface and user services. These will presumably employ authentica-
tion to tie account and services to users. The architecture presented in Figure 10.2 does
not include the web authentication services. We examined these issues previously in the
Web architecture example, Web-Sock-A-Rama.
What are shown are the other services that the software uses in an automated
fashion. If the user were forced to authenticate every time he or she accessed the URL,
I suspect that the security software would not have a very long or useful life on that
person’s device. Instead, the software will have to authenticate to the services. That’s a
slightly different architectural problem.
Should the application be the entity that authenticates? Or should it be the device?
Again, there are arguments back and forth about this design choice, which we won’t
go into in this example. There are strong reasons for authenticating the device. And
that’s the pattern that this system employs. If it’s the device that’s under management,
even if the software breaks down, the management service has some concept of the
device itself. If the device is lost, it can contain an identifier, regardless of whether the
software on it has been reset by a thief. The mobile carrier interacts with the device; the
management service, which is able to identify the device, may interact with the carrier
about the device, if necessary. Both the management services and the reputation service
authenticate the device, not the user. I reiterate that, in general, mobile operating sys-
tems and architectures assume a single user per device.
One way to accomplish the authentication is to issue a certificate to the device.
Along with the certificate, which can be validated, when the device is enrolled, its serial
number and the carrier’s identifier will be tied together. The communicator brings up a
TLS tunnel to the server, validating the server certificate, as per the TLS protocol. Then
the device certificate is presented. If validated, communications can flow. This means
that only enrolled devices are allowed reputation services and are under management.
The foregoing scheme does not protect the system against a rogue or compromised
device. It doesn’t protect the system against a malicious, paying customer. Other protec-
tions will need to be placed around the services in addition to the authentication. Along
with the authentication, all the protections together constitute a defense-in-depth. (The
SaaS architecture below will delve into other protections.)

294 Securing Systems
10.3.2 The Entire Message Stack Is Important
Once communications have been established, the mobility security software must vali-
date messages, commands, data, and software that come from the cloud services. This
is the same problem as was seen between the management console and the endpoint
security software. Data and software will have to be hashed and then signed such that
the signature over the hash can be validated on the device. This problem is no different
from what we explored previously.
Should the communicator authenticate the server? TLS is being used from the device
to the server. TLS always validates the server certificate before proceeding. However,
that authentication typically only validates that the certificate was issued by a known
Certificate Authority and that the certificate is valid for the IP address of the server. In
other words, any TLS server that has a valid certificate will successfully pass the server
side TLS authentication. What if our mobility security software gets directed through
DNS spoofing to a malicious site? And what if that malicious site presents a valid cer-
tificate? The TLS will start up with no errors.
Due to the details of server-side TLS certificate validation, in the absence of another
authenticator or authentication scheme, a device won’t necessarily know that it’s talking
to the true set of services. TLS server-side authentication usually happens somewhere
below the application on mobile devices.
It doesn’t make sense for each application to install TLS/SSL services on a device.
Mobile devices run on batteries (i.e., must do extreme power management) and tend
to be highly storage constrained. This means that the less code an application must
install to function, the better. Good mobile applications should use the system services.
The foregoing means that a TLS tunnel will be instantiated for an application, then
returned as an open connection (“tunnel,” “pipe,” or “socket”). The details of server-side
authentication are opaque to the application. The validation of the server’s certificate
takes place behind the scenes, from the application’s point of view.
Of course, an alternate or extra validation can be performed on the server certificate
to ensure that the connection is to the right set of services. That would be an extra
validation above the TLS validation provided in the protocol’s server-side authentica-
tion. In order to effect a validation of the server’s certificate (beyond that provided by
validating the chain of signing certificates), the server’s certificate must be stored for
comparison on the device. This validation must be performed after TLS has been suc-
cessfully instantiated (by the system, as described above).
Is an extra validation necessary in this instance? Will the overhead of an additional
certificate validation, and deployment and update of the server’s certificate to every
device, be worth the additional management issues and, more importantly, the extra
time taken to set up the communication channel?
I would argue, “No.” Why?
If all significant data and code is validated by hash and signature, what can an
evil site do even if connected? This means that validation of signatures over binaries

Mobile Security Software with Cloud Management 295
and messages has to be nearly bulletproof. And I’ve stated many times that a defense
should not rely on a single control. If the signature validation fails, then an attack
could proceed.
Often, the errors that crop up in cryptography are not issues in the cryptography but,
rather, “implementation errors.” That is, the implementer made a mistake while coding
the algorithm such that the cryptography has been weakened, perhaps significantly. Or,
over time, new mathematical understandings or increases in processing power render
a particular cryptographic algorithm weaker. These are the dangers of an overreliance
on a single cryptographic algorithm, such as a hash algorithm and signature, or relying
on a particular implementation. As long as the implementation has no errors and the
algorithm stands, the protection remains strong. Over the years, I’ve noticed that this
position of strength can be changed or watered down in almost an instant.*
Since these cryptographic change events are relatively rare, I might, in this case,
favor reliance on the signature across the binary. It’s a trade-off between performance
(customer acceptance) and security. As you encounter similar security situations, you
are likely to be faced with such trade-offs on a regular and repeating basis.
10.4 Just Good Enough Security
As was noted above, in this case, we need “just good enough security.” Let’s dig a little
into how I might determine how much “security” will be “good enough.”
In order to proceed, an attacker has to successfully line up a number of somewhat
difficult exploits: DNS spoofing, a valid server certificate proffered from a malicious
site, the signature failure. This is not an impossible scenario. But this set of coordinated
and interlinked, dependent attacks is not particularly easy, either. Given sufficient moti-
vation, as we have seen, this attack scenario (and more) can be completed. But I would
offer that this attack is probably extraordinary and would likely be highly targeted.
Depending upon the adversaries against whom you are protecting the device,
“good enough” might well be a reliance upon the message’s digital signature alone.
Alternatively, for high-security situations that require more assurance, the addition of
a validation of the TLS certificate itself could be added. In addition to validating the
certificate authority signing chain, also validate the certificate that is presented by the
server. Make it difficult to impersonate the reputation server. There is no “right” answer
to this question. It depends upon the security needs of the organization and how much
risk can be tolerated.
I invite you to consider this situation. Make up your own mind: What is enough
security? What’s too little? What might be too much?
* Really, the research that achieves these shifts often takes years. Still, once the research has
been announced, an algorithm can make a shift, as it were, from “safe” to unprotected very
rapidly, that is, in hours, globally.

296 Securing Systems
Figure 10.3 Cloud-hosted mobile device management using the Hardware Security
Module (HSM).

Mobile Security Software with Cloud Management 297
Since each device will be issued a certificate, device certificates will need to be
signed. The signature is generated from a private key so that the signature may be
validated with the public key. The private key, then, is an important secret that must
be protected from disclosure. If disclosed, anyone possessing the key may create a valid
device certificate. Doing so would then invalidate an important authentication, as
described previously.
How do you protect a secret like a private key?
The addition of a Hardware Security Module (HSM) can help to protect the private
key that will sign each device certificate and all the messages from the management
function to the devices (as specified above). But HSM’s are not a panacea, just like the
use of TLS is not a security cure-all. Deploying an HSM requires some forethought
and architecture.
If you glance at Figure 10.3, you’ll notice that the HSM has not been placed within
any of the outer layers where it would be more exposed. The HSM is placed within the
data layer, which only takes communications from the second layer, which lies behind
HTTP termination [the demilitarized zone (DMZ)]. The second layer, the “applica-
tion” layer, has a message flow only from the DMZ or outer, exposed layer. Attackers
must first gain the HTTP termination before they may proceed onwards to attack the
application layer. The HSM only accepts key signing requests from the application
layer. Thus, before an attack can be mounted against the HSM, both HTTP termina-
tion and application must be compromised.*
Since compromise of the private key will obviate a couple of the security controls in
the defense-in-depth, it will need protection, significant protection. A classic situation
then emerges: We add a security control, device certificates, which then cause a new
target to emerge (the private signing key), engendering attack surfaces that don’t exist
without the addition of device certificates.†
One protection, as described above, is to limit communications flows. Is that suf-
ficient? Probably not.
I would also require that certificate requests be constructed by the communicating
layer, in this case, Device Enrollment and Management (the application layer). We’ve
seen this mitigation before (Web-Sock-A-Rama and business analytics) as a method
for limiting exposure to inner components. The device certificate will be created and
then signed on the HSM. No data need be passed from the outside to the HSM. The
* Of course, all of these components have to be managed through their administrative
interfaces. Administrative communications are restricted as per the AppMaker architecture
presented earlier.
† In other words, adding device certifi cates fundamentally changes the architecture of the
system. Any architectural change should require a reassessment of the security of the system.
I go into some depth on this issue in Chapter 9 of Core Software Security, by James Ransome
and Anmol Misra (CRC Press, © 2014).

298 Securing Systems
untrusted device cannot originate the message requesting a certificate from the HSM.
(We’ll examine outbound message signing below.)
Here is the order of processing:
• The user enrolls the security application.
• The user enrolls a particular device.
• A device identifier is generated (must include sufficient entropy to be unpredictable).
• A certificate request is generated for the device identifier on behalf of the device.
• The certificate request is sent to the HSM.
• A signed certificate is returned. The public key of the signing key pair will be
included in the certificate.
• A device certificate for that particular device identifier is returned for installation
on the device.
• The device can use the public key to verify the certificate signature.
• After enrollment, communications from the device may not proceed before
the signature over the device certificate has been verified. This can be done
by comparison against a duplicate of the public key, which is kept within the
directory copy maintained within the authentication system’s restricted subnet.
No communication to the HSM is required in order to validate the certificate.
The duplicate public key does not require additional, extraordinary protections.
As you may see from the foregoing, as depicted in Figure 10.3, only the enrollment
function communicates with the HSM. The certificate request is entirely generated by
the enrollment function. No untrusted data is passed to the HSM.
10.5 Additional Security Requirements for a Mobile and
Cloud Architecture
In order to derive a more complete list of requirements, please return to the list of
requirements from the assessment in Chapter 9. Most, if not all, of those requirements
will be relevant to this architecture, as well. The following comprise the additional
requirements that must be added to those for an endpoint anti-malware application.
• Use TLS between the mobile device and cloud services. Server-side TLS
authentication is required.
• Each enrolled device will be issued a device identifier. Device identifiers must be
• Generate and install a certificate on each mobile device under management. The
certificate must be unique to each device. Certificate signature must be validated
before cloud services may be accessed. The private key used to sign the device
certificate must not be deployed to any device. A single private key may be used

Mobile Security Software with Cloud Management 299
for all devices. A better design generates a private signing key for each customer,
although consumers who individually purchase protection may all have their
certificates signed by the same private key.*
• The private key used to sign the device certificate must be stored in an HSM or
equivalent. The network must be configured such that the HSM will only accept
cryptographic operations from the enrollment and management service.
• All data (commands, reputations, policies, configurations, etc.) and all binaries
downloaded to devices must be hashed and signed. The device software will
validate the signature and the hash before any further processing.
* Discussions about Public Key Infrastructure (PKI) in general and certifi cate revocation in
particular have been avoided purposely. Issuing private signing keys would require a robust
PKI. When using X509 certifi cates, certifi cate revocation must be designed.

Chapter 11
Cloud Software as
a Service (SaaS)
In this, our last architecture, we examine the cloud “SaaS” that implements the reputa-
tion service that we encountered in the mobility example. “SaaS” is a cloud acronym for
“Software as a Service.” The meaning of SaaS should become clear through the analy-
sis. If you’re unfamiliar with reputation services for security software, it may be helpful
to return to the mobility example and refresh your understanding.
11.1 What’s So Special about Clouds?
Cloud computing is a model for enabling ubiquitous, convenient, on-demand network
access to a shared pool of configurable computing resources (e.g., networks, servers,
storage, applications, and services) that can be rapidly provisioned and released with
minimal management effort or service provider interaction.1
We will not analyze the management portion of the mobility service. Several example
architectures in this book have included management components. The cloud aspects
of the management service would be roughly equivalent to the other examples, save for
the distributed nature of cloud SaaS. The present example will explore distribution in
some depth. If this were a real system, security analysis of all the components would be
required, not just the reputation service in isolation.
In the mobility example, we represented the management of the mobility clients
and the reputation service as servers in the “cloud.” Of course, these services require far
more architecture than a single or even a horizontally scaled set of servers. Typically,
any service offered on the web that must process large data sets, what is commonly

302 Securing Systems
known as “big data,” will at the very least have an application layer to do the process-
ing, and some sort of data layer. Usually, because of the complexities of processing,
presentation, multitenancy, and performance, the application processing can’t be done
through a single execution environment. There are just too many tasks that need to be
accomplished to squeeze all that processing into a single, unitary application.
Even with horizontal scaling to increase the processing, there are distinct phases
through which data must flow in order to support an industrial-strength, commercial,
data-intensive service. The tasks in the processing chain are relatively distinct and fairly
easy to separate out. For this reason, there are more than just “layers.” As we shall see,
there are groupings of processing that can be assembled to form logical units. This
allows for a software architecture to emerge at a gross functional level, which fosters the
specialization of programmers in particular types of processing. And it also allows for
better security (and architectural) grouping.
11.2 Analysis: Peel the Onion
The data that forms the basis for making reputation decisions will likely come from
many sources.* Where data are collected from, and precisely what types of data are
involved, are deeply held trade secrets. And this proprietary mix is not germane to the
security of the system. From our perspective, we need to know that data are pulled and
received from many sources via a data-input function. That function, like the Data
Gatherer in the business analytics example, must normalize and parse many different
types of data so that the data may be kept in a unitary fashion that can be processed
for reputations. One logical function, therefore, is the receipt and initial processing of
the various data types. Neither of the figures of the SaaS architecture in this chapter
portray reputation data collection. We will not explore this aspect of the service. We
concern ourselves with the service as consumed by a mobile security application.
Once the data are in a form that can be processed, a continual process in the back-
end works through the data, continuously calculating and recalculating reputations.
There would be an architectural decision in the development of the reputation service as
to whether reputations were calculated upon request for a reputation (i.e., in “real time”).
Alternatively, a reputation calculation would perform its processing over every object
that had changed based upon incoming data. Each of these possible paths has its pluses
and minuses. The decision on which way to architect the system would be made based
upon the relative merits and challenges presented by each method. For our purposes,
neither of these paths presents a wildly different security problem. Let’s assume that
there was a decision to make a separate process for calculating the reputation. And that
process doesn’t receive traffic directly from the Internet.
* One of the sources for new objects will be requests about objects that have not been encoun-
tered by the service previously. Th ese will be entirely new, having no reputation calculated.
We will ignore this typical data input in this example, for simplicity’s sake.

Cloud Software as a Service (SaaS) 303
The key piece of security information here that I’ve just laid out is that messages do
not come directly from the Internet through to the processing function. If you remem-
ber from both the business analytics and Web-Sock-A-Rama examples, one of the key
defenses in both cases was a recomposition of requests before the request was passed
to an inner, more trusted layer. In this way, an attempt to attack the web layer stays at
the web layer and isn’t forwarded on further beyond that initial layer. Although such
a defense is not always possible, depending upon the nature of the applications in the
back-end, it’s relatively common that requests that will need to be processed by inner
layers are generated at the outer, less trusted layers rather than simply forwarding traffic
from even less trusted sources. In this case, the least trusted source would be a mobile
device. Although the entire system does attempt to keep mobile devices safe—that’s
its purpose—there’s no guarantee that some clever attacker hasn’t breached the device
through some means or other. Thus, an attacker could be in possession of a vector of
attack in the demilitarized zone (DMZ)–exposed system. Furthermore, as we know,
an attacker could very well just buy the software, put it on the attacker’s device, and
then use that device as a launching pad for attacks against the system. Hence, despite
the protections built into our mobile security software, including malware protection
and certificate authentication of the device to the service, the reputation service should
make no assumptions about the relative safety of communications from the device.
With the foregoing in mind, a more defensive security architecture will be one in
which messages are received and processed from devices, that is, requests for reputa-
tions via a bastion front end application. The front end “reputation service” to which a
device under management connects then regenerates a request for a reputation to the
reputation processor based upon the parameters sent from the device.
The URL or other object about which the user is asking must still be passed through
to reputation processing. The object data can be inspected, validated for proper form,
and sanitized. That processing will take place in the front end. Still, some data that
is received from the untrusted source will be moved through the front layer and on
to the next. Hold that in mind as we dig deeper through the architecture layers and
attack surfaces.
Because of the separation of functions, from a security perspective it doesn’t really
matter whether the reputation-calculating software performs the reputation calculation
when it gets a request or simply retrieves the reputation that’s been previously calculated
and stored. As long as reputations are kept up to date, that’s all that matters to the cus-
tomer: retrieving the most up-to-date calculation.
It is quite possible that there may be two back-end application functions: reputa-
tion calculation and reputation retrieval. For our purposes, this distinction isn’t really
relevant and doesn’t change the security picture of the system. Some piece of software
presents an attack surface by receiving requests from a less trusted layer. That’s the
important security-relevant piece of information. In order to keep our diagrams simple,
these two functions have been collapsed into a single component.
Figure 11.1 represents the cloud service as it may look, logically, from the user’s, that
is, the mobility security software’s, perspective. The “cloud” means that the service has

304 Securing Systems
multiple points of presence across the globe to be queried. Cloud also implies large scale
to support thousands, even millions, of clients. The mobile device software doesn’t need
to understand any of the details behind that service. In other words, the details of data
retrieval, processing, rating, and responding to queries are entirely opaque to the query-
ing software. This is my intention. Even in SaaS architectures, data and structure hid-
ing is a worthy design goal. As long as the messages remain consistent across versions,
there’s no need for the endpoint software to understand the details of how the query
Figure 11.1 SaaS Reputation Service architecture (in the cloud).

Cloud Software as a Service (SaaS) 305
gets answered. This is no different than an API hiding a library or an object hiding its
internal processing. This is nothing more than structured programming (a rather dated
term) at a very gross, logical architecture level.
Decomposing one of the points of presence, Figure 11.2 shows more detail. If you
recall the Web-Sock-A-Rama web store architecture, the application portion of the
Figure 11.2 Breakout of a SaaS application instance.

306 Securing Systems
reputation service looks surprisingly similar to the previous web example. This is not
to suggest that all web applications are the same. They decidedly are not. However, a
three-tier architecture, separating the web termination, authentication, and message
validation from front end processing logic, which is then separated from further pro-
cessing, is one very typical architectural arrangement.
In order to represent the many management and configuration functions that sup-
port a secure web presence, I have diagrammed a management network, administrative
interfaces, and configuration stores. These would all have to be implemented in order
to run a cloud SaaS service. Since these were explored in the earlier web example, please
refer to that if you have any doubts about the sorts of issues and resultant security archi-
tecture that will normally be put into place for the management of Internet-exposed
systems. A demilitarized zone (DMZ) management network is employed to ensure that
administrative access is highly controlled, just as we saw in the previous web architec-
ture. In this case, access to the management network is through a single constrained
point, which then proceeds to a management server. The management server is the
only source from which the administrative interfaces on the various DMZ components
can be accessed. As before, dual-factor authentication is required before access to the
management network is granted.
As we noted in our mobility example, all devices containing the mobility software,
which is under management, receive an X.509 certificate issued to that device. Only
queries for reputations that present a valid certificate signed by a private key issued by
the reputation service for the mobility software will be accepted. All other attempts
at communication will be rejected. In this way, all the dross attack attempts that take
place on the Internet are consistently rejected. This authentication does not mean that
all attacks are rejected. The value of an authentication as a security control is based on
its ability to reduce access, that is, reduce the attack surface, by rejecting “untrusted”
sources. But “untrusted” is a value judgment. The question that must be asked in order
to understand the value delivered by the authentication is how much does the issuance
of the authenticator tie to some level of trustworthiness? As we’ve noted a couple of
times, there is the distinct possibility that an attacker can become part of the popula-
tion of devices running the mobility software.
11.2.1 Freemium Demographics
Taking a slight tangent from our example, it may be interesting to note that different
levels of trust can be placed upon any particular “authenticated” population. In a busi-
ness model that encourages users to create accounts, such as a social media site, there’s
very little trust that can be placed into the entity that has authenticated access. And, in
fact, that’s precisely what can be seen empirically. Attackers routinely use their authenti-
cated access for malicious activities on social media sites. The authentication means very
little, except that the system has opened an account. I might point out that I personally

Cloud Software as a Service (SaaS) 307
have seen many accounts purporting to be pets on Facebook despite Facebook’s policy
of a single account only given to validated humans. This policy is very difficult to
enforce, especially where the revenue is based on customers’ enjoyment of the site.
Let’s compare social media with its relatively wide-open and easy access to a system
like a banking or investment site. In the latter case, it’s critical that an account be tied
to a validated, authentic human. But even more so, that human will have to be trusted
to be engaged only in legal activities. The possibilities opened by an attacker with
malicious intent who has normal customer access can be quite calamitous. Financial
institutions tend to take a very dim view of attempts to circumvent laws and regulations
through their services. Hence, a fair amount of due diligence takes place before issuing
accounts. And then a fair amount of due diligence and out-of-band authentication takes
place before issuing online access.
For a third example, security-conscious organizations, for instance, government
intelligence agencies or institutions of any kind requiring a fair amount of trust of the
employees, usually go through significant background checks to determine the trust-
worthiness of potential employees. Even after hire, responsibilities requiring significant
trust may be withheld until the new employee proves a level of trustworthiness.
In the last two examples, financial institution account holders and security- conscious
institution’s employees, an authentication will likely be tied to a much higher level of
proven trust; the authenticated entity has been validated through other means. In the
case of intelligence agencies, double agents do exist and occasionally get caught. In the
case of financial institutions, dishonest employees do sometimes get hired. And account
holders are not always as honest as the institution might hope them to be. Therefore, in
each case, we can infer that although authentication reduces the attack surface, some-
times a great deal of reduction, it is very rare that authentication eliminates the attack
surface. Perhaps, never?
One standard method for dealing with the fact that authentication does not protect
against all attacks is to log the actions taken on the system by the authenticated entity.
Logging system activity is a typical additional control. Monitoring is often employed
so that, at the very least, malicious actions can be tied to a user of the system. In order
to monitor, the activity first must get logged. At best, instantiation of malicious action
will be noticed and can be stopped before serious damage takes place. For social media
sites, logging of action sequences is critical to identifying malicious users and ejecting
them from the system before they can harm other users or the system itself.
Since for the reputation service we understand that attackers can gain access, all
actions taking place on the service by each device are logged. Anomalous sets of activi-
ties trigger an investigation from the 24×7, around-the-clock security monitoring team.
In Figure 11.2, you will see the authentication service. As in the eCommerce
example, the architecture has placed the authentication service within its own highly
restricted network segment. Only the web front end may issue authentication requests.
In this way, the authentication service cannot be attacked from the Internet. First, the
front end would need to be compromised.

308 Securing Systems
In addition, the credentials and entities (identity, i.e., device identifiers) are pushed
from a centralized master directory that lies within the most protected internal network.
In this way, even if an authentication service is lost, it cannot communicate inwards,
putting the master directory at risk. Communications are outbound from the master
directory only. (The authentication service must also be managed and con figured, just
as we have seen previously.)
11.2.2 Protecting Cloud Secrets
In the previous mobile security architecture example, we explored the use of Hardware
Security Modules (HSM) for protection of cryptography key materials and signing
operations. This reputation service has precisely the same problem. There are private
keys that are used to sign X.509 certificates that will be used to authenticate devices
requesting reputations. In addition, you may remember that all messages and data
going to the managed device must be cryptographically signed in order to establish
authenticity of the traffic after it has been received by software on the device. This is
true for reputation messages, as well as messages from the management service (which
we are not exploring in this example).
The use of an HSM is the same pattern, with the same challenges as we examined
earlier. Except, each node (point of presence) will either have to communicate with a
central HSM, or, alternatively, each node will require its own HSM. The latter archi-
tecture will be faster. But it does mean that the same private key will have to be pro-
visioned to each HSM.* If there is a change (e.g., a revocation or renewal) to the key,
every HSM will have to be updated. There are interesting timing issues when updating
multiple points-of-presence. We will not investigate these.
Of course, each HSM could generate a separate key. But that also has significant
issues for a global service. Each node across the globe would have to be able to validate
every public key (what is present in the certificate), which requires access to the private
key. This is because devices might be accessing from anywhere; devices travel with
their owners.
There are certificate chaining validation methods that address this problem. And
indeed, if each enterprise customer requires a separate signing key (and most will!), then
our cloud reputation service will have to employ a fairly sophisticated certificate signing
and validation implementation that accounts for multiple certificate signing keys. Still,
messages from the service may be signed with the same private key (so that every device
can employ the public certificate/key to validate).
* One use case is to generate the private key on the HSM and to perform signing operations
with that HSM. Th is way, the key never leaves the HSM. Th at isn’t the only possible HSM
use case.

Cloud Software as a Service (SaaS) 309
Introduction of public key cryptography (PKI) into a cloud scenario serving a
broad range of customer types requires a robust PKI implementation on the part of the
reputation service. Still, ignoring these problems (which are well described elsewhere),
an HSM is still the best method for protecting keying materials from both external
and internal attacks. This cloud service includes an HSM (for performance reasons)
deployed to each node in an architecture analogous to the previous mobility example.
11.2.3 The Application Is a Defense
A further protection at the front end of the reputation service must be the validation
of every request for reputation. Those are the only messages accepted by the service. If
the message is not in the proper form, in any manner, the system must reject it. Since
attackers might possess a device certificate, and thus be authenticated to the service,
rejecting any ill-formed request will provide protection of the front end and, thus, any
services with which the front end communicates, as well.
Even the best, most security-conscious development teams occasionally allow a seri-
ous security vulnerability to leak out into production software. So how can a service
like the reputation service ensure that all non-valid input is rejected? Security software
testing is beyond the scope of this book. Still, it may be worth mentioning that a typi-
cal testing strategy is to “fuzz” inputs that are likely to receive malformed messages (or
other input data). When we examined the file processing chain in the endpoint security
example, fuzzing was one of our recommended security controls. That software must
be made to withstand poor inputs; it will be examining malicious files. In a similar
way, the reputation service front end, which must receive requests from possibly mali-
cious sources, will benefit from rigorous fuzz testing. In order to protect services further
within the chain of processing, the reputation service vendor will want the front end to
be as bulletproof as possible. Fuzzing is an accepted test methodology to assure a robust
input defense.
It is worth noting that any web front end requires the same list of management and
hardening controls that we’ve seen in several examples already. The reputation service
front end is no different. Despite the fact that the front end will reject requests from
all but devices under management, all the usual and typical Internet-exposed controls
should be observed thoroughly and completely. I won’t reiterate those in this example.
If you’re unfamiliar with how web-exposed systems are set up and managed, please refer
back to any one of the web examples already given (see Chapters 3 and 6).
In order to speed up queries, if a reputation has already been requested for an object,
and the reputation hasn’t changed since having been previously retrieved from the repu-
tation processing function, it may be placed in the cache where it is accessible only by
the reputation query front end. Once the reputation is no longer valid in the cache
because it has changed, the reputation will need to be removed so that a query will
return the updated reputation for the object as retrieved from the reputation processing

310 Securing Systems
back-end. This is an interesting architectural problem, as the cache becomes a bridge
between trust levels. Both the bastion front end, and the replication processing back-
end (which needs protection) access the cache. Consequently, the cache represents an
opportunity to vector attacks from the front end to the next layer.
The sequencing of processing goes something like this:
1. A device requests a reputation for an object.
2. The query response process (front end) requests a reputation from the reputation
3. The reputation is returned to the device.
4. It is also placed in the cache for rapid retrieval, should a subsequent request for the
same information be made.
5. New data come in about the object.
6. In the back-end data store, the object reputation changes.
7. The reputation change triggers the reputation processing component to check the
cache for the presence of the reputation.
8. If found (i.e., it’s still in the cache), the reputation in the cache is deleted, forcing
the front end to refresh the reputation when it is next queried.
If the cache is not handled well, and, of course, there are bugs in the chain of pro-
cessing, a malicious reputation could be stuffed into the cache, which then would be
read by the back-end processing software in order to exercise a vulnerability somewhere
in the reputation processing component.
How should cache handling be accomplished in order to minimize the ability of the
attacker to deliver a payload through the cache? If the back-end processor reads what’s
been put into the cache, then it receives a payload. However, the back-end could query
for the presence of an object and, if present, order the cache to delete all reference to
the object’s reputation. With this logic, no data have actually been read from the cache.
The “if present, then delete” command could be implemented in an atomic way, such
that reputation processing simply issues the command to the cache. No communica-
tions are returned.
I hope you can see from the last example that the sequence of processing, what is
read and what is not accessed, determine whether an attack surface is created or elimi-
nated. If data are never read from the cache, then cache manipulation by the back-end
presents a very limited attack surface, if any surface is present at all. It’s important to
investigate processing sequences and data retrievals and flows thoroughly to uncover
attack surfaces. By designing in such a way that data flow is minimized or eliminated
when unnecessary, attack surfaces can then be removed.
There is, of course, a flow from the front end backwards. Reputations are requested.
We’ve already examined the controls placed at the front end in order to mitigate this
possibility. Still, vulnerabilities do crop up in operating systems, runtime environ-
ments, and the like such that an attacker has a potential credible attack vector until

Cloud Software as a Service (SaaS) 311
that vulnerability is closed. One of our security principles is to plan for the failure of
defenses. A good defense will plan for the compromise of the front end. Are the front
end defenses, as outlined, sufficient?
I would argue, “No.” An attacker might compromise the entire front end processing,
despite the steps that have been taken to prevent such a compromise. This reputation
service is a key component of mobile security software. It needs to have a strong defense
so that customers may rely upon it for their protection. That statement suggests plan-
ning for failure or compromise of exposed systems and infrastructure.
A front end compromise might not be due to software created by the vendor.
Vulnerabilities may crop up in networking equipment, protocols, operating systems,
open source software that’s been included, or any number of components over which
the security vendor may have little or no control. Therefore, a prudent defense takes
into account the loss of even key systems. Although relatively unlikely, the back-end
software should make some attempt to reject any request for reputation that does not
meet stringent and rigorous validation.
Because reputation requests should have been formed by the front end software, and
because the API is well defined, it should be relatively easy to deterministically validate
reputation requests. Additionally, the validation should be assured by a rigorous testing
program, precisely like that laid out for the front end input validations.
The data that are used to create the reputations are a key commercial resource.
Therefore, the defense-in-depth from device to reputation processing is, at least in part,
built to protect the reputation data. In the mobile security example, we explored some
of the reasons that an attacker might want to influence reputation. For at least these
reasons, the reputation data itself is considered a critical, and thus sensitive, resource.
11.2.4 “Globality”
One of challenges that will need to be addressed by a cloud service is updating points
of presence of the service across the globe. In Figure 11.2, you will see that the back-end
is actually not discrete but connected and replicated to all the points of presence across
the globe. This is very typical for a global SaaS service. It’s desirable to keep processing
as close to the consumer as possible. Consequently, all the reputations must be repli-
cated to each point of presence. This means that across the globe, in each data center
that hosts the service, the front end, the processing service, and the reputation data are
duplicated. If the reputation service is to be accurate everywhere, this replication of data
has to be very fast so that a reputation query in Singapore is as accurate and up-to-date
as a query in London or Hawaii. This need for the rapid dissemination of updates and
rigorous duplication of data everywhere implies that the back-end can be thought of
as one global system rather than discrete points. At the back-end, all the systems are
connected, as they must be to achieve the performance and accuracy required from a
global cloud service.

312 Securing Systems
Consequently, great pains must be taken to protect the global back-end. Failure to
do so could provide an attacker a launch pad for global attack. Of course, such an event
would be disastrous for a security software maker. As we have seen, the software maker
has built a defense-in-depth to keep attackers from getting through to the back-end
data layer.
The same rigor will need to be implemented for reputation data inputs. I reiterate:
This is very similar to the robust input protection we examined for both business ana-
lytics and the malicious file processor in the endpoint security example. Each of these
situations is an example of a higher-order pattern where untrusted data from a wide
variety of sources will need to be processed. If you need further detail, don’t hesitate to
re-read these earlier examples.
An additional challenge for cloud services with many customers is multitenancy. As
you consider the reputation service and the needs of customers or individual consumers,
as well as, perhaps, large organizations that are security conscious like our fictitious
enterprise, Digital Diskus, what will be the expectations and requirements of the cus-
tomers? Will consumers’ needs be different from those of enterprises? Who owns the
data that’s being served from the reputation service? And what kinds of protections
might a customer expect from other customers when accessing reputations?
Interestingly, the reputation service may present a rather unique multitenancy pro-
file. Consider for the moment that it is the software vendor who owns the reputations,
not the customers. Customers are purchasing the service that advises them about the
trustworthiness of various objects before they’re accessed, whether that’s the consumer
sitting in a café going to a URL on the Internet or an enterprise employee accessing a
web property from the corporate network. In each case, the response is unique to the
request, as provided by the service. Can you spot what part of this interchange the
customer owns?
The request can be tied to the user. And if the user is a member of the customer
organization, then the request can be tied to the organization, as well. Requests are
likely to have some value. If I know the sorts of web properties in which a consumer is
interested, I can direct sales efforts to those interests. And that is precisely how search
engines and social media services generate revenue. These services appear to be free,
but they are far from it. If the service knows that you’re passionate about skiing, it will
attempt to sell you skiing equipment, skiing vacations, and so forth based upon your
demonstrated interests. This is precisely how social media and search engine revenue is
generated; the advertising is highly targeted.
Furthermore, each customer, probably each user, won’t necessarily want to disclose
their web viewing and interaction habits to any other user. If the string of reputation
requests from a user were to be accessed by another user, not only would that be a
breach of privacy in many jurisdictions, it might also give the receiver a great deal of
information about the habits, interests, and activities of the individual.
In the case of an organization, the interests of its employees might be used com-
petitively to understand future directions, pricing models, customer interactions, sales

Cloud Software as a Service (SaaS) 313
prospects, and all manner of sensitive business activity. Perhaps any one individual’s
activities might not be all that useful in isolation from the rest of the organization?
But if an attacker could gather information about many of the organization’s employee
activities, significant information will likely be disclosed. For instance, if several of the
researchers or executives access the website of an interesting new startup, a competitor
might infer that the company was considering an acquisition. Any particular reputation
request in isolation may not be all that significant. But the aggregation of reputation
requests that can be tied back to an entity are probably quite valuable.
Hence, a reputation service does indeed have a multitenancy challenge. How can
the service make sure that no tenant (or other attacker) can have access to all the other
tenants’ request history?
One solution would be to toss all reputation requests once the request has been satis-
fied. If no reputation request history is kept, if all reputation requests are ephemeral,
then the best an attacker can get is a picture of the requests at any particular attack
moment.* The deletion of all reputation request history is certainly one viable strategy.
However, the software vendor may lose valuable data about the efficacy of the repu-
tation service. Certainly, the reputation service would need to keep metadata about
repu tation requests in order to compute performance statistics, log failures, spot anoma-
lies from expected usage, and similar telemetry information. Metadata are going to be
very useful in order to assess the workings of the service and to spot problems with the
service as quickly as possible.
Will metadata be valuable to attackers? Perhaps. But the metadata certainly will
not be as valuable as reputation request histories that can be tied to entities of interest.
Consequently, another viable security strategy might be to throw away all actual request
history and only keep metadata.
Consider the case where only metadata is kept, that is, the case where no data is
kept about objects for which a reputation request has been made. Once again, in this
strategy, valuable data needed by the software vendor about the running of the service
would be lost. For one thing, if there’s an error in reputation calculations, it may be
impossible to know how many customers were ill-informed. That might be another
acceptable business risk? Or not. In order to understand who may be affected, the ser-
vice would probably have to keep reputation request history for each entity.
If reputation history is kept for each user of the system, then the system has a signifi-
cant multitenancy challenge. Each customer expects that her or his reputation request
history is private to the customer. A number of international jurisdictions have enacted
laws, so-called “privacy” laws, which legislate exactly this premise: Browsing or other
evidence of personal activity is private and, thus, protected information.
Since our cloud service is global, it will be subject to at least some jurisdictions’ pri-
vacy laws wherever it serves protected customers. Compliance is, of course, one driver
* Attack success beyond a single point of presence for the service implies a global compromise
of all the front ends. Not a likely scenario.

314 Securing Systems
of security posture. I would argue that merely to comply is to miss the point of security.
There are other business drivers of an appropriate security posture. Large organizations,
enterprises, governments, and the like, tend to take a dim view of loss of employee data,
whatever that data may be. Meeting a customer’s expectation for protection of the cus-
tomer’s data should, in spirit at least, also meet many legal privacy requirements.
Having established the need to keep at least some reputation request history for each
user, how can user data protection be implemented? As noted above, this is a significant
design problem. We will explore this only in terms of general patterns. An actual design
for a specific product is out of scope.
We will not propose what some companies have attempted. Occasionally, a service
will build what I call “table stakes” security—firewalls, intrusion prevention, adminis-
trative privilege controls, the collection of typical and well-known security infrastruc-
ture—and then declare, “Your data are protected.” I’m not dismissing these controls.
But none of the typical controls deal with multitenancy. Standards such as NIST 800-
53 are based upon an implicit assumption that the controls are built for a single organi-
zation’s security posture.* And that is not the case when infrastructure and processing
is to be shared, in this case, highly shared. In the case of multitenant, shared services,
customers expect to be segregated from each other and to be protected from the service
vendor’s access, as well.
There are three architecture patterns that seem to be emerging to provide sufficient
tenant data protection.
1. Encrypt data as it enters the service; decrypt data when it exits.
2. Separate processing within the infrastructure. Each customer essentially receives
distinct infrastructure and processing.
3. Encapsulate data such that it remains segregated as it is processed.
Which of these patterns should be employed depends greatly upon what services are
to be shared and how the services are architected.
1. Encrypt data as it enters the service; decrypt data when it exits. Encryption as
data enter the service and decryption at exit is the preferred solution from the customer’s
perspective, as long as the keying materials are held by the customer or the customer’s
trusted third party. Encryption tends to be the obvious solution. Often, customer secu-
rity teams will ask for encryption because encrypting the data before it enters the cloud
* Th e assumption that control is for a single organization does not preclude handling others’
data. However, NIST 800-53 was originally drafted before many of today’s privacy standards
were enacted. It was certainly drafted before cloud systems become prevalent. In addition,
somewhat implicit in the standard is the United States’ assumption that once data are given,
the data belong to the recipient, not an “owner.” Privacy law in the United States is also chang-
ing. Perhaps older security standards may one day better refl ect current notions of data privacy.

Cloud Software as a Service (SaaS) 315
mitigates risk from multitenant compromise as well as vendor compromise. However, as
we have seen, proper key handling isn’t necessarily easy or obvious. If the vendor has the
keys, or worse, if the vendor stores key materials with the data, then encryption doesn’t
really offer much protection.
When used as a control for cloud data protection, encryption won’t work in many
situations. In particular, if the data must be manipulated during processing, then the
data cannot remain encrypted. For instance, consider a cloud-based business intelli-
gence application that must manipulate data in order to produce “intelligence.” Data
must be decrypted before processing, which assumes that the service also handles the
keys, or no processing can take place.
For cloud services that primarily offer storage of data, encryption from endpoint and
back again is ideal. The caveat must be that keys can be properly protected.
An interesting hybrid case is when at least some of the services can be rendered
based upon metadata alone. The data can remain encrypted because the processing
takes place based upon the metadata associated with and surrounding bits of data. At
least one shared meeting service works in precisely this manner. The visual (graphics)
and audio are assembled at the endpoint. Hence, if each endpoint possesses a shared
secret key, then meeting data can be encrypted between endpoints as the data traverse
networks and the service.
There are two downsides to be found within this hybrid model. First, protecting
secrets on endpoints isn’t trivial if the secret is kept between sessions. The solution to
that is to generate a key only for that meeting session. Once the session is over, the key is
destroyed. That still begs the question of a compromised endpoint. Still, if the endpoint
is compromised enough that a key in memory can be retrieved by the attacker, probably
all bets are off, “game over,” anyway. The attacker is in control of the endpoint.
The second downside to this hybrid model in which encryption protects data and
the cloud service (SaaS) only manipulates metadata is that the customer must forego
any additional services that entail the manipulation of data. In the case of the meet-
ing service cited above, if end-to-end encryption is used, then meeting recordings and
upload of shared files cannot be used. Either the service can manipulate data or it is
protected end-to-end by encryption.
2. Separate processing within the infrastructure. Each customer essentially receives
distinct infrastructure and processing. Separation at the infrastructure layer is very
popular with services that offer a “platform” and infrastructure, that is, Platform as a
Service (PaaS) or Infrastructure as a service (IaaS). Usually, these are highly virtualized
and automated such that a virtually separated set of services can be easily allocated to
each customer. Typically, the allocated services initialize in an entirely “closed” state,
closed to all communications except those from the management console. The cus-
tomer must open whatever communications will be needed, including ordering any
storage (which will also be separated at the virtual infrastructure layer). Each customer,
consequently, receives a separate and distinct set of services, almost as though a separate
network had been purchased.

316 Securing Systems
I have seen SaaS applications that use this model: Each customer gets a separate
instance of the service, and nothing is truly shared above the infrastructure. Where
shared processing will not deliver economies of scale, this model delivers reasonable
multitenant separation. A downside of this approach is that management of many sepa-
rate instances is not a trivial problem. Furthermore, the service vendor loses whatever
economies can be derived from scaling a highly shared application.
3. Separate data during processing. If the vendor requires economies acquired from
building a highly shared application (many SaaS vendors try to reap these economies),
then the simple, obvious solutions will not work. That is, neither #1 nor #2 above apply.
I’ll reiterate: No matter how strong the vendor’s perimeter and operational security
controls and practices are, the shared application and perhaps data layers might present
a comingling of tenant (customer) data unless the application is built such that data are
kept separated even during processing.
To solve the separated data flowing through a shared application, multitenant prob-
lem, the shared application must be carefully coded such that it becomes virtually
impossible for data from individual tenants to mingle or get mixed up while being
processed. The usual way to protect the integrity and confidentiality of each tenant’s
data during processing is to encapsulate each data message with identifier(s) for each
particular tenant.
Encapsulation is used in communications so that each protocol that rides upon a
lower (outer) protocol is held within a protocol-level header and footer (beginning and
end). This is how the Transmission Control Protocol (TCP) is encapsulated to ride
upon the lower IP layer (TCP/IP). HTTP, for instance, places an additional encapsula-
tion (header and footer) around the data that is to be routed over the TCP, which is,
of course, riding on top of the IP layer. There are numerous message layers that can be
placed within an HTTP message. Protocols are layered like a set of “Russian dolls,” one
layer (protocol) carried inside the message for a lower layer protocol.
Similar to network encapsulation, each tenant in a multitenant application will be
assigned either a header preceding data or a header and footer surrounding data. The
encapsulation identifies which tenant the data belong to and ensures that each tenant’s
flows remain separated during processing. Only a particular tenant’s data may flow
through a chain of processing intended for that tenant just as a single TCP message
will be routed to a single IP address and a unique TCP port number. The encapsulation
must be designed such that no tenant’s data can be mistaken for another’s. Figure 11.3
represents this data encapsulation visually.
Usually, the tag that identifies the tenant is a token tied to the tenant’s account, not
the actual tenant name or other public or well-known identifier. In this way, the appli-
cation has no notion of tenant identity. The tag is “just a number.” Some other, separate
process is used to tie processing to the tenant.
Better than using “just a number,” perhaps a predictable number, is to introduce
unpredictability, that is, entropy into the calculation of the tag token. In this way, should

Cloud Software as a Service (SaaS) 317
the application be breached and one or more flows become compromised, the attacker
will have more difficulty associating an unpredictable number to the vendor’s clients.
A step above utilizing a high-entropy token would be to add a bit of indirection
into the tag. A separate store might associate a client account to the hash. A second
temporary storage (perhaps in memory?) would associate the tag token to the hash. At
this point, an attacker must breach the hash-to-client store and the token-to-hash store.
These should be kept in segregated processing units, perhaps separate network seg-
ments? Capture of the flows delivers nothing but a bit of data that is being processed.
Wiring that data back to a client would require several more attacks, each successful.
I’ve seen a data store that used the above scheme to store customer data files.
Although the files were all comingled on disk, they were stored under the token name.
There were tens of thousands of individual files, each named by some high-entropy
number. The number-to-hash store resided outside the data layer. The hash-to-client
store was in yet another layer.
At this company there was 24×7 monitoring of data administrator activity. An alert
would be generated should any customer file be accessed by an administrator account
(administrators had no business reason to look at customer files).
Essentially, administrators who had access to the client/hash store didn’t have data
access. Data administrators couldn’t access the client key. In this way, customers were
protected from each other, from the accidental comingling of data, and from accidental
or malicious administrator access. A shared application must be designed to keep ten-
ants’ data separated sufficiently, both in processing and storage.
The reputation service must employ a scheme similar to that explained in the third
protection pattern above (#3) in order to protect reputation histories sufficiently for
each customer. Because the reputation histories are saved, they must be protected as
customer data when processed and in storage. A high-entropy token will be used as
a part of the tag for requests linking these to a particular tenant through a two-level
indirection scheme, as described above.
Figure 11.2 doesn’t decompose the internal systems that would make up the
“Distributed Big Data.” Nor have the reputation data collection services been repre-
sented. Although these do encompass significant security issues, the issues are akin
to those encountered with the business ecosystem of the enterprise architecture that
Figure 11.3 Multitenant data encapsulation.

318 Securing Systems
we examined. The internal cloud will require a closed network with highly restricted
access. As an architecture pattern, this is not radically different from any restricted net-
work and services. Therefore, these will not be explored again in this example.
Despite the opaque security nature of the internal network, we will consider the global
interconnections of all the points of presence for the application into the global data layer.
One typical implementation of distributed big data involves the use of products specifi-
cally designed to replicate data between instances. The open-source project Hadoop®
is one such product. The reputation service under consideration employs a distributed
data product. The details may remain opaque, since from a security perspective, we
have already discussed internal networks handling sensitive data.
An architecture specific to global cloud products with many points of presence is the
network interconnection. One way to achieve this is to establish a series of connections
using VPN or IPsec tunnels over the Internet. For a performance sensitive SaaS, the
vagaries of Internet routing will influence transmission times. Hence, although essen-
tially secure across the hostile Internet, such an architecture is likely to be unsuitable for
this purpose. Instead, where performance and security must both be served, a private
network is often the better choice.
Typically, each distinct and separate portion of the logical “internal” network will be
joined through the point-to-point link run through a common carrier. If the common
carrier network can be trusted, essentially, this architecture builds a distributed, pri-
vate network. Since common carriers tend to be highly regulated (we’re talking about
large telecommunication companies), this is how many internal distribution networks
are formed. In addition, the connections are point-to-point. Although the traffic does
move across the common carriers’ equipment, it isn’t routed across disparate networks.
Essentially, the point-to-point link is to be thought of analogously to a phone call. Only,
in this case, the call is connected continuously. And the line should have redundancy
and failover. Using point-to-point links, a private network is created over relatively
secure channels.
Having created the internal distributed network, the architect is faced with the
problem of whether to encrypt the traffic or not. I’ve seen this question come out both
ways. IPsec tunnels can be placed over the point-to-point links. Hardware encryption
might even be employed to speed up the traffic.
Alternatively, it may be that the common carrier is sufficiently secure to leave the
traffic in the clear. For a large distributed network with numerous point-to-point links,
the choice is not all encrypted or all clear. Any particular link traversing a less trusted
carrier can be encrypted. Links going over the more trusted link can remain in the
clear. Encryption can be implemented on a per link basis, creating a hybrid model.
Once the internal collection of links is established to the security posture desired,
the internal network can be taken as a whole to be at a certain level of security pro-
tection. In other words, though the network is actually composed of many disparate
points, it may be considered the “internal” network.
For the reputation service, private links connect all the points of presence across the
globe. These also connect two redundant data centers where the reputation data are

Cloud Software as a Service (SaaS) 319
stored and processed. It is from these two centers that data is replicated outbound into
the data layer we see in Figure 11.2.
11.3 Additional Requirements for the SaaS
Reputation Service*
The following list of requirements focuses on the different, cloud aspects of this
architecture. Earlier chapters covered in some depth other aspects, such as endpoint
requirements, web requirements, and administrative needs.
• The calculation processor will not read from the reputation cache. A message
will be sent from the calculation processor to the cache to execute a delete of
all references to an object in the cache. Failure of the deletion will be logged.
No acknowledgment will be returned by the cache in response to the deletion
• Reputation request messages must be constructed by the web front end. Only the
queried object will be passed to the calculation processor.
• The front end will sanitize and validate messages from the mobile device. Any
message that fails validation in form or data must be rejected.
• Before reputation requests will be received, a device certificate, signed by the
private key of the reputation service, must be validated by the reputation front
end. Certificate validation failure must cease all further communications.
• All customer reputation requests and request histories will be encapsulated with an
unpredictable token. The token will be related to a cryptographic hash. The hash
is the only value to be associated to service tenants. The hash-to-client relationship
must be kept in a separate network segment, under separate administrative control
(different team) from the application administrative team and from the storage
and data administrators. Hash-to-token correlation will be done by a separate
process running in the back-end, unexposed to any processing outside the internal
back-end networks. Wherever stored, tenant reputation histories will be tagged
with the token only. No mention of customer identity can be placed within that
administrative domain or network segment.
• Reputation data will be pushed from the data farm on the internal network to
each reputation data instance.
• Reputation data instances will exist on a segregated network that only receives
requests from the reputation calculation and processing module.
• There will be four distinct layers, as shown in Figure 11.2:
a. The DMZ, bastion layer containing the web service termination. Authentication
of device certificates will take place before a reputation request may be made.
* Please see the requirements from the endpoint anti-malware requirements in Chapter 9 and
the mobile security example in Chapter 10 for the relevant device requirements.

320 Securing Systems
b. A front end module that validates requests before any processing may occur.
Requests will be regenerated before they are passed on to subsequent layers.
Objects about which a reputation query is made may only be passed from the
front end after careful validation and format check. The reputation cache is
placed in this segment. It receives requests only from the front end.
c. The reputation calculator/processor, which only receives reformed requests
from the front end.
d. The local reputation instance for a point of presence, which will only receive
requests for data from the calculation module.
• The front end request processing input must pass rigorous fuzz testing before
• The calculation module’s request handling processing chain must pass rigorous
fuzz testing before release.
• An HSM will be employed at each node to perform certificate, message, and data-
signing operations. Multiple private keys, including customer specific keys, must
be supported. Every node must validate every device certificate.*
1. Mell, P. and Grance, T. (2011). Th e NIST Defi nition of Cloud Computing, NIST
Special Publication 800-145, Computer Security Division, Information Technology
Laboratory, National Institute of Standards and Technology, Gaithersburg. Retrieved
from .
* Th e architecture analysis ignores the issues of key synchronization, update, and revocation. A
“real-world” analysis would have to grapple with these details.

Part II
We have now examined the six distinct architectures for security using the ATASM
process. In each analysis, the recurring patterns of attack surfaces and security solu-
tions have been highlighted. After reading this chapter, it should be clear that simi-
lar situations arise in seemingly disparate architectures. Indeed, whether building an
online retail store, an internal business analytics system, the administration interface
for enterprise security software, or a cloud reputation service, the security basics around
administration and management make up one of the solutions that is required to build
a defense-in-depth.
This is not to presume that the precise solutions outlined in this section are the only
way to do it. In fact, depending upon the risk tolerance and business objectives in the
context, there is no right way to implement management for security. We will explore
this a little bit in the chapter on patterns. It is important to note, however, that although
we used the same approach in a number of different architecture examples in this sec-
tion, there are many ways to achieve the same ends, or rather, ends that are appropriate
for the context in which the system exists. Despite what some standards seem to imply,
security is always best implemented appropriately and not by meeting some formal
recipe that offers a binary “right or wrong” way. If you take nothing else away from
these analyses, hopefully, you will gather a sense of the craft of security architecture as
it is applied to differing systems.
In order to avoid adding even more length to the foregoing analyses, I purposely
dodged controversial subjects that are well covered elsewhere. Furthermore, in order
to demonstrate that distinct architectures may introduce new aspects and, quite often,
repeat the same patterns over and over, as well, I purposely reused the same solution
wherever possible in the examples. It may be prudent to remember that the wise security
architect often replies, “It depends.” That answer is correct so long as the architect can
articulate clearly what the dependencies are.

322 Securing Systems
We applied the ATASM process to six example architecture analyses. For the first
example, we followed the process closely, in precisely the order given.
The first step is to gather the context in which the architecture will function:
business, risk, objectives, and functions. Next, we analyze the overall architecture,
attempting to gain an holistic understanding of all the systems and components that
function and interact. A further aspect of understanding the architecture is the decom-
position and factoring process. The analysis must determine what level of granularity
will expose all the attack surfaces. Additionally, the architecture must be factored into
its defensible boundaries in order to apply mitigations appropriately. We listed all the
mitigations that existed in the architecture before the analysis in order to uncover and
prioritize against the enumerated threats those attack surfaces that require additional
protection. This last step generates the formal threat model of the architecture from
which the appro priate security requirements will be gathered.
The ATASM progression provides an ordering for the complex process of architecture
analysis for security. Such an analysis is meant to generate a threat model from which
security requirements can be understood. In our first example, we followed the process
carefully and in the order given: Architecture, Threats, Attack Surfaces, Mitigations.
However, a real-world analysis does not always proceed in a stepwise fashion. Often,
an analysis is more like peeling an onion. The analyst must deal with the outer layer.
And once that outer layer has been analyzed, successive layers are either discovered or
revealed through the analysis. Components and functions are “discovered” that were
not expressed in the first layer, the overall architecture diagram. New systems may be
uncovered as the assessment proceeds through the architecture’s “layers.”
To demonstrate in a more realistic fashion the way that analyses proceed, we dis-
pensed with the slavish adherence to the ATASM process. Instead, proceeding as it
might unfold on the job, once the context for the architecture was understood, we
proceeded through the architecture to understand the structure, uncover the attack
surfaces, and discuss the protection needs (risk tolerance), thus generating appropriate
security requirements. In other words, we applied a micro-ATASM process to each facet
of the analysis rather than to the architecture as a whole.
The danger to an approach that decomposes to generate the threat model can be a loss
of the whole picture. In these examples, we had the luxury of adequate, holistic informa-
tion from the start of the analysis.* As you apply the ATASM process to your analyses,
you may find that you have to bounce back and forth between analyzing the whole
architecture and analyzing particular areas. Once ingrained, architecture risk assessment
(ARA)/threat modeling does not require slavish adherence to any particular approach,
whether that be ATASM or any other. Essentially, one must make sure that every step is
covered for every facet and every component irrespective of ordering or method.
1 Th e information was adequate because these examples were created for this book. I had the
luxury of hindsight to generate precisely the preconditions required to draw the security con-
clusions presented.

Part II-Summary 323
One of the aspects of the craft of security architecture that I attempted to uncover
for the reader is the reasoning that goes into choosing one set of security controls over
another. Considerable space was devoted to an explanation about why a particular
defense was chosen as opposed to some other course. That should not imply that any
particular solution presented in these examples was the absolute “right” way. These
are typical solutions to often-encountered problems. But the actual solution that gets
implemented will most certainly involve local variation.
These lengthy explanations were intended to unpack, at least to some extent, the
reasoning that an AR A and threat model must include in order to arrive at appropriate
security within the context of an organization and its systems. The intention behind
digressions on enterprise authentication systems and PCI compliance (to name two
tangents) is to highlight the background behind the choice of one security alternative
versus another. Oftentimes, a security architect will have to investigate standards, tech-
nologies, and protocols with which she or he is entirely unfamiliar. Part of the “work”
of an assessment is to gain enough information to understand the trade-offs such that
appropriate decisions can be made. My forays were intended to help you develop skill
in identifying a situation in which more background is needed. To seed that, I regularly
asked you to consider the situation before the next portion of the analysis. I pray that
these questions sparked yo ur interest and creativity.
Hopefully, at this point in this book, you, the reader, have gained enough familiar-
ity with the art of securing systems that you can apply this knowledge to the systems
in your organization, and the assessments that you must conduct? For managers and
technical leaders, I hope that, at this point, you have a firmer grasp of what your secu-
rity architects are attempting as they perform your organization’s security assessments.

Part III

Part III
There’s more to securing systems than analyzing architectures for security require-
ments. If the process is near perfect in that upper-management support abounds, rela-
tionships with all the stakeholders are pristine, perhaps no more would need to be done
than to hand off the security requirements from an analysis and then move on to the
next system?
Unfortunately, in my humble experience, nothing works this well. If for no other
reason than the inevitable business trade-offs between rigorous security and delivering
products to customers, decisions will need to be made. This is why there is an entire
chapter devoted to risk in this volume. Such trade-off decisions will never be easy, and,
for the moment, they generally cannot be automated.
Another dimension is maintaining assessment velocity when scaled to large numbers
of assessments. In today’s highly competitive world, software is being produced at an
ever-increasing rate. Some of this can be accounted for by the increase in the number of
organizations producing software and the increase in the number of programmers who
write code. But I doubt these increases entirely account for the explosion in new software.
I don’t have hard data on the increase in software production. I do know that many
organizations are implementing methods to increase the velocity of their production of
code. The Agile movement is one such approach. Implemented well, many organiza-
tions experience a significant increase in the amount of code, often high quality, that
the same number of people who’d been working using Waterfall can produce within
an Agile environment. Hence, many organizations are chasing the Agile dream of self-
motivated, self-correcting, high-performing, nimble development teams.
Unfortunately, the kind of experience-based security architecture craft that we’ve
been exploring doesn’t scale very well. Though we will touch on some experiments in
Agile security architecture in the chapter about managing programs, it remains that at

328 Securing Systems
the present state of practice, security architecture requires significant human participa-
tion and engagement.
In Part III, we explore some of those areas that I’ve found help to create and main-
tain a program for architecture risk assessment (AR A) and threat modeling.
We will deepen our exploration of architecture patterns and their associated security
solutions. We have touched on this part of the practice from a practitioner’s perspective.
In Chapter 12, we examine how capturing, standardizing, and applying patterns and
standard solutions can help to increase efficiency and maintain delivery teams’ velocity.
Once the specter of standards arises, a program will likely have grown to a size
that exceeds that at which mere personal influence is an effective tool all by itself. It
should be obvious, if you stayed with me this far, that I consider personal skills to be
a critical factor and proficiency; make no mistake. However, when a certain size has
been reached, there have to exist some formal processes, even a few clear-cut rules, so
that everyone participating understands what must be done. If processes and activities
are not delineated clearly, delivery teams simply make up whatever is convenient to
their situation. Ensuring that the process has been followed is a matter of governance.
There are many books covering this area, so we will confine ourselves strictly to those
governance aspects that are relevant to security assessments of systems and security
architecture in general.
Finally, Chapter 13 will take up, in a somewhat foreshortened manner, those aspects
of building a security architecture practice that bear directly upon successful assess-
ments and building functional teams of security architects. Again, there are plenty of
fine programmatic works. I feel no need to repeat that which has been well covered else-
where. Hence, Chapter 13 has been tightly focused on the needs of this work: applied
security architecture.

Chapter 12
Patterns and Governance
Deliver Economies of Scale
For small organizations, or where there are a sufficient number of experienced security
assessors such that each practitioner can hold in his mind all the relevant patterns and
solutions that need to be applied, there may not be a need for standardized patterns.
When there are enough people to perform due diligence analysis on every proposed
system, these practitioners are the governance process for the organization. The need
for standards, a standards process, and governance of that process is generally a factor
of size, breadth of portfolio, and, sometimes, the necessity for compliance to regulations
imposed from the outside.
A well-known result from rigid, standardized processes and heavy governance of
those processes is a slowdown in delivery. When due diligence (i.e., security archi-
tects) resources are highly constrained, and there exist rigid processes that require those
shared resources to assess everything, due diligence will become a severe bottleneck
rather quickly. On the face of it and simplistically, it may seem intuitive to enact a
“law and order” and/or “command and control” process. Make everyone behave pro-
perly. But anyone who’s read the legendary book, The Mythical Man-Month: Essays on
Software Engineering, by Frederick P. Brooks, Jr.,1 and similar studies and essays, knows
that the more administration and bureaucracy an organization installs, the less work
actually gets done.
Hence, in this chapter, we take up some approaches to standards and their gover-
nance that have proven, at the very least, to put a minimal amount of drag on innova-
tion, creativity, and velocity in software and system delivery organizations. I make no
promise that these will absolutely work in your organization and under your regulatory
constraints. However, I have seen these things work. Therefore, I offer these for your

330 Securing Systems
consideration as you think about security architecture and risk assessment of systems in
larger or more complex organizations working on many different kinds of projects in
parallel, and delivering multiple, perhaps, many systems in each time period.
A key to Agile velocity is to prevent roadblocks and bottlenecks in the process. In
other words, a bottleneck will be introduced if every project must go through a small
number of security architects who must pass every single project no matter how small
or big. If, like an auditing function, every project must pass a standard checklist of
security items, whether any particular security item is relevant to the architecture or
not, then a significant bottleneck will have been built into the system. Even after my 15
years of experience working in and building security architecture practices,* I continue
to be surprised by organizations that insist upon trying to fit every project through the
same mesh, the same matrix of checks, no matter how irrelevant some of these may be.
They must have more money for resources and time than the organizations for whom
I’ve worked?
It is a truism that it is nearly impossible to capture even 80% of architecture patterns
into a checklist. Furthermore, flattening the decision tree into binary yes/no polari-
ties diminishes the ability to synthetically craft situational solutions, thus hampering
creativity and innovation.
The ideal model will empower and enable teams to be as self-sufficient as they can
be, while, at the same time, maintaining due diligence oversight of results. There is a
balance there between these two goals: entirely self-sufficient versus entirely dependent
upon expertise. It would be grand if there were enough security architects to participate
in every development project that has security needs. I hope this volume, of course,
increases the number of people who feel confident and capable of the work. Still, as of
this writing, there is a dearth of experienced practitioners; great security architects are
a rare breed, indeed.
So what can be done in the face of a scarcity of appropriate capability? When there
are standard patterns and solutions that are well understood and well described, many,
if not most, teams can simply “follow the bouncing ball” by architecting to standard
solutions. As Steve Atcheson† says, “make the easy path the secure path.”
In most organizations, programmers are typically incentivized to deliver a finished
product. If the security solutions provided to applications are vetted, well-documented,
and easy-to-use, the “easy” path will be to simply consume the known solution rather
than attempting an unknown implementation.‡ The tendency to build from known
* As of this writing, I have an additional 15 years in high tech, holding roles from developer to
technical leader to chief designer, and once as the Director of Software Development.
† Steve Atcheson, besides being one of my fi rst security architecture mentors, my one-time
colleague, and my friend, is also the author of “Th e Secure Shell (SSH) Frequently Asked
‡ Th ere does exist at least one case where even a competent developer may stray from the easy
path. I have seen developers insist upon including new and exciting technologies, even when

Patterns and Governance Deliver Economies of Scale 331
and trusted software “parts” means that supplying standards that include easy or easier
to consume solutions motivates developers to take the easy and secure path.
The foregoing suggests that standards and standard solutions can easily unburden
overloaded security architecture staff while, at the same time, empowering developers
to build correct solutions on their own. Standard approaches, then, become a critical
multiplier of velocity. Thus, standard approaches that cover a wide range of cases has
been effective at a number of organizations at which I’ve worked.
For instance, building a web environment that contains most, if not all, of the secu-
rity services required by a standard web application frees the developer to think about
the business logic that he or she is implementing—that is, authentication services, safe
storage, key management, TLS services, security layering, and unbreachable sandboxes
for each application. Providing these services will eliminate the possibility of error-
prone security implementations from the application programmer. When each of these
services (and, of course, others) are easily consumable, web programmers can focus on
the security aspects for which she or he is responsible, such as secure coding, and any
local input validation that must be performed beyond any standard input validation
libraries that are available. In addition, the environment will not have to account for
and tolerate all the errors that will creep into each local and unique implementation of
critical security requirements. That means less critical security vulnerabilities.
As we have seen in performing analyses, architectures differ, thus requiring dif-
ferent implementations of security controls at different granularities. For a low-level
BIOS programmer, or for a programmer writing to a hardware interface, the list given
for a web environment above is more or less irrelevant. Still, when a team writes BIOS
code for any length of time, the relevant security issues and solutions are going to start
forming repeatable patterns. The list will be different, but there is a list of patterns and
solutions, nonetheless.
Essentially, when a project can match the known solutions, follows the standard
patterns, and contains no exceptions, security review can be bypassed; the team then
gets a “get out of security jail free” card. In other words, the project or effort can
remain entirely self-sufficient, without the need for more intense security analysis.
Consequently, security architects who document their recurring patterns, who provide
the solutions to those patterns, and who help to get those solutions built into the avail-
able services and software from which projects may draw will increase the scale and
velocity at which the organization’s development teams can operate. In the ideal situ-
ation in which there are many easily consumable solutions, security architects mainly
handle the exceptional, the new, that which has never been seen before. Contrast this
doing so will create more work and perhaps even “torture” an architecture, misusing the new
technology in a place where it is not needed or does not really fi t. Th e developer insists upon
the use of the inappropriate technology so that it may be included on her or his resume. Th e
use has nothing to do with the fi tness of the technology or the “easy” path; the use is intended
to enhance the programmer’s career.

332 Securing Systems
situation in which standards are documented and services implemented with one in
which the security architect is forced to reiterate the same solutions over and over and
over again.*
What do architecture patterns look like?
In fact, if you follow along to at least some of the analyses given previously in this
book, you’ve noted that certain attack surfaces were encountered multiple times, per-
haps even across seemingly disparate systems.
One classic problem is how to deal with systems that will be exposed to the public
Internet. We know, without a doubt, that the public Internet is hostile and that hosts
on the public Internet will be attacked. To counter this omnipresent attack level, there
are typical solutions:
• Firewall allowing traffic only to the designated public interface that will be exposed
• Bastion, HTTP/S terminating host (or the equivalent, such as a load balancer or
virtual IP manager)
• Access restriction to and protection of management and administrative interfaces
• Network and protocol restrictions between traffic terminators and application
logic, between application logic and storage or databases. That is, multiple tiers
and trust levels
• Security configuration, hardening, patching of known vulnerabilities, and similar
• Authentication between layers of the automated processes and between trust levels
• Restriction to and protection of the networking equipment
The above list is not meant to be exhaustive, but rather representative. The point
is, if an organization is going to deploy a system, especially a complex system, to the
Internet, the solutions set that the organization will have to implement (whether pur-
chased or built) is fairly well understood. Some organizations will write the “rules” into
their policies or perhaps their standards. Others might make it impossible to deploy
Internet-facing systems without meeting these requirements. The point being that if the
solution for Internet-facing systems is well understood, any system that can follow the
well-trodden path probably won’t need a security assessment. Again, the “easy path” is
the secure path.
In the same manner, a standard solution could be written for critical and sensitive
systems management. Several times in the analysis examples we encountered a need
to restrict access, both of humans and of automated processes. For instance, using the
* I once interviewed for a job where it was clear that the hiring manager was looking for some-
one who was comfortable treating every project, and, indeed, every developer as a unique,
one-off situation. When I objected that a better solution would be to build a program, includ-
ing services and solutions, he got quite upset. I guess some practitione