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Extrusion Detection: Security Monitoring for Internal Intrusions Paperback – November 18, 2005

ISBN-13: 978-0321349965 ISBN-10: 0321349962

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Product Details

  • Paperback: 416 pages
  • Publisher: Addison-Wesley Professional (November 18, 2005)
  • Language: English
  • ISBN-10: 0321349962
  • ISBN-13: 978-0321349965
  • Product Dimensions: 9.1 x 7 x 0.8 inches
  • Shipping Weight: 1.6 pounds (View shipping rates and policies)
  • Average Customer Review: 4.6 out of 5 stars  See all reviews (12 customer reviews)
  • Amazon Best Sellers Rank: #290,173 in Books (See Top 100 in Books)

Editorial Reviews

About the Author

Richard Bejtlich is founder of TaoSecurity, a company that helps clients detect, contain, and remediate intrusions using Network Security Monitoring (NSM) principles. He was formerly a principal consultant at Foundstone--performing incident response, emergency NSM, and security research and training--and created NSM operations for ManTech International Corporation and Ball Aerospace & Technologies Corporation. For three years, Bejtlich defended U.S. information assets as a captain in the Air Force Computer Emergency Response Team (AFCERT). Formally trained as an intelligence officer, he is a graduate of Harvard University and of the U.S. Air Force Academy. He has authored or coauthored several security books, including The Tao of Network Security Monitoring (Addison-Wesley, 2004).

Excerpt. © Reprinted by permission. All rights reserved.

Welcome to Extrusion Detection: Security Monitoring for Internal Intrusions. The goal of this book is to help you detect, contain, and remediate internal intrusions using network security monitoring (NSM) principles. This book will guide security architects and engineers who control and instrument networks, help analysts and operators to investigate internal network security events, and give technical managers the justification they need to fund internal security projects. Extrusion Detection is the sequel to my first book, The Tao of Network Security Monitoring: Beyond Intrusion Detection. While Extrusion Detection is a stand-alone work, I strongly recommend reading The Tao first, or at least having it nearby as a reference.

Those of you who have read The Tao will recall that the book focused on outsiders gaining unauthorized access to Internet-exposed servers. This threat model reflected the predominant mode of Internet exploitation in the 1990s. The primary means for attackers to exploit targets during the 1990s involved server-side attacks. Intruders gained unauthorized access by exploiting services offered by Internet-facing victims. Typical targets included Web servers, e-mail servers, domain name resolution (DNS) servers, and other programs that wait to answer queries from Internet users. 1 If internal workstations were not obscured by network address translation (NAT) gateways or firewalls, they too could be attacked directly, but only if they offered services similar to the typical targets. Local file-sharing services employing Unix remote procedure calls (RPCs) or Windows Server Message Block (SMB) were high-priority targets.

With the advent of the firewall in the early 1990s and the adoption of private Request for Comments (RFC) 1918 space in the middle 1990s, internal workstations were seldom directly attacked, unlike their public server counterparts. Protection from the outsider threat required access control and limits on the exposure of Internet-facing hosts. Traditional monitoring efforts watched attacks from the Internet to exposed servers because intruders most often launched "server-side" attacks.

The current decade has seen this model turned inside-out. Beginning in 2000, and with increasing intensity since 2003, corporate and home users have been subjected to increasing numbers of "client-side" attacks. No longer are services offered by computers the only targets of attack. Now, the applications upon which users rely, such as Web browsers, e-mail clients, and chat programs are the targets.

Instead of an intruder attacking the Web server running on a company's Internet-facing server, the intruder attacks the Web browser of an internal user who surfs intentionally or accidentally to a malicious Web site. Alternatively, a user may receive a Trojan through a chat program and unwisely decide to run that executable while operating with administrator privileges. No longer is it sufficient for security staff to harden the network perimeter by limiting services exposed to the Internet. The perimeter network is still a crucial part of network infrastructure, despite calls for the "de-perimeterization" of enterprise networks. Now, software running on clients must be protected, and the traffic generated must be monitored for signs of compromise.

This book focuses on ways to deal with the threat to internal systems. By "internal systems," I mean those considered to be intranet, not Internet, hosts. Extrusion Detection is not about traditional hardening of internal hosts to the same degree as external hosts. Traditional internal host hardening means minimizing services offered by systems, thereby decreasing the likelihood of server-side attacks. In other words, I would not be offering new advice if I discussed how to control and detect attacks against the SMB server running on port 445 TCP on a Windows XP workstation. I may not address such practices in detail here, but reduction of server-side exposure is certainly a beneficial security practice.

Extrusion Detection explains how to engineer an internal network that can control and detect intruders launching server-side or client-side attacks. Client-side attacks are more insidious than server-side attacks, because the intruder targets a vulnerable application anywhere inside a potentially hardened internal network. A powerful means to detect the compromise of internal systems is to watch for outbound connections from the victim to systems on the Internet operated by the intruder. Here we see the significance of the word "extrusion" in the book's title. That is, in addition to watching connections inbound from the Internet, we watch for suspicious activity exiting the protected network.


This book is for architects, engineers, analysts, operators, and managers with intermediate to advanced knowledge of network security. Architects will learn ways to design networks better suited to surviving client-side (and server-side) attacks. Primarily using open source software, engineers will learn how to build solutions for controlling and instrumenting internal networks. Analysts and operators will learn how to interpret the data collected in order to discover and escalate indicators of compromise. Managers will read case studies of real malicious software and the consequences of poor internal security.

All readers will learn about the theory, techniques, and tools for implementing network security monitoring (NSM) for internal intrusions. Executives may use the material to assess the state of their networks in relation to the book's recommended best practices. Auditors can determine if their clients are collecting the network-based information that's needed for the appropriate control, detection, and response to intrusions.


I have attempted to avoid duplication of material presented in other books, including The Tao. My purpose here is to publish as much new thought on internal security as possible and to have this book be a complement to previously published books. I expect my audience to bring a certain amount of knowledge to the table.

Core skills readers should possess in order to get the most from the book are:

  • Scripting and Programming: Familiarity with simple shell scripting is helpful when automating certain tasks.
  • Weapons and Tactics: Knowledge of tools and techniques for network attack and defense is assumed.
  • System Administration: Readers should be comfortable with installing software on the operating systems they use.
  • Telecommunications: An understanding of Transmission Control Protocol/Internet Protocol (TCP/IP) networking is absolutely essential.
  • Management and Policy: Appreciation of the laws, regulations, and other restrictions associated with network security is highly recommended.

Readers who believe they may be lacking in any of these areas can benefit from my recommended reading list, which is constantly updated and available at http://www.bejtlich.net/reading.html.

If I were to recommend a single book to read prior to this one, it would be The Tao of Network Security Monitoring: Beyond Intrusion Detection. In many ways, Extrusion Detection is an attempt to extend The Tao to the addressing of internal threats. While Extrusion Detection will function as a stand-alone work, your network security monitoring operations will greatly benefit from your reading The Tao.

A Note on Operating Systems

Where possible, the reference platform for this book is FreeBSD 5.3 or 5.4 RELEASE. In the cases where Linux is required, I use Slackware Linux 10.0. Some of the latest innovations in host-centric access control are supported only on commercial operating systems such as Microsoft Windows.

Generally speaking, any tool that compiles on FreeBSD will work on the Unix variant you choose. Tools that are closely tied to the OS kernel, such as the Packet Filter (Pf) firewall (http://www.openbsd.org/faq/pf/), may not be available on any OS other than those specified later in the book.


Extrusion Detection is divided into three parts that are followed by an epilogue and appendices. You can focus on the areas that interest you, because the sections are modular. You may wonder why greater attention is not paid to popular tools like Nmap or Snort. With Extrusion Detection, I hope to continue breaking new ground by highlighting ideas and tools seldom seen elsewhere. If I don't address a widely popular product, it's because it has received plenty of coverage in another book.

Part I mixes theory with architectural considerations. Chapter 1 is a recap of the major theories, tools, and techniques from The Tao. It is important for readers to understand that NSM has a specific technical meaning and that NSM is not the same process as intrusion detection or prevention. Chapter 2 describes the architectural requirements for designing a network best suited to detect, control, and respond to intrusions. Chapter 3 explains the theory of extrusion detection and sets the stage for the remainder of the book. Chapter 4 describes how to gain visibility to internal traffic. Part I concludes with Chapter 5, original material by financial security architect Ken Meyers that explains how internal network design can enhance the control and detection of internal threats.

Part II is aimed at security analysts and operators; it is traffic-oriented and requires basic understanding of TCP/IP and packet analysis. Chapter 6 offers a method of dissecting session and full content data to unearth unauthorized activity. From a network-centric perspective, Chapter 7 offers guidance on responding to intrusions. Chapter 8 concludes Part II by demonstrating principles of networ...

More About the Author

Richard Bejtlich is Chief Security Strategist at FireEye, and was Mandiant's Chief Security Officer when FireEye acquired Mandiant in 2013. He is a nonresident senior fellow at the Brookings Institution, a board member at the Open Information Security Foundation, and an advisor to Threat Stack, Sqrrl, and Critical Stack. He is also a Master/Doctor of Philosophy in War Studies Researcher at King's College London. He was previously Director of Incident Response for General Electric, where he built and led the 40-member GE Computer Incident Response Team (GE-CIRT). Richard began his digital security career as a military intelligence officer in 1997 at the Air Force Computer Emergency Response Team (AFCERT), Air Force Information Warfare Center (AFIWC), and Air Intelligence Agency (AIA). Richard is a graduate of Harvard University and the United States Air Force Academy. His fourth book is "The Practice of Network Security Monitoring" (nostarch.com/nsm). He also writes for his blog (taosecurity.blogspot.com) and Twitter (@taosecurity), and teaches for Black Hat.

Customer Reviews

4.6 out of 5 stars
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This book is a good read and a very good book to keep in one's reference library.
Bob Burd
Bejtlich starts out by doing an overview of Network Security Monitoring, referencing his earlier book as a more in-depth treatise on NSM.
Joshua Brower
Conclusion: This book was informative and an enjoyable to read, I highly recommend it.
Jon Schipp "Keisterstash"

Most Helpful Customer Reviews

14 of 14 people found the following review helpful By Christos Partsenidis on December 5, 2005
Format: Paperback
Following the success of 'The Tao of Network Security Monitoring' last year, world renowned security expert Richard Bejtlich raises once again the standard for security professionals, this time by focusing on analyzing threats coming from within our network - a kind of underestimated area.

Traditionally, the point of network security is about keeping the bad guys out of a network ¡V ¡¥out¡¦ being where we hope they are to start with. Possible points of entry are considered to be devices accessible from the outside in some way, mostly servers and perhaps routers. Workstations with no address on the network have no apparent footprint that would betray their existence, so if potential intruders don't even know the hosts exist, and are unable to make any connection to them, how could they possibly exploit them? The truth is they can, in many ways, using not only technical skills but imagination and ability to exploit the human factor - against which no automated procedure or device can defend for long.

Furthermore, many administrators put all their effort and resources into trying to design an impenetrable network infrastructure, but ignore the fact that every prevention measure is bound to fail at any moment. These administrators put little or no thought into the possibility of a real intrusion and, as a result, when it occurs the network infrastructure they've built doesn't allow them to cut their losses to a minimum, regain control in a timely manner and collect credible evidence that may lead to a future investigation.

This, Richard Bejtlich's second book on the subject of network security, attempts to establish into readers' minds a solid grounding on how things are, while emphasizing common misconceptions of the past.
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Format: Paperback
First, this book should be called The Engineers Guide to Implementing Security to Detect and Prevent Malicious Traffic in Your Network. This is a very thorough book on how to detect malicious traffic leaving a network (hence Extrusion), with great illustrations and walkthroughs. There are chapters on planning, deployment, tuning and other key, often overlooked, aspects surrounding the wonderful world of Intrusion Detection.

The first hint that this book was a bit different is noticed in the Foreward. Marcus Ranum wrote the forward, or I should say guided the direction of the Foreward. Marcus opts for an interview with the author, versus "telling you a bunch of stuff about the book". The Foreward is a must when browsing this book. Very creative, something perhaps missing in the world of Information Security these days.

After the foreward, chapters include Defensible Network Architecture, a brief overview of IDS, Enterprise network Instrumentation (packet captures, tools and some techniques), Layer 3 Network Access Control, Traffic Threat Assessment, Network Incident Response, Network Forensics, and Internal Intrusions that discuss Traffic Threat Assessment Case Study and Malicious Bots. There are several Appendixes as well (a requirement for all technical books) that include how to Collect Session Data, minimal Snort Installation Guide, Enumeration Methods (identifying systems on a network), and Open Source Host Enumeration (doing it for free).

The author uses firewall technology, proxy technology, and IDS technology to define how to monitor and control traffic entering or leaving a network. Specific configurations that could be copied line by line and implemented into a network are provided.

Richard leaves nothing to the imagination in this book.
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10 of 10 people found the following review helpful By Dr Anton Chuvakin on December 1, 2005
Format: Paperback
Now, they say that one way to be happy is to have low expectations about stuff. In case of me and this book, the opposite has happened: my expectations were really high. In fact, I was counting days until the book's arrival. Do not get me wrong, it's an excellent book, but it seems to fall slightly short of my lofty expectations.

My first unmet expectation was the term `extrusion' itself. I suspected that the book will have more coverage of real insider attacks, and not just infected misbehaving client PCs. The author does say that some use the term `extrusion' to refer to intellectual property theft (or `IP leakage') in his section on the `History of Extrusion Detection', but does not follow up on that. His definition of `extrusion detection' seem to be closer to the `detection of consequences of intrusion in the form of outbound connections', such as after a client-targeting attack, rather than a separate phenomenon of a trusted insider attack.

The second thing I did not quite like was too much overlapping material with Richard's previous book, `Tao of Network Security Monitoring.' For example, security process and security principles sections seem to be taken from his Tao book (which is a superb book, by itself!). Similarly, in my opinion, an in depth coverage of NSM methodology and `network forensics', presented in the Tao book should not have been repeated since the differences between applying NSM for intrusion and extrusion detection are really minor.

And, I liked pretty much everything else: detailed examples of `traffic threat assessment', unmatched technical accuracy, easy to follow style, etc. Coverage of bots in chapter 10 deserves a favorable mention as well as a strategy for network incident response.
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