Book excerpt: Exploiting Software

Editor's Note: This article was first posted on March 10, 2004

In this excerpt, from Chapter 3 of their new book Exploiting Software, authors Greg Hoglund and Gary McGraw explain the concepts and methods of reverse engineering and the tools that can be used to exploit it. The excerpt is posted with permission from Addison-Wesley Professional.

Most people interact with computer programs at a surface level, entering input and eagerly (impatiently?!) awaiting a response. The public facade of most programs may be fairly thin, but most programs go much deeper than they appear at first glance. Programs have a preponderance of guts, where the real fun happens. These guts can be very complex. Exploiting software usually requires some level of understanding of software guts.

The single most important skill of a potential attacker is the ability to unravel the complexities of target software. This is called reverse engineering or sometimes just reversing. Software attackers are great tool users, but exploiting software is not magic and there are no magic software exploitation tools. To break a nontrivial target program, an attacker must manipulate the target software in unusual ways. So although an attack almost always involves tools (disassemblers, scripting engines, input generators), these tools tend to be fairly basic. The real smarts remain the attacker's prerogative.

When attacking software, the basic idea is to grok the assumptions made by the people who created the system and then undermine those assumptions. (This is precisely why it is critical to identify as many assumptions as possible when designing and creating software.) Reverse engineering is an excellent approach to ferreting out assumptions, especially implicit assumptions that can be leveraged in an attack. [ 1 ]

Into the House of Logic

In some sense, programs wrap themselves around valuable data, making and enforcing rules about who can get to the data and when. The very edges of the program are exposed to the outside world just the way the interior of a house has doors at its public edges. Polite users go through these doors to get to the data they need that is stored inside. These are the entry points into software. The problem is that the very doors used by polite company to access software are also used by remote attackers.

Consider, for example, a very common kind of Internet-related software door, the TCP/IP port. Although there are many types of doors in a typical program, many attackers first look for TCP/IP ports. Finding TCP/IP ports is simple using a port-scanning tool. Ports provide public access to software programs, but finding the door is only the beginning. A typical program is complex, like a house made up of many rooms. The best treasure is usually found buried deep in the house. In all but the most trivial of exploits, an attacker must navigate complicated paths through public doors, journeying deep into the software house. An unfamiliar house is like a maze to an attacker. Successful navigation through this maze renders access to data and sometimes complete control over the software program itself.

[1] A friend at Microsoft related an anecdote involving a successful attacker who made use of the word "assume" to find interesting places to attack in code. Unsuspecting developers assumed that writing about what they assumed would be OK. This is a social-level attack pattern. Similar searches through code for BUG, XXX, FIX, or TODO also tend to work.)

Software is a set of instructions that determines what a general-purpose computer will do. Thus, in some sense, a software program is an instantiation of a particular machine (made up of the computer and its instructions). Machines like this obviously have explicit rules and well-defined behavior. Although we can watch this behavior unfold as we run a program on a machine, looking at the code and coming to an understanding of the inner workings of a program sometimes takes more effort. In some cases the source code for a program is available for us to examine; other times, it is not. Therefore, attack techniques must not always rely on having source code. In fact, some attack techniques are valuable regardless of the availability of source code. Other techniques can actually reconstruct the source code from the machine instructions. These techniques are the focus of this chapter.

Reverse Engineering

Reverse engineering is the process of creating a blueprint of a machine to discern its rules by looking only at the machine and its behavior. At a high level, this process involves taking something that you may not completely understand technically when you start, and coming to understand completely its function, its internals, and its construction. A good reverse engineer attempts to understand the details of software, which by necessity involves understanding how the overall computing machinery that the software runs on functions. A reverse engineer requires a deep understanding of both the hardware and the software, and how it all works together.

Think about how external input is handled by a software program. External "user" input can contain commands and data. Each code path in the target involves a number of control decisions that are made based on input. Sometimes a code path will be wide and will allow any number of messages to pass through successfully. Other times a code path will be narrow, closing things down or even halting if the input isn't formatted exactly the right way. This series of twists and turns can be mapped if you have the right tools. ...

Generally speaking, the deeper you go as you wander into a program, the longer the code path between the input where you "start" and the place where you end up. Getting to a particular location in this house of logic requires following paths to various rooms (hopefully where the valuables are). Each internal door you pass through imposes rules on the kinds of messages that may pass. Wandering from room to room thus involves negotiating multiple sets of rules regarding the input that will be accepted. This makes crafting an input stream that can pass through lots of doors (both external and internal) a real challenge. In general, attack input becomes progressively more refined and specific as it digs deeper into a target program. This is precisely why attacking software requires much more than a simple bruteforce approach. Simply blasting a program with random input almost never traverses all the code paths. Thus, many possible paths through the house remain unexplored (and unexploited) by both attackers and defenders.

Why Reverse Engineer?

Reverse engineering allows you to learn about a program's structure and its logic. Reverse engineering thus leads to critical insights regarding how a program functions. This kind of insight is extremely useful when you exploit software. There are obvious advantages to be had from reverse engineering. For example, you can learn the kind of system functions a target program is using. You can learn the files the target program accesses. You can learn the protocols the target software uses and how it communicates with other parts of the target network.

The most powerful advantage to reversing is that you can change a program's structure and thus directly affect its logical flow. Technically this activity is called patching, because it involves placing new code patches (in a seamless manner) over the original code, much like a patch stitched on a blanket. Patching allows you to add commands or change the way particular function calls work. This enables you to add secret features, remove or disable functions, and fix security bugs without source code. A common use of patching in the computer underground involves removing copy protection mechanisms.

Like any skill, reverse engineering can be used for good and for bad ends.

Should Reverse Engineering Be Illegal?

Because reverse engineering can be used to reconstruct source code, it walks a fine line in intellectual property law. Many software license agreements strictly forbid reverse engineering. Software companies fear (and rightly so) that their trade secret algorithms and methods will be more directly revealed through reverse engineering than they are through external machine observation. However, there is no general-purpose law against reverse engineering.

Because reverse engineering is a crucial step in removing copy protection schemes, there is some confusion regarding its legality. Patching software to defeat copy protection or digital rights management schemes is illegal. Reverse engineering software is not. If the law changes and reverse engineering is made illegal, then a serious blow will be dealt to the common user of software (especially the common and curious user). A law completely outlawing reverse engineering would be like a law making it illegal to open the hood of your car to repair it. Under such a system, car users would be required by law to go to the dealership for all repairs and maintenance.[ 2 ]

Software vendors forbid reverse engineering in their license agreements for many reasons. One reason is that reverse engineering does, in fact, more obviously reveal secret methods. But all this is a bit silly, really. To a skilled reverse engineer, looking at the binary machine code of a program is just as good as having the source code. So the secret is already out, but in this case only specialists can "read" the code. Note that secret methods can be defended through means other than attempting to hide them from everyone but specialists in compiled code. Patents exist specifically for this purpose, and so does copyright law. A good example of properly protecting a program can be found in the data encryption algorithms domain. To be acceptable as actually useful and powerful, encryption algorithms must be published for the cryptographic world to evaluate. However, the inventor of the algorithm can maintain rights to the work. Such was the case with the popular RSA encryption scheme. Also note that although this book is copyrighted, you are allowed to read it and understand it. In fact, you're encouraged to do so.

[2] Although this may not sound so bad to you, note that such a law may well make it illegal for any "nonauthorized" mechanic to work on your car as well.

Another reason that software vendors would like to see reverse engineering made illegal is to prevent researchers from finding security flaws in their code. Quite often security researchers find flaws in software and report them in public forums like bugtraq. This makes software vendors look bad, hurts their image, and damages their reputation as upstanding software vendors. (It also tends to make software improve at the same time.) A wellestablished practice is for a security specialist to report a flaw to the vendor and give them a reasonable grace period to fix the bug before its existence is made public. Note that during this grace period the flaw still exists for more secretive security specialists (including bad guys) to exploit. If reverse engineering is made illegal, then researchers will be prevented from using a critical tool for evaluating the quality of code. Without the ability to examine the structure of software, users will be forced to take the vendor's word that the software is truly a quality product. [ 3 ] Keep in mind that no vendor is currently held financially liable for failures in its software. We can thus trust the vendor's word regarding quality as far as it impacts their bottom line (and no farther).

The Digital Millennium Copyright Act (DMCA) explicitly (and controversially) addresses reverse engineering from the perspective of copyright infringement and software cracking. For an interesting view of how this law impacts individual liberty, check out Ed Felten's Web site at http://www. freedomtotinker.com.

When you purchase or install software, you are typically presented with an end-user license agreement (EULA) on a click-through screen. This is a legal agreement that you are asked to read and agree to. In many cases, simply physically opening a software package container, such as the box or the disk envelope, implies that you have agreed to the software license. When you download software on-line, you are typically asked to press "I AGREE" in response to a EULA document displayed on the Web site (we won't get into the security ramifications of this). These agreements usually contain language that strictly prohibits reverse engineering. However, these agreements may or may not hold up in court [Kaner and Pels, 1998]\.

The Uniform Computer Information Transactions Act (UCITA) poses strong restrictions on reverse engineering and may be used to help "click through" EULA's stand-up in court. Some states have adopted the UCITA (Maryland and Virginia as of this writing), which strongly affects your ability to reverse engineer legally.

Reverse Engineering Tools and Concepts

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