The Art of Software Security Assessment: Identifying and Preventing Software Vulnerabilities
| Before you perform the actual review, you need to help scope and plan the assessment. This process involves gathering key pieces of information that assist you in later phases of your review. By gathering as much information as you can before starting the assessment, you can construct a better plan of attack and achieve more thorough coverage. Scoping
When tasked with an application security review, first you need to ask what your goal is. This question might seem simple, but numerous answers are possible. Generally, a vulnerability researcher's goal is to find the most significant vulnerability in the shortest time. In contrast, an application security consultant is usually concerned with getting the best application coverage the project's budget allows. Finally, a developer or security architect might have a more generous schedule when conducting internal reviews and use that time to be as thorough as possible. The goal of a review might also be heavily colored by business concerns or less tangible factors, such as company image. A company certainly isn't inclined to devote extensive time to a product that's close to or even past its end of life (EOL). However, a review might be required to meet regulatory concerns. That same company might also want a thorough review of its newest flagship financial management application. When businesses commit to more thorough reviews, often you find that their interests aren't what you expect. A business is sometimes more concerned with easy-to-detect issues, regardless of their severity. Their goal is more to avoid the negative stigma of a published security issue than to address the ultimate technical security of their product or service. So you aren't meeting your client's (or employer's) needs if you spend all your time on complex issues and miss the low-risk but obvious ones. Focusing on low-risk issues seems like blasphemy to most technical security people, but it's often a reasonable business decision. For example, assume you're performing a source-code-based assessment on a bank's Web-facing account management application. What is the likelihood of someone blindly finding a subtle authentication bypass that you found only by tracing through the source code carefully? In contrast, think of how easily an attacker can find cross-site scripting vulnerabilitiesjust with normal user access. So which issue do you think is more likely to be identified and leveraged by a third party? The obvious answer is cross-site scripting vulnerabilities, but that's not what many auditors go after because they want to focus on the more interesting vulnerabilities. That's not to say you should ignore complex issues and just get the easy stuff. After all, that advice would make this book quite short. However, you need to understand the goals of your review clearly. You also need to have an appreciation for what you can reasonably accomplish in a given timeframe and what confidence you can have in your results. These details are influenced by two major factors: the type of access you have to the application and the time you have available for review. Application Access
Application access is divided into the five categories listed in Table 4-1. These distinctions are not, of course, absolute. There are always minor variations, such as limited source access or inconsistencies between test environments and deployment environments. However, these distinctions work well enough to cover most possibilities.
This book focuses primarily on source-code-based application review. Although the techniques discussed in this chapter can be applied to other types of reviews, more information is generally better. The ideal assessment environment includes source-based analysis augmented with access to functioning binaries and a live QA environment (if appropriate). This environment offers the widest range of assessment possibilities and results in the most time-effective review. The remaining types of access in Table 4-1 are all viable techniques, but they generally require more time for the same degree of thoroughness or have an upper limit on the degree of thoroughness you can reasonably hope to achieve. Timelines
In addition to application access, you need to determine how much time can be allotted to a review. The timeline is usually the most flexible part of a review, so it's a good way to adjust the thoroughness. The most commonly used measure of application size is thousands of lines of code (KLOC). It's not an ideal way to measure an application's complexity and size, but it's a reasonable metric for general use. A good reviewer ranges between 100 to 1,000 lines of code an hour, depending on experience and details of the code. The best way to establish an effective baseline for yourself is to keep track of how much time you spend reviewing different components and get a feel for your own pacing. Code type and quality have a big impact on your review speed. Languages such as C/C++ generally require close examination of low-level details because of the subtle nature of many flaws. Memory-safe languages, such as Java, address some of these issues, but they might introduce higher-level complexity in the form of expansive class hierarchies and excessive layering of interfaces. Meanwhile, the quality of internal documentation and comments is a language-independent factor that can seriously affect your review pacing. For this reason, you should look at some samples of the application code before you attempt to estimate for your pace for a specific codebase. Overall code size affects the pace at which you can effectively review an application. For instance, reviewing a 100KLOC application doesn't usually take twice as much time as a 50KLOC application. The reason is that the first 50KLOC give you a feel for the code, allow you to establish common vulnerability patterns, and let you pick up on developer idioms. This familiarity enables you to review the remainder of the application more efficiently. So be sure to account for these economies of scale when determining your timelines. In the end, balancing coverage with cost is usually the ultimate factor in determining your timeline. In a perfect world, every application should be reviewed as thoroughly as possible, but this goal is rarely feasible in practice. Time and budgetary constraints force you to limit the components you can review and the depth of coverage you can devote to each component. Therefore, you need to exercise considerable judgment in determining where to focus your efforts. Information Collection
The first step in reviewing an application is learning about the application's purpose and function. The discussion of threat modeling in Chapter 2 included a number of sources for information collection. This component of your review should encapsulate that portion of the threat model. To recap, you should focus on collecting information from these sources:
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