Scoping a security code review is a critical step in ensuring a successful engagement. Without proper scoping, you risk falling into one of two traps: either you have too little time to conduct a thorough review and miss critical issues, or you spend excessive time and resources analyzing code with diminishing returns. The key is to balance depth, coverage, and cost to maximize value for the client and your team.
Understanding the Goals of a Security Code Review
A well-scoped security code review starts with understanding the client’s goals. Are they looking for exploitable bugs, a strengthened security posture, or simply a clearer understanding of potential risks in their application? Clients might seek a code review for various reasons: they may have already subjected the application to rigorous penetration testing, heard about the benefits of security code reviews, or want to use the review process as a foundation for internal developer training. Each of these scenarios will dictate a different approach to the review, affecting how much time and effort you invest in different areas of the codebase.
Evaluating Application Complexity and Size
The complexity and size of the application are also major factors. Reviewing a small CRUD application built with Rails is vastly different from auditing a highly complex system implementing zero-knowledge algorithms or innovative cryptography. The larger and more intricate the application, the more time and expertise are required. A codebase with 100,000 lines of code demands a different approach from one with a million lines. Adding to this, the programming language can impact the cost and effort—it’s easier to find experienced reviewers for popular languages like Golang than for more niche languages like Scala.
Knowing Where to Stop
One of the most challenging aspects of scoping a security code review is deciding where to stop. Code reviews can spiral into an endless rabbit hole—checking dependencies, dependencies of dependencies, or even the underlying framework itself. Knowing when to draw the line is crucial. While it might seem appealing to audit every aspect of the system, this isn’t realistic or cost-effective for most clients. The focus should remain on areas of the application that are most likely to yield impactful findings based on the client’s goals and the application’s threat profile.
Tailoring the Review to Threat Profiles
Speaking of threat profiles, a proper threat assessment can help scope a security code review tactically. An internal application with features accessible only to trusted users requires a different approach than a public-facing application with self-registration. In the first case, you may prioritize reviewing authentication mechanisms and session management, whereas in the latter, your focus might shift to input validation and public API endpoints. Tailoring the scope to address specific threats ensures that your efforts are directed toward the areas that matter most.
Addressing Deployment and Testing Challenges
Another factor to consider is the deployment and testing environment. If the code is difficult to deploy or cannot be run with full debugging enabled, your ability to trace behaviors and confirm vulnerabilities will be limited. A lack of debugging capabilities can slow down the review process significantly. Similarly, if it takes several days to set up the test environment, this delay should be factored into the timeline. A seamless and functional environment allows reviewers to map code to application behavior more effectively, which is critical for identifying and verifying issues.
Setting Client Expectations
The client’s expectations for findings also play a major role in shaping the review. Some clients may value detailed explanations of risks and recommendations for improvement, while others might want fully working exploits to demonstrate the impact of vulnerabilities. Writing exploits takes time, especially when bypassing multiple layers of defense, and this effort must be accounted for in the scope. Additionally, consider what deliverables the client expects from the review. Some clients can benefit from tailored artefacts, such as a set of Semgrep or SAST rules created to address recurring issues in their codebase. These artefacts not only provide immediate value but also help establish long-term safeguards for maintaining security. Clarifying these expectations upfront will prevent misunderstandings and ensure that the review delivers what the client truly needs.
Timeboxing and Collaboration for Efficiency
Timeboxing is often the most practical way to scope a security code review. For very small engagements, a one-week review may suffice, while more comprehensive reviews typically require at least two to four weeks. Iterative reviews can be particularly effective for large or complex systems. After completing an initial cycle, evaluate the findings and decide whether further review cycles are warranted. This approach ensures that the review remains focused and cost-effective, avoiding situations where you continue reading code without meaningful security improvements. Unfortunately, doing this can be a nightmare for scheduling.
A successful security code review also benefits from multiple reviewers. Having at least two reviewers allows for cross-validation of findings and ensures that nothing critical is overlooked. This collaborative approach is particularly important for longer engagements, where fresh perspectives can make a significant difference.
Educating Clients About Security Code Reviews
Educating clients is another essential part of the scoping process. Not all clients have a clear understanding of what a security code review entails or how to scope it effectively. Some may overestimate the importance of certain areas or underestimate the value of targeted reviews. For example, a client requesting a comprehensive review of a fragile system might benefit more from prioritizing foundational fixes before diving into deeper analysis. If the client has not conducted a penetration test yet, this might be a more efficient or cost-effective starting point. Alternatively, a "grey box" approach, where the testers have access to the code during a penetration test, can offer a balanced middle ground—combining the strengths of both methodologies. By guiding clients toward a more strategic approach, you can help them achieve better outcomes while avoiding unnecessary costs.
Conclusion
Scoping a security code review is both an art and a science. It requires balancing technical depth with practical constraints, understanding the client’s goals, and tailoring the approach to deliver maximum value. While it may seem daunting at first, scoping becomes more intuitive with practice and experience. By focusing on the right areas and setting clear expectations, you can ensure a successful review that meets the client’s needs and strengthens their application’s security.
