Secure code review

What is secure code review?

Secure code review is the examination of an application's source code to identify security flaws or vulnerabilities. These appear in the software development lifecycle (SDLC) and must be closed or fixed to strengthen the security of the code. Secure code review can take place at any point in the SDLC, but within the DevSecOps culture, it is most valuable to use it from the early stages. This is a procedure that can be performed either manually or automatically.

Code review vs. secure code review

It's crucial to distinguish between a standard code review and a secure code review.

Secure code review ensures that security is treated as a core property of code quality, preventing the introduction of weaknesses that could otherwise compromise data or functionality.

How does the secure code review work?

The most robust approach to secure code review is a blend of manual and automated reviews to leverage their particular capabilities. The automated review (like SAST) provides speed and breadth, while the manual review provides depth and accuracy.

The manual secure code review is conducted with great attention to detail. One or more security analysts scrutinize code, understanding what they are evaluating, keeping in mind its use and context, the developers' intentions, and the business logic.

On the other hand, the automated secure code review is a process in which more code is examined in less time, but in which the above factors are not considered. The tools work with a predefined set of rules, are restricted to certain types of vulnerabilities, and suffer, some more than others, from the defect of reporting false positives (i.e., saying that something is a vulnerability when it is not).

Automated secure code review tools, with their quick and initial assessment, act as an assistant to the human reviewer, making it easier for security analysts to focus on identifying more complex and business-critical vulnerabilities.

Among the most commonly used methods in automated secure code review are static application security testing (SAST) and software composition analysis (SCA; understand how one differs from the other in this blog post).

How to conduct a secure code review?

A professional secure code review follows a methodical process to maximize its impact:

1. Planning and scope definition

The process begins by defining the scope. Since secure code review is time-consuming, it is essential to prioritize the review based on risk. This involves:

• Defining clear objectives: What types of vulnerabilities are we trying to detect (e.g., compliance with PCI DSS, or flaws in a new authentication feature)?

• Gathering context: Understanding the application's architecture, business requirements, and functionality; gathering threat models and previous security findings.

• Prioritizing code: Focusing on critical assets and high-risk functions such as authentication modules, payment processing, access controls, and new features.

2. Tool-augmented execution

The human review is supported by technology to increase efficiency:

• Pre-review scanning: Run automated tools (SAST/SCA) first to rapidly detect known security issues in the code baseline and third-party components.

• Triage: Use the automated findings to filter false positives and guide the deeper manual investigation to specific, high-risk code paths.

• Manual analysis techniques: The reviewer uses techniques like code path tracing (following execution paths) and trust boundary mapping (analyzing security control points) to apply their domain expertise.

3. Deep dive and checklist validation

The core of secure code review involves a detailed, line-by-line review, with a focus on areas that require contextual and business logic understanding. During this phase, the expert reviewer leverages standardized security checklists (often based on OWASP, CWE, or internal policies) to ensure a systematic examination of key categories like input validation, authentication, and access control.

4. Reporting, remediation, and verification

Findings are documented with precise details, including:

• Vulnerability description: A clear explanation of the flaw and its security mapping.

• Exploitability and impact: An assessment of the risk level (using a standardized framework) to aid in prioritization.

• Precise location: File name and line number, confirmed by human validation.

• Detailed proof of concept: Providing a step-by-step demonstration of how the vulnerability can be exploited to facilitate developer understanding.

• Remediation guidance: Specific, secure code suggestions for developers.

After remediation, a follow-up review (re-scan/re-test) must be conducted to verify that the fix successfully eliminated the vulnerability and did not introduce new flaws.

The human advantage: what experts look for

Security experts who apply secure code review best practices, also considering the threat actors' perspective, can assess code to recognize the security issues that contribute most to the risk exposure of the target of evaluation. For example, in our latest annual State of Attacks report, we shared that 71% of the total risk exposure in the assessed systems was reported by the manual method.

Secure code review by experts excels at finding issues that are business logic blind to automated tools. The review focuses on critical areas such as the following:

Business logic flows

The reviewer analyzes the application's unique processes to look for:

• Workflow integrity: Opportunities to bypass state transitions, steps, or validation in multi-step processes.

• Race conditions: Timing-based vulnerabilities in concurrent operations where multiple users interact with the same resource.

• Resource limits: Ensuring rate limiting and resource quotas are implemented to prevent denial of service or resource exhaustion.

Authorization and access control

The reviewer checks the correctness and completeness of security enforcement:

• Server-side enforcement: Verifying that all access controls are enforced on the server, not just on the client side.

• Fail-safe defaults: Ensuring a default deny access policy is used.

• IDOR prevention: Checking for Insecure Direct Object References where a user can manipulate a parameter (e.g., an ID) to access another user's data or unauthorized resources.

Input validation and output encoding

While SAST can find basic injection patterns, the expert ensures contextual correctness:

• Server-side validation: All user and external input is validated regardless of client-side checks.

• Allowlist validation: Using allowlists (accepting only known good input) rather than blocklists (rejecting known bad input) for robustness.

• Context-appropriate encoding: Ensuring data is properly encoded (HTML, JavaScript, URL, SQL) before output to prevent injection attacks like XSS or SQL Injection.

Cryptography and secrets management

• Strong algorithms and key management: Verifying the use of modern, tested algorithms (e.g., AES-256, RSA-2048+) and secure key generation, storage, and rotation.

• Hardcoded secrets: Identifying where a developer made the mistake of leaving confidential information (e.g., API keys, tokens, credentials) inside the code, including configuration files.

Secure code review tools

For the automated implementation of secure code review, there are corresponding tools available that quickly identify known security vulnerabilities.

• SAST tools are programs that automatically scan source code or object code of applications —while these are not running— to detect vulnerabilities that match those stored in databases.

• SCA tools are programs that automatically scan applications to inventory their third-party software components and their dependencies and identify vulnerabilities in them that coincide with those registered in databases.

Secure code review vs. application security testing

Application security testing (AST) is a broader concept than secure code review. In fact, the latter is part of the former. AST, apart from SAST and SCA, involves assessment methods such as dynamic application security testing (DAST), pentesting as a service (PTaaS), and reverse engineering. While secure code review can be applied at any stage of software development, DAST and PTaaS, for instance, are generally employed when the application can run in order to evaluate its behavior through attack vectors.

Secure code review is a foundational element that, when combined with DAST (which checks for runtime and configuration errors) and PTaaS, ensures a comprehensive and defense-in-depth approach to security.

When should you implement secure code review?

Secure code review is part of a preventive approach, which should be addressed first, rather than a reactive approach. Applying this method as soon as the first lines of code are written makes it possible to identify and remediate vulnerabilities before going into production. Staying one step ahead of malicious hackers and blocking in the code any possible entry for improper uses, even simple shenanigans, is undoubtedly a very effective strategy to reduce the likelihood of catastrophes caused by cyberattacks.

A holistic strategy involves two main timing strategies:

• Continuous review: Implementing automated tools in integrated development environments (IDEs) and during pull requests is the most impactful way to "shift security left." This allows developers to receive near real-time feedback and fix issues when they are freshest in mind.

• Targeted review: Reserving an exhaustive manual code review for strategic points in the SDLC:

  • Project initiation: Comprehensive security assessment of a new or legacy codebase.

  • Major releases: Before significant version releases to ensure large-scale changes are secure.

  • Architecture changes: When fundamental design or technology changes occur.

  • Compliance cycles: Periodic extended reviews for regulatory requirements (PCI DSS, HIPAA, etc.).

Why is secure code review important?

A successful development team, committed to the security of its products, always has secure code review as a pillar. Any organization that develops software should have it among its constant practices, from the early stages of the SDLC, paying attention to the small changes that the members of its team gradually make to the code.

Security in general and common weaknesses in software and their exploitation are not usually taught to developers in their academies and workplaces. And even the most experienced developers, due to factors such as burnout or carelessness, can make coding mistakes and end up generating vulnerabilities such as those listed in the OWASP Top 10 and CWE Top 25. For reasons such as these, source code should usually remain under review by security experts.

Detection of vulnerabilities in source code and components

Secure code review identifies the absence of safe coding practices, lack of appropriate security controls, and violation of compliance standards such as PCI DSS and HIPAA. Secure code review providers may find, for instance, missing or erroneous validation of inputs (verification that they comply with specific characteristics) coming from different sources that interact with the application (e.g., users, files, data feeds). They may discover that a developer made the mistake of leaving confidential information (e.g., tokens, credentials) inside the code, having forgotten to remove it after putting it there without a reasonable justification. They may see that the information that needs to be stored and transferred doesn't pass through proper encryption algorithms. Likewise, they may find that user authentication processes are pretty weak, requiring, for example, short passwords with little variety in their characters. And that authorization controls are poor and end up giving unnecessary access to any user without requesting permission.

An important issue often discovered with secure code review, using SCA tools, is vulnerabilities within third-party and open-source software components. Application development today heavily depends on open-source software components, which are imported from various sources and serve as support for what is intended to be built, which often turns out to have little originality. The dependency also exists between some components and others. So when using one of them, for instance, the developer may not be aware of the relation of this one to the others. Cybercriminals have among their desired targets these dependencies to look for vulnerabilities to exploit.

Application of best coding practices

The experts and tools responsible for a secure code review will be in charge of verifying whether or not the developers of the software under evaluation have been employing secure coding practices. Though we have two blog posts that deal more extensively with secure coding practices (i.e., "Go Over and Practice Secure Coding" and "Secure Coding in Five Steps?"), which we invite you to read, here are some of those practices that should always be a point of reference for code development and review.

Specifically, ensure that your software meets objectives such as the following:

• Validate inputs from untrusted sources and accept only those that meet specific characteristics.

• Verify the identity of users or entities seeking access to private resources and, in critical operations, request multi-factor authentication.

• Require users to create sufficiently complex passwords.

• Restrict access to specific high-value resources to only a few authorized users.

• Give users access by default only to the resources needed to accomplish certain tasks.

• Establish relatively short session inactivity timeouts.

• Use well-known, tested, and up-to-date encryption algorithms for sensitive information in transit and at rest.

• Do not possess sensitive data such as comments within its code.

• Do not reveal valuable information to attackers in error messages resulting from invalid activities.

• All its components, elaborated by third parties, must be updated to their latest versions.

For more information on secure coding practices, you can also check OWASP recommendations for developers on their Code Review Guide.

Other benefits of secure code reviews

Cost efficiency

Secure code review allows the number of errors or vulnerabilities found in the final stages of the SDLC, through procedures such as pentesting, to be lower. Therefore, the time developers have to spend on remediation processes in these stages can also be reduced. Fixing a large number of vulnerabilities shortly before going into production becomes a thorn in the developers' side.

Always keep in mind that it is easier and less expensive to do code fixes in the development environment than in production. With a continuous secure code review, you are closer to the cause of the problem and can fix it immediately, avoiding any buildup.

Fostering a security culture

Thanks to an early secure code review, developers can start to assume a commitment not only to remedy the security issues identified in their products but also to make their results better every day. This can be a chain process. Certain groups of developers, with the help of the security teams and their tests or reviews, can pass on knowledge, inspire others to improve their practices and productivity, and make the transition to a mindset in which everyone in the organization is responsible for security.

The feedback loop helps developers learn from the patterns and practices that led to the error, enhancing their skills and contributing to high-quality code. Those security missteps that so often give rise to vulnerabilities can become less frequent over time.

Compliance and reputation

Organizations that decide to implement secure code review in their software development processes recognize the responsibility to comply with established standards in their industries. They seek to offer products and services that guarantee security for their operations, data, and other resources, mainly those of their customers or users. This success generates trust and reflects commitment and quality, positively affecting their competitiveness and reputation.

How to choose your secure code review team?

While a team of developers can do their own code reviews, such as when a developer asks a teammate to peer review their build to avoid logical or stylistic errors, it is recommended that, in security issues, experts in the field be involved. Security engineers, code reviewers, and pentesters specialize in identifying vulnerabilities. They bring a broader perspective and threat modeling mindset to spot subtle, structural security flaws that a developer might overlook. Moreover, reviews by an external agent can ensure that all flaws are reported while maintaining an unbiased view.

The main elements of secure code review within a good cybersecurity team should be a widely certified group of security experts and automated tools showing low false positive and false negative rates. Their assessments should be able to be performed in a broad range of programming languages, be based on multiple international security standards, and report findings on a single dashboard that prioritizes, encourages, and facilitates remediation.

Fluid Attacks' secure code review

At Fluid Attacks, we offer our secure code review as a comprehensive and accurate review of your software's source code, combining manual (by our certified pentesters) and automatic procedures based on methods such as SAST and SCA. With us, you can apply secure code review from the earliest stages of your SDLC in a continuous manner. You can solve your security issues promptly (prioritizing those that represent the highest risk exposure) in favor of your development team's productivity and the security of your products.

Our secure code review supports many programming languages, including C, C#, C++, HTML, Java, JavaScript, PHP, Python, Ruby, and Swift. We have among our review requirements those present in more than 60 international security standards, including CERT, CVE, CWE, HIPAA, NIST, OWASP, and PCI DSS. And we adjust to specific requirements for your application and business logic, all constantly reviewed and updated.

We integrate our CI Gate into your pipelines to break the build when there are policy violations and open vulnerabilities. And we report everything to you on our platform, where you can thoroughly understand and analyze your security issues, as well as receive recommendations and manage remediation processes. Additionally, your developers can have our IDE extensions at hand for faster recognition of affected lines of code and GenAI-based remediation suggestions. All this is part of our Continuous Hacking, which also integrates security testing methods such as DAST, CSPM, PTaaS, and reverse engineering.

Do not hesitate to contact us if you want more information about our secure code review and other AppSec solutions. Click here to try our Continuous Hacking Essential plan free for 21 days.