Security Code Review - Catching Bugs Before Release

About 2 min read

Code review is a quality management practice in which source code written by one developer is inspected by another to find bugs, vulnerabilities, and design problems. According to research by IBM, code review can detect more defects than testing, with a defect removal rate reaching 60 to 90%. From a security perspective, a major strength is that human eyes can detect flaws in authorization logic and business-logic vulnerabilities that are difficult for automated tools to catch. As of 2025, the importance of code review is growing, including from the standpoint of verifying the security quality of code generated by code-completion tools such as GitHub Copilot.

The Flaws Automated Scanners Miss

The value of code review shows most clearly with the kinds of flaws that static analysis (SAST) tools struggle to detect. A missing authorization check on an API endpoint, for example, is code that runs perfectly well, so mechanical pattern matching has nothing to flag. IDOR (Insecure Direct Object Reference) — where merely being logged in is enough to reach someone else's data — is the classic case: deciding that it is a flaw requires understanding whose data a record is and who is allowed to touch it. Human review can read that gap between the specification and the implementation, which is why it complements automated scanning rather than being replaced by it.

Reviewing from a Security Perspective

In a security code review, you focus on missing input validation, SQL injection and XSS vulnerability patterns, hardcoded credentials, and improper error handling (such as exposing stack traces). The OWASP Code Review Guide recommends prioritizing four areas for inspection: authentication, authorization, session management, and encryption.

How to Conduct It Effectively

The optimal amount of code to review in a single session is considered to be 200 to 400 lines; beyond that, concentration drops and oversights increase. Use a checklist for reviews and systematically verify items aligned with the principles of secure coding. Asynchronous reviews using tools, such as GitHub pull request reviews and GitLab merge request reviews, are mainstream, but synchronous reviews in a pair-programming style are also effective for complex logic. Set up branch protection rules that prevent merges without review approval, and protect the accounts that hold approval rights with a strong random password. If an account that can approve changes is taken over, the review gate itself is bypassed.