Source code review is a critical practice for identifying vulnerabilities and weaknesses in application code before deployment. By thoroughly analyzing source code, development teams can detect flaws like injection attacks, improper validation, and other security issues that put applications at risk. This comprehensive guide provides developers with the key concepts, strategies, tools and best practices needed to conduct effective source code reviews that enhance security. By following the techniques outlined here, teams can remediate vulnerabilities early, reduce risk, and build more resilient software.

Planning Your Review  

Before diving into code analysis, it’s important to define the goals and scope of the review. This planning sets the stage for an efficient, targeted and high-value review process.

Objectives and Scope

• Define specific objectives like identifying high risk vulnerabilities or evaluating compliance with security standards. Keep the goals focused on security issues vs general code quality.
• Determine scope by selecting target code sections, features or system components to review. Prioritize critical functions like authentication, session management, and sensitive data processing. 
• For large codebases, focus on modules that handle significant security risks rather than attempting comprehensive review.

Understanding the Codebase

• Review the application’s architecture and get familiar with the main components before analyzing code. 
• Identify key modules, third party libraries, dependencies and frameworks in use. Understanding these codebuilding blocks provides context.
• Perform a walkthrough of the most security-critical functions in the codebase. This primes reviewers on how key features like login are implemented.

Manual Code Review Techniques

Manual code review entails human analysis of source code line-by-line. This hands-on process allows detection of vulnerabilities that automated tools may miss.

Line-by-Line Analysis

• Read through code thoroughly line-by-line without skimming. Reviewers should manually parse each line of code.
• Watch for common weaknesses like SQL injection and cross-site scripting during line-by-line inspection. Identifying these flaws requires human judgment.
• Scrutinize areas where user-supplied data is handled, since improper validation is a major source of weaknesses.

Data Flow Analysis

• Trace the flow of data through the application to understand how user inputs are processed. Follow the path from entry points like forms to backends like databases. 
• Verify validation, encoding and sanitization of data at each stage of processing. Flaws often arise from incomplete sanitization.
• Watch for “tainted” untrusted data entering security-sensitive functions like SQL queries. This helps catch injection attack vectors.

Targeted Spot Checks

• Check code for hardcoded secrets like API keys, passwords or encryption keys, which create risk if exposed.
• Review error handling logic for improper handling of exceptions. Erroneous or verbose errors can aid attackers.
• Inspect access controls and authorization checks for issues enforcing least privilege and other security principles.

Automation and Tooling

Automated code scanning complements manual review by identifying vulnerabilities at scale. Integrating automation into the review process boosts efficiency, consistency and coverage.

Static Analysis Security Testing (SAST)

• SAST tools scan codebases to systematically detect security flaws like injection issues, unvalidated redirects, weak cryptography and more.
• Leading commercial SAST options include Veracode, Checkmarx and Synopsys. Open source tools like SpotBugs and SonarQube also available.
• Configure tools to minimize false positives and focus on high confidence results. Integrate into build processes for rapid feedback.

Specialized Tools

• Use grep and other search tools to identify the use of banned APIs, unsafe functions like `strcpy()`, and other undesirable code patterns.
• Fuzz testing tools mutate inputs to detect crashes and anomalies that could signal flaws like buffer overflows.
• Linters highlight style violations and unused code based on predefined rule sets. This maintains code hygiene.

Orchestration and Integration 

• Combine manual code review with automated scanning for maximum coverage. Humans identify logic flaws automation may miss.
• Integrate code analysis tools into developer IDEs to provide constant secure coding feedback during initial development.
• Incorporate code scanning into CI/CD pipelines to shift security left and prevent flawed code from being deployed.

 Best Practices for Effective Reviews

Following secure coding review best practices helps teams maximize value while minimizing disruption to existing processes.

Focused and Constructive Feedback

• Provide specific, actionable recommendations on how to fix identified vulnerabilities and other issues.
• Maintain a respectful, collaborative tone when delivering feedback. The goal is improving code quality, not criticizing developers.

Iterative Process   

• Schedule code reviews frequently throughout the development lifecycle rather than just before launch. This allows incremental improvement.
• Treat code review as an evolving process. Continuously assess and refine techniques to increase efficiency and results over time.  

Knowledge Sharing

• Document review findings internally for developer reference. Discuss results post-review to spread awareness.
• Use reviews as teaching moments. Highlight examples of secure coding done well to promote best practices. 

Conclusion

By following a strategic approach to source code review, development teams can uncover vulnerabilities early, enhance code quality and promote a culture of security. The methodologies and tools covered in this guide provide a practical blueprint for conducting reviews that deliver meaningful risk reduction benefits. Consistent use of these source code review best practices will lead to more secure software that better safeguards data and withstands real-world attacks.