What is the difference between SAST and DAST, and how do they complement each other in the testing process?

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SAST and DAST are two complementary approaches to security testing that examine your application from different angles. Understanding both is essential for building a comprehensive security testing strategy.

Static Application Security Testing, or SAST, analyzes your source code, bytecode, or binary without executing the application. It works like a specialized code review that looks for patterns known to introduce vulnerabilities. SAST tools scan through the codebase looking for issues like hardcoded credentials, SQL queries built with string concatenation, missing input validation, insecure cryptographic functions, and buffer overflow potential.

The advantages of SAST include early detection since it can run as soon as code is written, even before the application is deployable. It identifies the exact line of code where the vulnerability exists, making remediation straightforward. It can cover the entire codebase including code paths that are difficult to reach through testing. Popular SAST tools include SonarQube, Snyk Code, Semgrep, and CodeQL. Many of these integrate directly into IDEs, giving developers real-time feedback.

The limitations of SAST are that it produces false positives because it cannot determine whether a vulnerable code pattern is actually exploitable in the running application. It also cannot detect configuration issues, authentication flaws in the deployment environment, or vulnerabilities that emerge from the interaction between components at runtime.

Dynamic Application Security Testing, or DAST, tests the running application from the outside, simulating how an attacker would interact with it. DAST tools send malicious requests to the application's endpoints and analyze the responses to detect vulnerabilities. They probe for SQL injection by sending injection payloads, test for XSS by injecting script tags, check for security headers, and attempt to access unauthorized resources.

DAST advantages include low false-positive rates because it tests actual exploitability, not just code patterns. It finds configuration and deployment issues that SAST misses. It does not require access to source code, making it useful for testing third-party components. OWASP ZAP and Burp Suite are the most widely used DAST tools, with ZAP being free and open source.

DAST limitations include that it can only test what it can reach. Code paths that require specific conditions or complex user journeys might not be tested. It also cannot pinpoint the exact source code location of a vulnerability, making it harder for developers to fix issues.

The complementary nature is why mature security testing programs use both. SAST catches code-level vulnerabilities early in development, providing fast feedback to developers. DAST validates that the deployed application is actually secure, catching configuration issues and runtime vulnerabilities that SAST cannot detect. Together, they provide comprehensive coverage. I recommend running SAST in the CI pipeline on every pull request for fast feedback, and running DAST scans against deployed environments nightly or as part of release testing.