SAST vs DAST: Choosing the Right Security Testing Approach

SAST (Static Application Security Testing) analyzes source code for vulnerabilities without running the application. DAST (Dynamic Application Security Testing) tests a running application by sending crafted requests and analyzing responses. They are complementary approaches that find different classes of vulnerabilities at different stages of the development lifecycle.

SAST operates on source code, bytecode, or binaries. It reads the code, builds structural models (ASTs, control flow graphs), and identifies vulnerable patterns. SAST finds issues like SQL injection, XSS, hardcoded secrets, and weak cryptography by analyzing code constructs directly. It runs without a deployed environment.

DAST operates on a live application. It acts as an external attacker, sending malicious inputs to endpoints and observing how the application responds. DAST finds issues like authentication flaws, server misconfigurations, runtime injection, and exposed error messages. It requires a running application accessible over the network.

The fundamental difference: SAST sees the code but not the running behavior. DAST sees the running behavior but not the code. This means each approach has blind spots the other covers. A comprehensive application security program uses both.

The practical differences between SAST and DAST affect how, when, and what they test:

Timing in the SDLC

• SAST: Runs as early as the coding phase. Can execute on every commit, PR, or pre-commit hook. No deployment needed.
• DAST: Runs after deployment to a test or staging environment. Requires a running, accessible application.

What They Detect

• SAST excels at: Injection flaws (SQL, XSS, command), hardcoded secrets, weak cryptography, insecure deserialization, dependency vulnerabilities, coding standard violations.
• DAST excels at: Authentication and session management issues, server misconfiguration, runtime access control failures, CORS misconfigurations in deployed environments, response header issues.

False Positive Profile

• SAST: May flag code that appears vulnerable but is protected by runtime controls (middleware, WAF). Higher false positives on code patterns, lower on dependency issues.
• DAST: May flag transient behaviors or test environment artifacts. Lower false positives on confirmed exploitable issues, but limited to tested endpoints.

Coverage

• SAST: Analyzes all code paths, including error handlers and unused code. Coverage is codebase-wide.
• DAST: Only tests endpoints it can discover and reach. Coverage depends on crawling depth and endpoint enumeration.

The choice between SAST and DAST is not either/or—it is a question of when each provides the most value:

Use SAST when:

• You want security feedback at the earliest possible stage (pre-commit, pull request)
• You need to scan code before a deployable environment exists
• You need CWE-mapped, line-level findings for developer remediation
• Compliance requires evidence of source code security analysis (PCI-DSS, SOC 2)
• You want to catch dependency vulnerabilities and hardcoded secrets before they reach any environment

Use DAST when:

• You need to validate that runtime defenses (WAF, middleware, authentication) are working correctly
• You want to test deployed configurations (headers, TLS, CORS) that only exist at runtime
• You need to simulate external attacker behavior as part of penetration testing
• You are testing third-party or closed-source components where source code is unavailable

Most mature security programs run SAST on every pull request and DAST on staging deployments, with penetration testing on a scheduled cadence. SAST catches 80% of issues before deployment; DAST catches the runtime-specific 20% that SAST cannot see.

CodeSlick is a SAST tool designed to be the first security gate in your pipeline. It complements any DAST tool you already use by covering the source code analysis layer:

• 294 security checks across JavaScript, TypeScript, Python, Java, and Go, covering 95% of the OWASP Top 10
• Pre-deployment detection: Runs on pull requests and pre-commit hooks, catching vulnerabilities before they reach any environment
• Dependency scanning: Checks npm, pip, Maven, and Go module dependencies for known CVEs, covering the SCA layer alongside SAST
• SARIF output: Exports findings in SARIF format for integration with GitHub Security Tab and other security management platforms

Teams using DAST tools like OWASP ZAP, Burp Suite, or Qualys get the most complete coverage by pairing them with CodeSlick as the SAST layer. CodeSlick catches code-level issues at the PR stage; DAST catches runtime issues in staging.