DAST vs SAST: What Is the Difference?

The Detailed Answer

SAST and DAST approach the same goal, finding security vulnerabilities, from opposite directions. SAST reads the code and reasons about what it could do. DAST runs the application and observes what it actually does. The distinction matters because each approach has blind spots that the other covers, and understanding those blind spots is essential for building a security testing program that does not leave gaps.

SAST operates on the source code, bytecode, or binary. It builds a model of the application's data flows, tracking how user input moves through functions, assignments, and API calls until it reaches a sensitive operation like a database query, file system access, or HTML rendering. When user input reaches one of these operations without passing through proper sanitization, SAST flags it as a potential vulnerability. The analysis happens without executing the code, which means SAST can run before the application is deployed, even before it compiles, catching vulnerabilities at the earliest possible stage.

DAST operates on the running application. It does not see source code. Instead, it sends HTTP requests to the application's endpoints, manipulating parameters, headers, cookies, and form data with payloads designed to trigger vulnerabilities. If the application's response indicates that a payload was processed unsafely, such as SQL error messages after injection attempts, reflected script content after XSS payloads, or unauthorized data after access control bypass attempts, DAST reports the finding. The test runs against the application as deployed, including all its real-world dependencies: the web server configuration, the database, the middleware, the CDN, and the infrastructure.

What SAST Catches That DAST Misses

SAST sees the code, which gives it visibility into areas that DAST cannot reach.

Dead code vulnerabilities. If a vulnerable function exists in the codebase but is not currently reachable through any URL or API endpoint, DAST will never test it because DAST can only test what it can reach. SAST flags it because the code exists and could become reachable through a future change. Dead code with SQL injection is a vulnerability waiting for a route to be added.

Hardcoded secrets. API keys, database passwords, encryption keys, and other credentials embedded in source code are invisible to DAST because they never appear in HTTP responses (usually). SAST pattern-matches against known credential formats and flags them. This category is critical because a leaked credential gives an attacker direct access without exploiting any code vulnerability.

Insecure cryptographic patterns. Using MD5 for password hashing, ECB mode for encryption, or Math.random() for security tokens are code-level decisions that DAST cannot detect from the outside. SAST checks for known weak patterns in cryptographic function calls and flags them during code review.

Second-order injection. If user input is stored in a database on one page and later retrieved and used unsafely on a different page, DAST may not connect the two interactions because it tests endpoints independently. SAST can trace the data flow from storage to retrieval to use and flag the complete vulnerability chain.

Race conditions and concurrency issues. Code that is not thread-safe, such as a check-then-act pattern without locking, is a code-level issue that DAST can only find by chance (sending many concurrent requests and hoping to trigger the race). SAST analyzes the code structure and identifies patterns known to be unsafe in concurrent execution.

What DAST Catches That SAST Misses

DAST sees the running application, which gives it visibility into areas that SAST cannot assess.

Server configuration issues. Missing security headers (Content-Security-Policy, Strict-Transport-Security, X-Frame-Options), directory listing enabled, verbose error messages exposing stack traces, outdated TLS versions, and insecure cookie flags are all server-side configuration decisions that do not appear in application source code. DAST checks response headers and server behavior directly.

Authentication and session management. Session token randomness, session expiration behavior, and cookie security flags are runtime behaviors that SAST cannot evaluate from code alone (it can flag patterns that look insecure, but it cannot verify actual token entropy). DAST captures real session tokens and analyzes their randomness, tests whether expired sessions are actually rejected, and verifies that cookies carry the right flags in actual HTTP responses.

Third-party component behavior. If a web server, reverse proxy, or WAF modifies request handling in ways that introduce or mask vulnerabilities, only DAST sees the effect because it tests the full stack as deployed. SAST analyzes application code only and has no visibility into the middleware or infrastructure that processes requests before they reach the application.

CORS and CSRF in practice. Cross-origin resource sharing policies and CSRF protections involve both server configuration and application logic. SAST can check for missing CSRF token validation in code, but DAST tests whether the actual responses include permissive CORS headers, whether CSRF tokens are properly validated in the deployed environment, and whether the browser would actually allow a cross-origin attack.

Business logic vulnerabilities. DAST can be scripted (or operated manually) to test business logic: submitting a negative quantity, skipping checkout steps, applying discounts beyond their valid date. While SAST can flag some known anti-patterns, business logic vulnerabilities are by definition specific to the application's purpose, and SAST rules are generic by design.

SAST Strengths and Weaknesses

Strengths: Runs early in development, before deployment. Covers the entire codebase including unreachable code. Pinpoints exact file and line number. Scales to large codebases. Runs fast in CI pipelines. Catches vulnerability patterns at the moment they are introduced.

Weaknesses: High false positive rate because it reasons about what could happen rather than what does happen. Cannot see runtime behavior, server configuration, or infrastructure issues. Requires source code access. Results depend heavily on rule quality and configuration. Language-specific, each language needs its own analyzer.

Popular SAST tools: Semgrep (free, fast, developer-friendly), CodeQL (free for open source on GitHub), SonarQube (free Community Edition), Checkmarx (enterprise), Fortify (enterprise). See our full tool comparison for detailed evaluations.

DAST Strengths and Weaknesses

Strengths: Tests the real application as deployed. Language and framework agnostic, tests the HTTP interface regardless of implementation. Finds runtime, configuration, and infrastructure issues. Lower false positive rate than SAST because findings are based on observed behavior. Does not need source code access.

Weaknesses: Cannot pinpoint the source code location of a finding. Can only test code paths reachable through the application's interface. Slower than SAST because it makes actual HTTP requests. Requires a running environment. May miss vulnerabilities behind complex authentication flows or in code paths that require specific state. Can cause side effects (data modifications, account lockouts) during active scanning.

Popular DAST tools: OWASP ZAP (free, open source), Burp Suite (free Community, paid Professional), Nuclei (free, template-based), HCL AppScan (enterprise), Invicti (enterprise). Browser automation tools like Playwright also serve as application-specific DAST when scripted for security scenarios.

IAST: The Hybrid Approach

Interactive Application Security Testing (IAST) combines elements of both approaches. Agents embedded in the application runtime monitor code execution while the application handles traffic, correlating HTTP inputs with code paths. When user input reaches a dangerous function without sanitization, IAST reports the finding with both the HTTP request details (like DAST) and the exact code location (like SAST).

IAST produces fewer false positives than SAST because it observes actual execution rather than reasoning about possible paths. It produces more precise results than DAST because it sees the internal code handling. The tradeoff is deployment complexity: IAST agents must be installed in the application runtime, which adds overhead and requires compatibility with the application's language and framework.

IAST works best during QA testing and staging deployment, where real or realistic traffic exercises the application through its normal workflows. It does not replace SAST (which catches issues in code not yet deployed) or DAST (which tests the full stack including infrastructure), but it adds a layer of precision between them.

When to Use SAST vs DAST

Building a Combined SAST and DAST Program

The strongest security testing programs layer both approaches with clear roles for each.

SAST in pull requests: Run Semgrep or CodeQL on every pull request. Block merge on critical findings (SQL injection, command injection, hardcoded credentials). Log medium and low findings as warnings. Maintain a suppression file for confirmed false positives, reviewed quarterly.

DAST against staging: Run OWASP ZAP or Burp Enterprise against the staging environment after each deployment. Authenticate the scanner so it can test protected pages. Generate a findings report and compare against the previous scan to identify new issues. Block promotion to production on critical findings.

Dependency scanning in builds: Run Snyk, npm audit, or pip-audit on every build. This is neither SAST nor DAST but SCA (Software Composition Analysis), and it catches the third major vulnerability category: known vulnerabilities in libraries you depend on.

Manual penetration testing annually: Hire external testers who bring fresh perspectives and current attack expertise. Their scope includes everything that automated tools cannot test: business logic, complex multi-step attacks, and the combination of small findings that individually seem low-risk but together create an exploitable chain.

This layered approach, SAST for code, DAST for deployment, SCA for dependencies, and manual testing for everything else, provides comprehensive coverage that no single tool or approach can match.