Application security testing comes in two fundamental flavors: SAST (Static Application Security Testing) analyzes your source code without running it. DAST (Dynamic Application Security Testing) attacks your running application from the outside. Both find real vulnerabilities, but they find different ones.
If you already follow a web application security checklist, understanding when to use SAST vs DAST helps you fill gaps in your testing coverage.
SAST: Scanning Your Code
SAST tools read your source code and flag patterns that indicate vulnerabilities — SQL injection, cross-site scripting, hardcoded secrets, insecure deserialization, and hundreds of other issues. They run without deploying your application, which means they integrate naturally into your CI/CD pipeline as a build step.
The major advantage is early detection. SAST finds vulnerabilities before the code is deployed, merged, or even committed. Running SAST in pre-commit hooks or PR checks means developers see security feedback at the same time they see lint errors and test results.
Popular SAST tools include Semgrep (open-source, rule-based, excellent for custom rules), SonarQube (comprehensive code quality and security), and Snyk Code (AI-powered, integrates with Snyk’s dependency scanning).
Semgrep deserves special mention. Its rule syntax is intuitive — you write patterns that look like the code you want to find, with metavariables for flexible matching. Writing custom rules for your codebase’s specific patterns takes minutes, not hours. The community ruleset covers OWASP Top 10 and common framework-specific issues.
SAST limitations are real. False positive rates can be high — 30-50% is common without tuning. The tools flag code patterns that look dangerous but may be safe in context. Tuning rules and suppressing false positives takes ongoing effort. SAST also cannot find runtime issues — authentication bypass, business logic flaws, or configuration errors do not show up in source code analysis.
DAST: Attacking Your Application
DAST tools interact with your running application the way an attacker would — sending malicious inputs, probing endpoints, checking for misconfigurations. They do not need access to source code, which means they find issues that SAST misses: server misconfiguration, exposed admin panels, missing security headers, and runtime injection vulnerabilities.
OWASP ZAP (free, open-source) is the most widely used DAST tool. It provides automated scanning, a proxy for manual testing, and an API for CI/CD integration. The Active Scan mode tests every parameter and endpoint it discovers, reporting confirmed vulnerabilities with reproduction steps.
Burp Suite (commercial) is the professional standard. The scanner is more thorough than ZAP, the manual testing tools are more refined, and the reporting is better suited for compliance requirements. Burp Suite Professional costs $449/year per user.
Nuclei (open-source, by ProjectDiscovery) takes a template-based approach. The community maintains thousands of vulnerability templates for known CVEs, misconfigurations, and exposed services. It excels at finding known issues quickly across many targets.
DAST limitations include speed (scanning a large application takes hours), potential for disruption (active scanning can trigger rate limits, create test data, or crash fragile endpoints), and the requirement that the application must be running and accessible. DAST also cannot pinpoint which line of code causes a vulnerability — it tells you the symptom, not the location.
IAST: The Middle Ground
Interactive Application Security Testing (IAST) instruments your running application to monitor security-relevant behavior from the inside. It combines SAST’s code awareness with DAST’s runtime context. Tools like Contrast Security and Hdiv detect vulnerabilities as the application processes requests, with lower false positive rates than either approach alone.
IAST requires agent installation in your application runtime, which adds complexity and a small performance overhead. It works best for Java and .NET applications where runtime instrumentation is mature.
What to Use When
Use SAST in your CI/CD pipeline on every pull request. It catches the common issues early with fast feedback. Semgrep is the best starting point — it is free, fast, and the custom rules let you enforce your team’s security patterns.
Use DAST against your staging environment before each release. Schedule automated scans with OWASP ZAP or Nuclei. This catches configuration issues, missing headers, and runtime vulnerabilities that SAST cannot see.
Combine both with dependency scanning and AI code review for comprehensive coverage. No single tool catches everything — layered security testing is the only approach that works.
Verdict
Start with SAST (Semgrep) in your CI/CD pipeline. The early feedback and low setup cost make it the highest-ROI first step.
Add DAST (OWASP ZAP) against staging before releases. The runtime perspective catches what static analysis misses.
Consider IAST if you need lower false positive rates and your stack supports it. The runtime context produces more accurate findings at the cost of setup complexity.
The goal is not perfection — it is coverage. Each layer catches different vulnerabilities, and together they reduce your attack surface significantly. Implement Zero Trust principles alongside testing for defense in depth.
About Brian Detering
Brian Detering is a software engineer, educator, and tech writer based in Los Angeles. He teaches programming and software engineering at the University of Southern California, where his work spans programming languages, systems architecture, and applied AI. With over a decade of hands-on experience building production systems, Brian writes about the tools and workflows that actually make developers more productive — from CI/CD pipelines and containerization to API testing and security best practices. When he's not teaching or writing code, he's usually benchmarking the latest dev tools or tinkering with homelab infrastructure.
