SAST vs DAST: What Is the Difference?
Understand the difference between SAST static code scanning and DAST live application testing, when each runs, and how they work together.
Expected Interview Answer
SAST (Static Application Security Testing) scans an application’s source code, bytecode, or binaries without executing them to find vulnerabilities like SQL injection patterns or hardcoded secrets, while DAST (Dynamic Application Security Testing) attacks a running application from the outside, sending real requests to find vulnerabilities that only appear at runtime.
SAST tools parse the codebase and build an abstract syntax tree or control-flow graph, then trace tainted data from untrusted input sources to dangerous sinks such as a raw SQL query or an unescaped HTML output, flagging the exact file and line number. Because it runs early in the pipeline, often on every pull request, SAST catches issues before the code ever ships, but it cannot see runtime configuration issues, authentication flows, or how the app actually behaves under a live server and browser, and it tends to produce more false positives. DAST instead crawls and probes a deployed, running instance the way an attacker would — fuzzing form fields, tampering with headers, and testing for injection through HTTP responses — so it catches real exploitable issues including misconfigurations and environment-specific bugs that static analysis misses. DAST runs later, typically in staging, is language-agnostic since it treats the app as a black box, but it cannot point to a specific line of code and is slower to run than SAST.
- Combining both shifts security left while still validating runtime behavior
- SAST gives fast, line-level feedback directly in the pull request
- DAST catches real exploitable issues static analysis cannot see
- Together they reduce both false positives and blind spots
AI Mentor Explanation
SAST is like a coach reviewing a bowler’s action frame by frame on video before any match is played, spotting a flawed wrist position that could cause a no-ball. DAST is like an umpire watching the bowler actually deliver live balls in a real match, catching problems like overstepping that only show up in a genuine delivery under match pressure. The video review is fast and precise about exactly which frame is wrong, but it cannot catch how the bowler reacts to a hostile crowd. The live umpire check is slower and cannot rewind, but it validates what truly happens on the field.
Step-by-Step Explanation
Step 1
Run SAST in CI
Scan source code on every pull request to catch vulnerable patterns before merge.
Step 2
Triage static findings
Review flagged lines, filter false positives, and fix confirmed issues like injection sinks or hardcoded secrets.
Step 3
Deploy to staging
Ship the build to a running staging environment so it can be tested as a live target.
Step 4
Run DAST against the live app
Crawl and attack the running staging instance to surface runtime and configuration vulnerabilities before production.
What Interviewer Expects
- Clear distinction: static code analysis vs live runtime attack testing
- Understanding of what each approach can and cannot detect
- Awareness that SAST runs early/fast, DAST runs later/black-box
- Knowledge that mature pipelines use both, not one or the other
Common Mistakes
- Treating SAST and DAST as interchangeable or redundant
- Believing SAST can catch runtime misconfigurations
- Forgetting DAST cannot point to an exact vulnerable code line
- Running only one of the two and assuming full security coverage
Best Answer (HR Friendly)
“SAST checks our source code for risky patterns before we ever run the app, so we catch problems early and cheaply, right in the pull request. DAST actually attacks the running application like a real attacker would, so it catches issues that only show up once everything is deployed and live. We use both, because each one catches things the other one simply cannot see.”
Code Example
stages:
- sast
- deploy_staging
- dast
sast_scan:
stage: sast
script:
- semgrep --config=auto --error src/
deploy_staging:
stage: deploy_staging
script:
- ./deploy.sh staging
dast_scan:
stage: dast
script:
- zap-baseline.py -t https://staging.example.com -r dast-report.htmlFollow-up Questions
- What is IAST and how does it relate to SAST and DAST?
- How would you reduce false positives from a SAST tool?
- Why does DAST require a running, deployed application?
- How do SAST and DAST fit into a shift-left security strategy?
MCQ Practice
1. What does SAST analyze?
SAST is a static technique that inspects code artifacts directly without running the application.
2. What is a key limitation of DAST?
DAST treats the application as a black box, so it can confirm a vulnerability exists but cannot map it back to a specific code line.
3. When does SAST typically run in a CI/CD pipeline?
SAST is designed to run early and fast in the pipeline, giving developers feedback before code merges.
Flash Cards
What does SAST test? — Source code, bytecode, or binaries, without running the application.
What does DAST test? — A live, running application, attacked from the outside like a real user or attacker.
Main SAST weakness? — Cannot see runtime configuration or environment-specific issues.
Main DAST weakness? — Cannot point to the exact source code line causing the vulnerability.