SAST vs DAST: what each finds and how to use them together
Two acronyms come up in almost every conversation about application security: SAST and DAST. They are two different ways of finding security problems, and they find different things. Most teams need both, used at different points.
The short version
- SAST (static application security testing) reads your source code without running it, looking for patterns that lead to vulnerabilities.
- DAST (dynamic application security testing) tests the application while it runs, from the outside, the way an attacker would.
One looks at the plans; the other tries the doors.
What SAST is good at
- Finding problems early. It can run on every commit or pull request, before the code is deployed.
- Pointing to the exact line. Developers see where the problem is and can fix it in the same change.
- Covering code that is hard to reach from the outside, such as admin functions or rarely used paths.
- Common coding mistakes: unsafe database queries, unescaped output, hard-coded secrets, weak cryptography.
Where SAST falls short
- False positives. It flags code that looks risky but isn't, and someone has to review each finding.
- No view of the running system. It can't see server configuration, missing security headers, or how services behave together.
- Language and framework coverage varies by tool.
- Business logic. It can't tell that a user shouldn't be able to see another user's order.
What DAST is good at
- Testing what is actually deployed, including configuration, headers, cookies, TLS and the web server.
- Fewer false positives for the issues it does find, because it observes real behaviour.
- Language independence. It doesn't care what the application is written in.
Where DAST falls short
- Late in the process. It needs a running application, usually in a test or staging environment.
- Coverage depends on what it can reach. Pages behind logins, multi-step flows and APIs need setting up, or they get missed.
- No line numbers. It tells you what is wrong, not where in the code.
- Business logic, again. Automated scanners struggle with "this user should not be allowed to do this".
Side by side
| Compared | SAST | DAST |
|---|---|---|
| Looks at | Source code | The running application |
| When | During development | In test or staging, before or after release |
| Finds | Coding flaws, secrets, unsafe patterns | Configuration, runtime and exposure issues |
| Shows where to fix | Yes, file and line | No, the symptom |
| Main weakness | False positives | Coverage behind logins and in complex flows |
What neither does well
Both are automated. Neither reliably finds broken access control, which leads the OWASP Top 10 for web applications, or flaws in business logic, such as applying a discount twice or reading someone else's records by changing an ID in the URL. Those need a person who understands what the application is supposed to do, testing it deliberately. That is manual penetration testing.
How to use them together
A practical setup for a growing team:
- SAST in the pipeline. Run it on every pull request, start with a small set of high-confidence rules, and fail the build only on serious findings. Add secret scanning and a check of your open-source dependencies.
- DAST against staging. Run it on a schedule and before major releases, with logins configured so it reaches the important pages and APIs.
- Manual testing periodically, and before big launches, focused on access control and business logic.
- One list of findings, triaged by severity, with an owner and a date for each fix.
Where to start
If you have neither, start with SAST and dependency scanning in your pipeline: they are cheap to run and catch problems before they ship. Add DAST once you have a stable test environment, and plan a manual test before your next significant release.
Our application security assessment combines automated scanning with manual testing of your web app, mobile app and APIs, and Application security covers setting this up as an ongoing practice.
Sources
Want help implementing this?
Book a free 30-minute call about your business. No obligation.