SAST vs DAST: Semgrep vs OWASP ZAP and Other Tools for Application Security Testing
Use SAST early, use DAST before release, and use both if you like sleeping at night. Semgrep is great at reading your code before it runs. OWASP ZAP is great at poking your app after it runs. They catch different bugs. So picking only one is like wearing a helmet but no shoes.
TLDR: SAST checks source code, while DAST attacks a running app from the outside. For example, a team with 12 APIs might run Semgrep on every pull request in under 3 minutes, then run OWASP ZAP each night and find 6 broken access issues in staging. SAST is best for fast developer feedback. DAST is best for real attack paths, login flows, headers, cookies, and exposed endpoints.
SAST vs DAST in plain English
SAST means static application security testing. It reads code, config files, and sometimes dependencies. It asks, “Could this line cause trouble?”
DAST tests a live app. It sends requests, clicks around, follows links, and tries weird inputs. It asks, “Can I break this thing from the outside?”
Think of a restaurant.
- SAST checks the recipe before cooking.
- DAST tastes the meal and complains loudly.
Both are useful. One checks the plan. One checks the result.
Semgrep: the friendly code detective
Semgrep is a popular SAST tool. It scans code for risky patterns. It works with many languages, including JavaScript, TypeScript, Python, Go, Java, Ruby, PHP, and more.
Its big win is speed. You can run it in a terminal. You can run it in CI. You can run it on a pull request. Developers get feedback before bad code gets merged.
Semgrep is also easy to teach. Rules look like code. That matters. Security teams can write custom checks without building a giant compiler in a cave.
Example rule idea:
- Find hardcoded secrets.
- Find weak hash functions like MD5.
- Find unsafe SQL string building.
- Find missing authorization checks in key handlers.
The catch is… Semgrep does not run your app. It may not know what really happens after routing, middleware, database layers, and user roles get involved. It can warn you about risky code. It cannot always prove the bug is exploitable.
OWASP ZAP: the web app poking machine
OWASP ZAP is a free and open source DAST tool. It tests web apps and APIs by sending traffic to them. It can spider pages, replay requests, scan for issues, and act as a proxy while you use the app.
ZAP looks for things like:
- Missing security headers.
- Cross site scripting.
- Open redirects.
- Cookie flags that are missing.
- Exposed files.
- Some injection bugs.
ZAP shines when your app is deployed to a test environment. It can catch problems that code scanners miss. For example, your code might look fine, but your staging server may expose a debug route. ZAP can spot that. Semgrep may not.
Honestly, it feels like ZAP enjoys finding the one admin panel you forgot existed. Rude. Helpful, though.
Quick comparison
| Question | Semgrep | OWASP ZAP |
|---|---|---|
| Type | SAST | DAST |
| Needs running app? | No | Yes |
| Best time to run | Pull request | Staging or nightly build |
| Best at | Code patterns | Real web behavior |
| Main pain | False alarms | Setup and noisy scans |
Which bugs does each tool catch?
SAST tools like Semgrep are good at catching code smells that often become security bugs.
- SQL injection patterns: string-based query building.
- Secrets: keys, tokens, and passwords in code.
- Bad crypto: weak algorithms or unsafe random values.
- Unsafe functions: risky eval calls or command execution.
- Policy checks: “All admin routes must call this function.”
DAST tools like ZAP are good at catching issues in the working app.
- Bad headers: missing content security policy or frame options.
- Auth mistakes: pages reachable without login.
- Session issues: weak cookie settings.
- Input bugs: reflected script payloads.
- Server leaks: stack traces, debug pages, old paths.
Other SAST tools worth knowing
CodeQL is powerful. It treats code like a database and lets you query it. GitHub supports it well. The downside is that custom queries can feel brain-melty at first.
SonarQube mixes code quality and security checks. It is popular with engineering teams. It is good for dashboards and trends. It may need tuning so developers do not drown in low-priority findings.
Snyk Code focuses on developer-friendly scanning. It fits nicely with dependency scanning too. It is useful when teams want one vendor for several security checks.
Checkmarx and Veracode are common in larger companies. They offer broad language support, reports, and policy controls. They can be heavier to roll out.
Other DAST tools worth knowing
Burp Suite is the classic web security toolkit. The free edition is useful. The professional version is loved by penetration testers. It is strong for manual testing and deep web app work.
Nuclei is fast and template-based. It is great for known exposures, misconfigurations, and quick checks across many targets. It is not a full replacement for ZAP, but it is very handy.
StackHawk is built for CI-style app scanning. It is developer-friendly and works well with APIs. Teams that want DAST inside pipelines often like it.
Acunetix is a commercial scanner. It provides polished reports and broad web scanning features. It is often used by teams that want less setup work.
Where each tool fits in your workflow
A simple setup looks like this:
- Developer writes code.
- Semgrep runs on the pull request.
- Secrets and obvious code bugs get blocked.
- The app deploys to staging.
- OWASP ZAP scans staging at night.
- Serious findings create tickets.
This flow is boring. That is good. Security should be boring most days.
Expect to waste time on tuning at first. A fresh scanner can shout about everything. Some findings are real. Some are “technically true, but please stop yelling.” Create rules. Set severity levels. Ignore junk with care.
API testing changes the game
Modern apps often talk through APIs. ZAP can test APIs if you give it an OpenAPI file, Postman collection, or recorded traffic. This helps it find endpoints that a normal spider may miss.
Semgrep can also help with API code. It can check that every route has authorization. It can check that user input goes through validation. This is where custom rules become fun.
False positives and false negatives
No scanner is magic. Sorry. I also wanted the magic one.
A false positive means the tool reports a bug that is not really a bug. This happens a lot with SAST. The scanner sees risky code but lacks full context.
A false negative means the tool misses a real bug. This happens with both SAST and DAST. ZAP may miss a hidden route. Semgrep may miss a logic flaw spread across ten files.
That is why humans still matter. Tools catch patterns. People understand intent.
Best choice for small teams
If you are a small team, start simple.
- Use Semgrep in pull requests.
- Use OWASP ZAP against staging.
- Add dependency scanning with tools like Dependabot or Snyk.
- Add secret scanning in Git.
- Review critical findings weekly.
Do not buy five tools before fixing the first ten bugs. That is security theater with invoices.
The final verdict
Semgrep and OWASP ZAP are not rivals. They are teammates. Semgrep guards the code door. ZAP shakes the running app until loose parts fall out.
Use Semgrep for fast feedback. Use ZAP for real app checks. Add CodeQL, SonarQube, Burp Suite, Nuclei, or commercial tools when your risk and budget justify it.
The best setup is simple. Scan early. Scan after deploy. Fix the boring stuff. Repeat until attackers have a worse day than you do.
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