Static analysis reads your code. Dynamic analysis attacks your running app. You need both.
| SAST | DAST | |
|---|---|---|
| Full name | Static Application Security Testing | Dynamic Application Security Testing |
| Analyzes | Source code, bytecode, or binaries | Running application via HTTP |
| When in SDLC | During development (shift left) | After deployment or in staging |
| Access needed | Source code | Just a URL |
| Finds well | Hardcoded secrets, injection sinks, insecure functions | Misconfigurations, auth flaws, runtime behavior |
| Misses | Runtime issues, business logic, auth flows | Code-level flaws it can't reach via HTTP |
| False positives | High — flags code paths that may never execute | Lower — it tests real behavior |
| Speed | Fast — runs in CI in minutes | Slow — crawls and tests every endpoint |
| Examples | Semgrep, CodeQL, Bandit, SonarQube | Burp Suite, OWASP ZAP, Nuclei |
SAST reads code that isn't running. DAST attacks code that is. They fail in opposite directions, which is the entire reason teams run both: SAST sees every path including the ones nobody can reach, and DAST sees only what it can find but knows that what it found is real.
The question is rarely "which is better" — it's "which one fits what my team can act on this quarter". Three things decide it.
Do you have the source, and do you own it? If you're securing an application your team builds, SAST is available to you and belongs in CI. If you're assessing something you only have a URL for — a vendor product, an acquired system, a partner integration — DAST is the only one of the two you can run at all.
How fast is your release cycle? SAST runs in minutes on a diff, so it fits a pull-request gate. A full DAST crawl of a large application takes hours, which means it belongs on a nightly or pre-release schedule, not on every commit. Teams that try to gate merges on DAST usually end up disabling it.
Who triages the output? This is the one that actually kills tool rollouts. SAST's false-positive rate is high — it flags code paths that may never execute with data that may never be attacker-controlled — and a queue nobody triages is worse than no tool, because it trains the team to ignore security alerts. If you have no one to own the queue, start with SAST rules scoped narrowly to high-confidence classes (hardcoded secrets, known-dangerous functions) rather than a full ruleset.
A Django application builds a reporting query by string concatenation inside a management command, and exposes a report endpoint that calls it with a user-supplied date range and a sort column.
SAST flags the concatenation immediately. It sees cursor.execute(f"... ORDER BY {sort}"), recognizes the sink, and reports it on the pull request that introduced it — before it ever ships. What it cannot tell you is whether sort is attacker-reachable, or whether a decorator upstream restricts the endpoint to administrators. That determination is yours.
DAST knows nothing about the code. It crawls, finds the report endpoint, fuzzes the parameters, notices that a quote in sort changes the response, and confirms injection by timing. When it reports, the finding is real and already has a reproduction. But it only got there if the crawler reached the endpoint at all — and if the report is behind a login, a multi-step wizard, or a client-side route the crawler can't follow, it found nothing.
That's the trade in one bug: SAST found it three weeks earlier and couldn't prove it; DAST proved it and might have missed it entirely.
SAST. Semgrep is the usual starting point — rules are readable, the free tier is genuinely usable, and you can write project-specific rules in an afternoon. CodeQL is more powerful for real dataflow analysis and is free for public repositories on GitHub. Language-specific options (Bandit for Python, gosec for Go, Brakeman for Rails) have low setup cost and are a reasonable first step. Commercial platforms mainly buy you triage workflow and compliance reporting.
DAST. OWASP ZAP is the open-source standard and automates well in CI. Burp Suite Professional is the tool most practitioners actually test with, and its scanner is strong, though it's built around an operator rather than a pipeline. Nuclei occupies a different niche — template-driven checks for known issues and misconfigurations, extremely fast, not a general crawler.
Whatever you pick on the DAST side, the single highest-value configuration step is authentication. An unauthenticated scan tests your login page. Most of your attack surface is behind it.
Business logic. Neither tool knows that a user shouldn't apply the same coupon twice, that this API should only return rows for the caller's organization, or that a refund endpoint shouldn't accept a negative amount. Those are the findings that show up in bug bounty reports and pentest deliverables, and no amount of scanner tuning surfaces them — the tool has no model of what your application is supposed to mean.
Access control is the important special case. Broken object-level authorization is consistently the most common serious finding in real applications, and it is invisible to SAST (the code looks fine; the check is simply absent) and mostly invisible to DAST (which has no concept of which records belong to whom). Testing it takes two authenticated sessions and someone deliberately replaying one against the other.
If you build and run your own applications, yes — they find genuinely different bugs, and the overlap is smaller than vendors imply. If you only have a URL, DAST is the one available to you. If you have neither budget nor triage capacity, one narrowly-scoped SAST ruleset in CI beats two tools nobody reads.
SAST, in most cases. It runs on a diff in minutes, it fits a pull-request gate, and it catches issues before they reach an environment. Add DAST against staging once the SAST queue is actually being worked.
It reasons about code paths without knowing which ones execute or which inputs an attacker controls. A dangerous function reached only by an internal admin script looks identical to one reached from a public endpoint. Narrowing rulesets and tuning to your codebase matters more than switching tools.
Yes, but not by crawling — give it an OpenAPI or Postman definition so it knows the endpoints and schemas. Point a crawler at a bare JSON API without a specification and it will find almost nothing.
IAST instruments the running application, so it sees both the request and the code path it took — roughly DAST's confidence with SAST's visibility. It needs an agent in your runtime and language support. See SAST vs DAST vs IAST vs RASP for the fuller comparison.