SAST vs DAST: Which Security Test Should You Use, and When?
## The short answer
Use **SAST** (static application security testing) early, while you're writing code, to catch insecure patterns directly in the source. Use **DAST** (dynamic application security testing) later, against a running version of the app, to find flaws that only appear at runtime. They are complementary, not competing: SAST tells you *where* in the code a problem lives; DAST tells you *whether* an attacker can actually exploit it. Mature teams run both.
## What SAST actually does
Static analysis inspects your source code, bytecode or binaries without executing them. It parses the code, builds a model of how data flows through it, and flags risky patterns such as untrusted input reaching a database query, hard-coded credentials, or weak cryptographic calls.
Its strengths:
- **Runs early.** It can sit in your IDE or build pipeline and give feedback before anything is deployed.
- **Pinpoints location.** Findings come with a file and line number, which makes fixing fast.
- **Full coverage of code paths.** It can examine branches that rarely execute at runtime.
Its weaknesses:
- **False positives.** It flags patterns that look dangerous but may be safe in context, so findings need triage.
- **No runtime view.** It cannot tell whether a flaw is reachable or exploitable once the app is deployed and configured.
- **Language-specific.** A scanner must understand the language and framework you use.
## What DAST actually does
Dynamic analysis treats the running application as a black box. It sends crafted requests, manipulates inputs and observes responses, much as an external attacker would. It needs no source code.
Its strengths:
- **Tests the real, deployed system,** including its configuration, server and runtime environment.
- **Few language constraints.** It interacts over HTTP, so it doesn't care what the app is written in.
- **Lower false-positive rate for certain classes,** because it confirms a behaviour rather than inferring one.
Its weaknesses:
- **Runs late.** You need a deployed, working app, so feedback comes later in the cycle.
- **Limited code visibility.** It can tell you a page is vulnerable but not which line of code is responsible.
- **Coverage gaps.** It only tests the paths it can reach and exercise; hidden features may go untested.
## A side-by-side comparison
| Dimension | SAST | DAST |
|---|---|---|
| Needs source code | Yes | No |
| Runs against | Code at rest | Running app |
| Stage in lifecycle | Early | Later |
| Pinpoints code location | Yes | No |
| Sees configuration flaws | No | Yes |
| Typical false positives | Higher | Lower |
## When to use each
Think in terms of where the cost of a missed flaw is lowest:
1. **During development:** SAST in the IDE and on every pull request. Developers fix issues while the code is fresh in their minds.
2. **In the build pipeline:** SAST as a gate, plus dependency scanning, to stop known-bad patterns merging.
3. **Against staging:** DAST to test the assembled, configured system before it reaches production.
4. **Before major releases:** both, followed by manual penetration testing for business-logic flaws neither tool catches.
Neither tool finds logic flaws well, for example a checkout process that lets you apply a discount twice. That still needs human testing.
## What about IAST?
Interactive application security testing instruments the running application from the inside, so it watches code execute during your normal functional tests. It blends SAST's code-level visibility with DAST's runtime accuracy, and it can reduce false positives. The trade-off is added complexity and the need to instrument your runtime. For many teams it is a useful third layer once SAST and DAST are established, rather than a starting point.
## Avoiding the common mistakes
- **Don't treat scanner output as a to-do list.** Triage by severity and exploitability first; a wall of low-priority findings causes teams to ignore the tool entirely.
- **Don't run SAST only at release.** Its whole value is early feedback.
- **Don't assume DAST covers everything it can reach.** Provide it with authenticated sessions and good route coverage, or it tests only the front door.
- **Don't skip manual testing** because the scanners are green.
Building enterprise-grade products means layering these techniques rather than betting on one. At neart.ai we treat SAST, DAST and human review as parts of one pipeline.
## Practical takeaway
SAST and DAST answer different questions. SAST reads your code early and tells you exactly where a risky pattern lives; DAST attacks the running app and tells you whether a flaw is actually reachable. Run SAST on every change for fast feedback, run DAST against staging before release, add IAST as a third layer when you're ready, and reserve manual penetration testing for the logic flaws no scanner can find. Use both, at the right stage, and triage ruthlessly by severity.