SAST vs DAST vs IAST vs RASP is really a question of when in an application's lifecycle each technique looks at your code, and that timing determines what each one can and can't catch. SAST examines source code before it runs; DAST attacks a running application from the outside like a real attacker would; IAST instruments the application to watch its internal behavior while tests exercise it; and RASP sits inside a live production application, blocking attacks in real time. None of the four replaces the others — mature application security programs run several of these in combination because each one closes a gap the others leave open.
What does SAST catch that the others miss?
Static Application Security Testing analyzes source code, bytecode, or binaries without executing the program, tracing data flow through the codebase to find patterns like unsanitized input reaching a database query or a hardcoded credential sitting in a configuration file. Its biggest advantage is timing: SAST can run on every pull request, flagging a SQL injection pattern or an insecure cryptographic call before the code ever merges, which is dramatically cheaper to fix than the same bug caught later. As part of a broader SAST/DAST program, static analysis is usually the first gate in the pipeline precisely because it doesn't need a running environment to produce results.
The tradeoff is that SAST reasons about code structure rather than actual runtime behavior, so it tends to produce more false positives than dynamic approaches — it might flag a pattern that looks dangerous in isolation but is actually neutralized by validation logic elsewhere in the call chain that the tool didn't fully trace. It also can't see issues that only manifest through the interaction of your application with its real runtime environment, like a misconfigured server header or a business logic flaw that only shows up when you actually exercise the running app.
How is DAST different, and what does testing a running app add?
Dynamic Application Security Testing treats the application as a black box, sending crafted requests at a running instance — often a staging or test deployment — and observing the responses for signs of vulnerability, the same way an external attacker with no access to source code would probe a target. Because DAST doesn't need source code at all, it works against applications built in any language or framework, and it naturally catches configuration and deployment issues that only exist once the application is actually running, like a missing security header, a exposed debug endpoint, or an authentication bypass that only manifests through the real HTTP request/response cycle.
The downside mirrors SAST's advantage: DAST runs later in the lifecycle, against a deployed instance, so vulnerabilities surface after code has already merged and typically require more rework to fix than a pre-merge SAST finding. DAST scans also tend to be slower than static analysis and can struggle to reach deeply nested application states without significant configuration — a multi-step checkout flow gated behind authentication, for instance, needs to be explicitly modeled for a DAST scanner to reach the code behind it.
Where does IAST fit between the two?
Interactive Application Security Testing instruments the application itself — typically via an agent running alongside the application in a test environment — so it can observe actual code execution, data flow, and library calls while the application is exercised, whether by automated functional tests, a QA team, or a DAST scanner driving traffic against it. This hybrid position is the whole appeal of IAST RASP-adjacent tooling: because IAST watches real execution rather than either reasoning about static code (like SAST) or only observing external inputs and outputs (like DAST), it tends to produce fewer false positives than SAST while pinpointing the exact vulnerable line of code, something pure DAST usually can't do since it only sees the HTTP-level symptom.
IAST's limitation is coverage: it can only report on code paths that actually get exercised during testing, so an under-tested feature or an unusual input combination that your test suite doesn't happen to trigger stays invisible to IAST just as it would to a human tester who never ran that scenario.
What does RASP add once an application is already in production?
Runtime Application Self-Protection is the odd one out in this comparison because it's not a testing technique at all — it's a runtime defense. A RASP agent embeds in the running production application and monitors its behavior for attack patterns, then actively blocks or alerts on detected exploitation attempts, such as an injection payload reaching a database call or an attempt to load an unexpected file. Comparing RASP vs DAST directly is a bit of an apples-to-oranges question: DAST is a testing technique used pre-production (or against a non-production instance) to find vulnerabilities before release, while RASP is a protective control that operates continuously in production, defending against exploitation of vulnerabilities that may still be present, found or not.
That makes RASP a compensating control rather than a replacement for finding and fixing bugs upstream — it's valuable as a last line of defense, particularly for legacy applications where remediation is slow or third-party code can't easily be patched, but it doesn't reduce the number of vulnerabilities in your codebase, only the odds that they get successfully exploited.
Which combination should a team actually run?
Most mature programs layer these rather than picking one: SAST on every pull request for fast, cheap, pre-merge feedback; DAST against staging or pre-production environments to catch runtime and configuration issues SAST structurally can't see; IAST where the tooling and test coverage support it, for pinpoint-accurate findings during QA cycles; and RASP as a production safety net, especially for internet-facing applications or legacy systems where the first three haven't caught everything. Budget and tooling maturity usually dictate the order teams adopt these in — SAST and DAST first, since they map cleanly onto existing CI/CD and staging environments, with IAST and RASP added as programs mature and specific gaps (slow triage, legacy app protection) become priorities worth solving.
FAQ
Is DAST or SAST better for catching SQL injection?
Both can catch it, but from different angles — SAST flags the code pattern where untrusted input reaches a query before you ever deploy, while DAST confirms the injection is actually exploitable by attacking the running endpoint. Running both gives you pre-merge coverage and a real-world confirmation, which is why many teams don't treat this as an either-or choice.
Does IAST require source code access like SAST does?
Not necessarily — IAST typically instruments the running application (via a language runtime agent) rather than analyzing source directly, so it can work in environments where SAST-style source analysis isn't practical, though having source access helps map findings back to exact code locations.
Can RASP replace the need for SAST or DAST entirely?
No. RASP protects a running application against exploitation of vulnerabilities that already exist; it doesn't find or fix those vulnerabilities in the first place. Removing SAST and DAST from a program and relying solely on RASP leaves you with an unknown and growing number of unpatched bugs, purely dependent on the RASP agent catching every exploitation attempt in real time.
Which of these four should a small team start with if they can only pick one?
SAST paired with DAST is the most common starting combination, since together they cover both pre-merge code-level issues and post-deployment runtime and configuration gaps without requiring the additional instrumentation overhead that IAST and RASP typically involve.