DAST, SAST, IAST, and SCA each test a different layer of an application, and none of them alone covers the full risk surface. The confusion that trips up a lot of teams building their first appsec program isn't which one is "best," it's that the four are frequently discussed as competing choices when they're actually complementary layers that catch different bug classes at different points in the pipeline. This question shows up in search as "IAST DAST SAST" or "SAST IAST DAST," as if there's one correct sequence, but in a working program these four run in parallel across different pipeline stages rather than as a single linear order.
What does each acronym actually test?
SAST (static application security testing) analyzes source code without running it, looking for patterns like SQL injection, hardcoded secrets, and unsafe deserialization by walking the code's structure and data flow. It runs early, in the IDE or at commit time, and it doesn't need a running application.
DAST (dynamic application security testing) tests a running application from the outside, sending crafted HTTP requests the way an attacker would and observing the responses. It catches things SAST can't see, misconfigurations, authentication bypasses, issues that only exist in the deployed runtime, but it requires a deployed target and generally runs later in the pipeline.
IAST (interactive application security testing) sits inside the running application, usually via an instrumentation agent, and observes actual code execution as real or simulated traffic flows through it. It combines some of SAST's code-level visibility with DAST's runtime accuracy, tracing whether a specific line of code was actually exercised by a request that also triggered a vulnerable condition.
SCA (software composition analysis) doesn't test your code at all, it inventories your open-source dependencies and cross-references them against known vulnerability databases, flagging when you're running a package version with a disclosed CVE.
Where do they overlap, and where are the gaps?
The overlap is smaller than people assume. SAST and DAST both can theoretically detect SQL injection, but SAST finds it by tracing tainted data through code paths, which means it can flag issues before the vulnerable code ever runs, while DAST finds it by actually sending a payload and observing behavior, which means it can only detect what's reachable and triggerable in the deployed configuration it's testing against.
The genuine gaps are the important part. SAST can't tell you about vulnerabilities introduced by misconfiguration, an exposed admin endpoint, a missing security header, weak TLS settings, because those aren't visible in source code in a way a static analyzer reasons about. DAST can't see into your dependency tree at all; it has no idea a library three levels deep in your package.json has a known CVE unless that CVE happens to manifest as an externally observable behavior. And neither SAST nor DAST tells you anything about license risk or the provenance of a package, which is squarely SCA's job.
How does IAST change the picture?
IAST is often pitched as a way to get SAST's precision with DAST's runtime accuracy, and that's a fair description of what it aims for. Because it instruments the running application, it can confirm that a vulnerable code path identified by static analysis is actually reachable and exercised by real traffic, which cuts down on the false positives that plague pure SAST tools flagging code that's technically vulnerable but never executed with attacker-controlled input in practice.
The tradeoff is deployment complexity. IAST needs an agent running inside your application or test environment, which means it only sees what your test traffic, manual or automated, actually exercises. A code path that isn't hit during testing gets no IAST coverage at all, whereas SAST would still have flagged it from the source. It's a complement to SAST and DAST, not a replacement for either.
How should these actually compose in a pipeline?
A reasonable ordering, and the one most mature appsec programs converge on, looks roughly like this: SCA and SAST run on every commit or pull request, because they're fast and don't need a deployed target, catching known-vulnerable dependencies and code-level bugs before merge. IAST, where adopted, runs during integration or QA testing, riding along with the test suite that's already exercising the application. DAST runs against a staging or pre-production environment, closer to release, catching runtime and configuration issues that only show up once the whole system is assembled and running.
The mistake to avoid is treating this as a single tool decision. Teams sometimes ask "should we buy a SAST tool or a DAST tool," as if the answer is exclusive, when the honest answer is that a mature program eventually wants coverage from all four categories, layered at the point in the pipeline where each one is cheapest to run and most likely to catch its specific class of issue. Whichever order you write it in — the IAST DAST pairing during QA, or the full DAST IAST SAST chain read right to left — the pipeline placement above, not the acronym order, is what actually determines cost and coverage. SCA and SAST/DAST tooling that shares a single findings pipeline, so a SQL injection flagged by SAST and confirmed exploitable by DAST show up as one prioritized issue instead of two disconnected tickets, is worth more in practice than four best-in-class point tools that don't talk to each other.
FAQ
Do I need all four to have a real appsec program?
Not on day one. SCA plus SAST covers the highest-volume, cheapest-to-fix issues (known-vulnerable dependencies and code-level bugs) and is a reasonable starting point. DAST and IAST add real value but are worth adding once the first layer is running reliably in CI.
Which one catches business logic flaws, like a broken authorization check?
Realistically, none of them catch business logic flaws reliably on their own. SAST and IAST can sometimes flag missing authorization checks if the pattern is structural, and DAST can catch some via automated testing of role-based access, but subtle logic flaws generally need manual penetration testing or threat modeling to catch consistently.
Is IAST worth adopting if we already have SAST and DAST?
It's worth evaluating if your SAST tool produces a high false-positive rate that's burning reviewer time, since IAST's runtime confirmation is specifically good at filtering "technically vulnerable but never actually reachable" findings out of that queue.
Does SCA replace the need for SAST on open-source code we've forked or modified?
No. Once you've modified a dependency, SCA's CVE-matching against the original package version becomes unreliable, and the forked code should be treated like first-party code and run through SAST.