SAST, DAST, and IAST are the three core approaches to application security testing, and they differ in where they look, not just what they look for: SAST analyzes source code without running it, DAST attacks a running application from the outside with no code visibility, and IAST instruments a running application to observe its internals from the inside while it's exercised by tests. Each catches vulnerability classes the others structurally can't, which is why mature AppSec programs run more than one, not because any single approach is incomplete on its own but because they're answering genuinely different questions.
How does SAST actually find vulnerabilities?
Static Application Security Testing parses source code, bytecode, or binaries and traces data flow through the program without ever executing it, flagging patterns like unsanitized input reaching a SQL query, hardcoded credentials, or insecure deserialization. Because it works on source directly, SAST can run as early as a developer's IDE or a pre-merge CI check, catching issues before code is even merged, and it can pinpoint the exact file and line number responsible. Its blind spot is anything that only manifests at runtime — a misconfigured server header, an authentication bypass that depends on how two services interact, or a vulnerability introduced by the deployment environment rather than the code itself. Safeguard's SCA and SAST tooling under SAST/DAST run this analysis directly against pull request diffs so new findings surface before merge, not after a separate scheduled scan.
How does DAST find what SAST can't?
Dynamic Application Security Testing sends real HTTP requests at a running application — crawling it, submitting forms, calling API endpoints — and observes the actual responses, which means it catches configuration and runtime issues invisible to source analysis: missing security headers, session handling flaws, injection points that only become exploitable given the live application's actual routing and validation logic. DAST doesn't need source code access at all, which makes it useful for testing third-party components or services you don't own the code for, but it also means findings come with less precision about root cause — a DAST tool can tell you a parameter is injectable, not which line of code introduced the flaw. It also can't test paths it never discovers, so DAST coverage depends heavily on how well the tool crawls and authenticates into the application.
How does IAST work, and why is it less common than the other two?
Interactive Application Security Testing runs an agent inside the application (via language runtime instrumentation) while functional or QA tests exercise it, giving IAST visibility into both the code path executed and the actual runtime data flowing through it — combining SAST's code-level precision with DAST's confirmation that a flaw is actually reachable and triggered. This hybrid position is IAST's real advantage: because it observes an actual code path being hit with actual tainted data, it tends to produce far fewer false positives than SAST alone. It's less common in practice mainly because it requires runtime instrumentation support for your specific language and framework, and it only finds issues in code paths your test suite actually exercises — a bug in an untested branch stays invisible to IAST regardless of how good the tool is.
Do you need all three, or can you get away with two?
Most teams start with SAST and DAST together, since they cover the two ends of the pipeline (pre-merge code review and pre-deploy runtime testing) with tooling that's well understood and easy to integrate into standard CI/CD flows. IAST is usually a later addition, valuable when a team already has strong automated test coverage to drive it and wants tighter confirmation on findings before they reach a security team's triage queue — without a solid test suite, IAST has little to instrument against and adds cost without much signal. The honest answer for most engineering orgs is that SAST plus DAST, run consistently and wired into CI rather than run occasionally, catches the large majority of what these three approaches together are capable of finding.
How do findings from the three approaches actually relate to each other?
They frequently point at variations of the same underlying weakness from different angles — SAST might flag a data flow pattern in code that DAST later confirms is exploitable through a live request, and IAST might confirm the same flow gets hit during a specific test case. Without a shared findings pipeline, that overlap looks like three separate, unrelated tickets instead of one vulnerability triangulated three ways, which is exactly the kind of duplication a combined SAST/DAST platform is designed to collapse into a single, prioritized finding.
FAQ
Which of SAST, DAST, or IAST should you run first if starting from nothing?
SAST first, since it runs earliest, requires no live environment, and catches the largest volume of clearly fixable code-level issues before anything is deployed.
Does DAST need source code access to run?
No. DAST tests the running application over HTTP the same way an external attacker would, which is exactly why it can also test third-party components you don't have source for.
Is IAST the same thing as RASP?
No, though they share runtime instrumentation. IAST is a testing tool used during development and QA to find vulnerabilities; RASP is a production defense mechanism that blocks or alerts on attacks against a live, deployed application.
Can SAST, DAST, and IAST replace manual penetration testing?
Not entirely. All three are automated and pattern-based; manual testing still catches business-logic and multi-step attack chains that automated tools, regardless of type, are weaker at reasoning through.