The SAST and DAST full form is straightforward: SAST stands for Static Application Security Testing, and DAST stands for Dynamic Application Security Testing. Both are ways of finding security defects in software, but they work from opposite ends. SAST reads your source code without running it. DAST attacks your application while it runs, without seeing the source. Understanding that difference is the key to using them well, because neither one replaces the other.
If you have seen these acronyms in a security checklist and wondered what they expand to and why teams run both, this covers it.
SAST: testing from the inside
Static Application Security Testing analyzes an application's source code, bytecode, or binary without executing it. Because it can see the code, it can reason about how data moves through the program. Its signature technique is taint analysis: it follows untrusted input from where it enters (a source, like an HTTP parameter) to where it is used (a sink, like a database query or a shell command), and flags the flow when nothing sanitizes the data along the way.
That inside view gives SAST two big strengths. It runs early, before the application is even deployable, so you can catch issues in a pull request. And it points directly at the offending line, which makes fixes fast. It is strong on injection flaws, hardcoded secrets, unsafe cryptographic calls, and other patterns visible in the code itself.
Its weakness is the mirror image of its strength. Because it never runs the app, it cannot see runtime behavior, configuration, or how components actually interact in production. It also produces false positives: a flow that looks dangerous in isolation may be safe because of a validation the tool could not follow. Tuning SAST to keep signal high is part of the job.
DAST: testing from the outside
Dynamic Application Security Testing takes the attacker's perspective. It interacts with a running application over its real interfaces, sending crafted HTTP requests, malformed inputs, and unexpected sequences, then observes the responses for signs of a vulnerability. It does not need the source code. It only needs a running target.
Because DAST exercises the deployed system, it catches things SAST structurally cannot: server misconfiguration, authentication and session weaknesses, issues that only appear once the app is wired to its real dependencies, and problems in code you did not write, like a misconfigured web server. When DAST reports a finding, it is usually a real, reachable behavior, which means lower false-positive rates for the issues it does surface.
Its limits are also the flip side. DAST runs late, after you have something deployable, so feedback arrives later in the cycle. It only tests the paths it can reach, so coverage depends on how well it can crawl or be driven through the app. And when it finds a problem, it tells you the symptom at the boundary, not the exact line, so remediation takes more digging.
The core difference in one view
The cleanest way to remember it: SAST is white-box testing (it sees inside), DAST is black-box testing (it sees only the surface). SAST asks "is this code written unsafely?" DAST asks "does this running system behave unsafely when I poke it?"
Those are different questions, and they surface different bugs. A stored SQL injection reachable through a validated form might be flagged by SAST from the code and confirmed by DAST from the outside. A TLS misconfiguration or a session cookie missing the Secure flag will show up in DAST and be invisible to SAST. That non-overlap is exactly why mature programs run both rather than picking one.
Where IAST and SCA fit around them
Two neighbors come up constantly, so it is worth placing them. IAST, Interactive Application Security Testing, instruments the running application from inside and watches real execution during functional or DAST-driven tests, blending code-level visibility with runtime confirmation. SCA, Software Composition Analysis, is a different axis entirely: it inventories your open-source dependencies and flags known vulnerable versions. Since most of a modern codebase is third-party code, an SCA tool covers ground that neither SAST nor DAST touches, because those two focus on the code and behavior of the app you wrote.
Using them together in a pipeline
The practical pattern places each tool where its feedback is cheapest to act on. SAST and SCA run early, on every pull request, because they need only code and dependency manifests and they point at specific lines. DAST runs against a deployed build in a staging environment, on merge or on a schedule, because it needs a running target.
A sensible flow looks like this:
commit -> SAST + SCA in CI (fast, per-PR, source + deps)
merge -> deploy to staging
staging -> DAST against the running app (behavior + config)
Do not gate the entire pipeline on raw counts from any single tool. Triage findings by real risk and reachability, and let the tools cover their respective blind spots rather than competing. If you want a deeper walk-through of how these testing types complement each other, the Academy breaks it down with examples.
FAQ
What does SAST stand for?
SAST stands for Static Application Security Testing. It analyzes source code, bytecode, or binaries without running the application, tracing untrusted data to dangerous sinks to find defects like injection and hardcoded secrets.
What does DAST stand for?
DAST stands for Dynamic Application Security Testing. It tests a running application from the outside, sending crafted requests and observing responses to find vulnerabilities such as misconfiguration, authentication weaknesses, and reachable injection flaws.
Should I use SAST or DAST?
Both, if you can. They find different classes of defect and have complementary blind spots. SAST catches code-level issues early; DAST catches runtime and configuration issues in a deployed app. Neither replaces the other.
How are SAST and DAST different from SCA?
SAST and DAST test the application you wrote, from the inside and outside respectively. SCA, Software Composition Analysis, inventories your third-party open-source dependencies and flags known vulnerable versions, which the other two do not do.