Static Analysis vs. Dynamic Analysis: What Each Actually Catches

SAST and DAST show up in the same sentence often enough that it's easy to assume they're two flavors of the same thing. They're not - one reads your source code without ever running it, the other attacks a running application without ever seeing its source. They catch almost entirely different classes of problems, which is the actual reason most serious security programs run both instead of picking one.

Static Analysis (SAST) - Reading Code Without Running It

A SAST tool parses your source code (or compiled bytecode) and analyzes it through pattern matching and data-flow tracing, following how a value moves from an input source to a potentially dangerous function without it ever executing. This is how it catches things like a hardcoded API key, string concatenation feeding directly into a SQL query, or user input flowing unsanitized into an eval() call.

The strength of SAST is speed and timing - it can run on every commit or pull request, in seconds, before anything is even deployed, and it points to an exact file and line number. The weakness is that it's reasoning about code in the abstract: it can't know your runtime configuration, can't know if a flagged code path is actually reachable given how the application is really used, and tends to produce a meaningful volume of false positives from patterns that look dangerous in isolation but are safe in context.

Dynamic Analysis (DAST) - Attacking a Running App

A DAST tool treats the application as a black box - it doesn't read a single line of source code. Instead, it runs against a live, deployed (or staged) instance, sending real requests with malicious payloads - SQL injection strings, XSS probes, malformed input - and observes how the application actually responds. This catches an entirely different set of problems: misconfigured security headers, authentication bypasses that only manifest at runtime, session handling flaws, and confirmation that a vulnerability is genuinely exploitable rather than theoretically present in the code.

The tradeoff runs the opposite direction from SAST - DAST needs a running environment to test against, so it's inherently slower and happens later in the development cycle. And because it only sees the application from the outside, it reports a symptom ("this endpoint returned an error consistent with SQL injection") rather than a specific line of code, leaving someone to trace it back to the actual source.

What Each One Misses

• SAST misses: runtime configuration and infrastructure issues, environment-specific misconfigurations, and whether a flagged vulnerable pattern is actually reachable given real application flow rather than theoretically possible in isolated code.
• DAST misses:anything not exercised by its crawl or test scripts - a form or API endpoint it never discovers goes completely untested. It's also blind to logic bugs that don't produce an observable symptom in the response, and reaching deeper, authenticated parts of an application usually requires custom scripting rather than working automatically out of the box.

Where IAST and SCA Fit In

Two related but distinct terms worth not conflating with either of the above: Interactive Application Security Testing (IAST) runs an agent inside the application itself while it's being exercised - by tests or real traffic - combining runtime visibility with code-level reporting, aiming at the gap between SAST and DAST. Software Composition Analysis (SCA) is the specific job of checking your dependencies against known-vulnerability databases - what tools like npm audit, Snyk, and Dependabot do - which is neither reading your own code's logic (SAST) nor attacking a running app (DAST), just cross-referencing package versions against CVE data.

Why Most Teams Need Both

SAST belongs in the pull request loop - fast, automated, catching a class of mistake before it ever reaches a deployed environment. DAST belongs as a periodic, realistic check against what's actually running, catching the runtime and configuration issues that no amount of source code review will ever surface, and confirming that findings are genuinely exploitable rather than theoretical. Neither replaces the other because they're answering different questions - "does this code contain a dangerous pattern" versus "can this running application actually be broken into."

Our repo scanner runs static analysis across your codebase on every scan, while our security scan takes the dynamic, external view against your live site - covering both sides of this without needing to stitch two separate tools together. See our guide to automating code quality audits in CI for where static analysis fits into a real pipeline.