SAST vs DAST: Static and Dynamic Analysis
Same goal, opposite vantage points
SAST and DAST are the two workhorse categories of automated application security testing, and the acronyms hide a simple difference in vantage point.
SAST (static application security testing) analyzes your source code, bytecode, or binaries without running the program. Think Semgrep, CodeQL, SonarQube’s security rules, Checkmarx. It parses your code into an abstract syntax tree, traces how data flows from sources (user input) to sinks (queries, shell commands, HTML output), and flags paths that match known-vulnerable patterns.
DAST (dynamic application security testing) probes your running application from the outside with no knowledge of the source. Think OWASP ZAP, Burp Suite’s scanner, StackHawk. It crawls the app, throws crafted payloads at every input it finds, and watches responses for signs of injection, reflection, or leakage.
White box versus black box. Reading the blueprints versus rattling the doors. Both are automation, which means both are fast, repeatable, and blind in ways worth understanding before you trust either one. The glossary entries on SAST and DAST cover the definitions; this page covers the decision.
Side by side
| Dimension | SAST | DAST |
|---|---|---|
| Analyzes | Source code, without executing it | The running application, without seeing source |
| Pipeline stage | Early: on every commit or pull request | Later: against a deployed staging or test environment |
| Typical finds | Injection-prone code paths, hardcoded secrets, weak crypto usage, tainted data flows, unsafe deserialization | Injection confirmed via response, XSS, misconfigured headers and TLS, server misconfiguration, auth weaknesses at the surface |
| Findings point to | Exact file and line | URL and parameter, no line number |
| False positive rate | High: flags reachable-in-theory paths that are guarded in practice | Lower per finding, but coverage gaps mean more false negatives |
| Language dependence | Needs a parser per language; coverage varies by stack | Language-agnostic: only sees HTTP |
| Catches deployment issues | No: config, TLS, headers, and infrastructure are out of scope | Yes: it tests what is actually deployed |
| Speed | Seconds to minutes per scan | Minutes to hours per crawl and attack cycle |
Where SAST earns its keep
The strongest argument for SAST is feedback latency. A finding that appears on the pull request that introduced it costs minutes to fix. The same flaw found by DAST in staging costs a context switch; found in production by an attacker, it costs an incident.
SAST also sees code that never renders a URL: background jobs, message consumers, admin tooling, library code. DAST cannot attack an input it never discovers, and crawlers miss plenty.
The weakness is precision. Static analysis reasons about possible execution, not actual execution, so it flags paths that a validation layer or framework guard makes unreachable in practice. Teams that turn on a SAST tool with default rules and no triage process get a wall of noise, mute the tool within a month, and end up with worse security than before because now the alerts are ignored by policy. If you adopt SAST, budget triage time and tune the ruleset. A tuned Semgrep config with twenty rules the team trusts beats an untuned scanner with two thousand.
Where DAST earns its keep
DAST tests the only thing that matters at the end: the deployed system. It catches the entire class of problems that never appear in source code, including missing security headers, verbose error pages, weak TLS configuration, and framework debug modes left on in an environment.
Because it works over HTTP, one DAST setup covers a polyglot estate. The Rails monolith, the Go microservice, and the PHP thing nobody admits to owning all get the same treatment.
The weakness is coverage and depth. DAST only exercises what it can reach: pages behind complex auth flows, multi-step wizards, and single-page apps with heavy client-side routing all resist crawling. And when DAST does find something, it hands you a URL and a payload, not a file and a line. Someone still has to trace it back through the code.
What neither one catches
This is the part vendor comparisons skip. Both SAST and DAST detect pattern-shaped vulnerabilities: code or responses that match known-bad signatures. Neither understands what your application is supposed to do, which means both are structurally blind to:
- Authorization logic flaws. User A fetching user B’s invoice by changing an ID is a perfectly valid request with a perfectly valid response. No pattern matches. This is why auditing authentication and authorization is fundamentally human work.
- Business logic abuse. Negative quantities in a cart, replayed coupons, race conditions in a checkout. The code is syntactically fine and the responses look normal.
- Design-level problems. Trust boundaries in the wrong place, secrets management strategy, multi-tenant isolation decisions.
In our experience, the highest-severity findings in real codebases are exactly these logic-shaped flaws, and they are found by people reading code with intent. The tooling handles volume; humans handle judgment. We wrote up the division of labor in manual vs automated code audits.
The verdict
This is not a versus decision in the way vendors frame it. SAST and DAST have disjoint blind spots, both are cheap relative to an incident, and a serious pipeline runs both.
Best for SAST: teams that own their source and want fast, line-level feedback in CI; codebases in well-supported languages; catching flaws before they ship.
Best for DAST: validating what is actually deployed, including config and infrastructure behavior; polyglot estates; teams that cannot integrate per-language tooling.
If you can only adopt one this quarter, start with SAST plus dependency scanning in CI, because feedback on the pull request changes developer behavior in a way a monthly staging scan never will. Then add DAST when you have a stable staging environment to point it at. And schedule a human review for everything the scanners cannot see.
Get a free code audit
Scanners will tell you where your code matches known-bad patterns. They will not tell you whether your authorization model holds up or whether your architecture is sound. For that, get a free code audit: a Webisoft engineer manually reviews your codebase and delivers a written report of what we find. It is genuinely free, and it covers exactly the logic-shaped problems SAST and DAST leave behind.