Secret Scanning vs SAST vs DAST: Which Do You Actually Need?

Quick answer
Secret scanning looks for credentials committed to the repository or exposed in build artifacts. It is the highest-value scanner to add first for an AI-built project.
SAST reads source code without running it, looking for patterns such as injection and missing access checks. It finds issues earlier and produces more false positives.
DAST runs against a deployed application and probes it like an attacker. It finds real runtime behaviour and misses anything not reachable from a request.
They are complementary rather than alternatives: different inputs, different findings, and they do not overlap much.
Add in this order for most small projects: secret scanning to block leaks, dependency scanning for known vulnerable packages, then SAST and DAST as the app grows.

What does each scanner actually look at?

Secret scanning inspects text — repository contents, commit history, and sometimes build logs and container layers — for strings that look like credentials. Its input is the artifact, not the behaviour, and it does one thing. Because leaked credentials are immediately exploitable by anyone who finds them, and because generated code has a known habit of hardcoding keys, this is usually the first scanner worth installing.

SAST takes source code as input and analyses it without executing it, matching patterns and data flows to known flaw shapes: an unvalidated value reaching a query, a route without an authorisation check. It can run on every commit and can point at a specific line. Its weakness is understanding: without running the code it cannot know whether a suspicious path is reachable, so it reports more findings than are real.

DAST takes a running application as input and sends requests, looking for behaviour that indicates a vulnerability. It sees the truth of what the deployed system does, including configuration and framework behaviour that source analysis cannot infer, but only for what it can reach. It cannot see an internal function, a scheduled job, or missing authorisation on an endpoint it never discovered.

Side-by-side comparison

The table is the short version. The row that decides the ordering for a small project is the time to first useful finding.

| Attribute        | Secret scanning        | SAST                    | DAST                     |
|------------------|------------------------|-------------------------|--------------------------|
| Input            | Repo, history, artifacts| Source code            | Running application      |
| Runs             | On commit / in CI      | On commit / in CI       | Against a deployed stage |
| Finds            | Committed credentials  | Flaw patterns, data flow| Runtime behaviour         |
| False positives  | Low                    | High                    | Low                      |
| Needs a deploy   | No                     | No                      | Yes                      |
| Blind spots      | Anything not in text   | Unreachable code, config| Undiscovered endpoints   |
| Fix speed        | Minutes (rotate + purge)| Hours                  | Hours                    |

Which one first, and why

For a small or newly generated project: secret scanning first, because the finding is immediate, unambiguous, and cheap to fix, and because the consequence of missing it is a live credential in public history. Dependency scanning is next and often bundled with the same tooling, since known-vulnerable packages are a similarly concrete finding. SAST comes third, once there is enough code for pattern matching to be worth the noise. DAST is usually last, because it requires a deployed environment and a stable surface to probe.

That ordering reflects the ratio of value to effort, not severity in the abstract. A leaked key and a vulnerable dependency are both real and both found mechanically. SAST and DAST surface issues that also matter, and they demand more setup, more triage, and more judgement about which findings are true. Installing them before the cheaper controls means the noise arrives before the habit of acting on findings.

Getting one of them working beats installing all four

The failure mode is a pipeline with four scanners in report-only mode, a growing findings list nobody reads, and no behaviour change. One scanner that fails the build on a specific class of finding changes behaviour; four that produce a dashboard do not. Start with a single control wired to block, tune it until it is quiet, then add the next.

Two practical rules make that work. Fail the build only on the classes you have triaged and agreed are real, keeping everything else visible without blocking, so the pipeline stays credible. And handle the finding properly rather than suppressing it: a leaked secret must be rotated, not merely deleted from the current commit, because history and clones retain it. The remediation is part of the control, and a scanner without a remediation habit is a report generator.

Where this bites vibecoders
Generated code is likely to contain a hardcoded key at some point, and a generated CI pipeline may install a scanner with no policy attached, producing either noise or nothing. The two changes that matter most are small: a secret scan that runs on the full commit history, not just the working tree, and an agreed short list of findings that actually fail the build. Everything else can come later without much loss.

Where AI coding assistants get this wrong

Installing several scanners in report-only mode and never acting on any finding.
Scanning only the working tree for secrets and missing them in history.
Deleting a leaked secret from the latest commit without rotating it.
Adding SAST with default rules and treating the resulting noise as the security posture.
Running DAST against production instead of a staging environment.
Assuming one scanner covers the others’ findings.

Checklist

Enable secret scanning across the full commit history.
Rotate any credential the scanner finds, then remove it from history.
Add dependency scanning for known-vulnerable packages.
Add SAST and fail the build on a short, triaged rule set.
Add DAST against a staging deployment once the surface is stable.
For every finding class, write down the remediation, not just the detection.

FAQ

Do I need all three scanners?

Eventually, most teams benefit from all of them, because they find different things. You do not need them at once. Secret scanning and dependency scanning give the most concrete findings for the least setup, and starting there builds the habit of acting on results before the noisier tools arrive.

Can SAST replace DAST?

No. SAST reads code and cannot confirm that a suspected path is reachable or that the deployed configuration behaves as the code implies; DAST sees actual runtime behaviour but only for what it can reach. Each misses what the other sees, which is why mature pipelines run both.

Why is secret scanning the first one to add?

Because the finding is unambiguous and the consequence is immediate: a committed credential is usable by anyone who finds the repository, and generated code has a well-documented tendency to hardcode keys. Detection is cheap, the fix is well defined, and missing it is not recoverable by a later scan.

Should scanners block the build?

For a small set of triaged, high-confidence classes, yes — otherwise findings accumulate and nothing changes. Keep the blocking set short so the pipeline stays credible, and review it periodically as the false-positive rate changes. A pipeline that blocks on everything gets disabled, which is worse than one that blocks on nothing.