Security Regression Tests

Rules (for AI agents)

ALWAYS

• Prove the test fails without the fix. Revert the patch, or mutate the guard, and confirm the new test goes red — then restore. A test written after the fix can be vacuously green: wrong route, wrong fixture user, wrong assertion target, a typo in the payload. It will stay green forever and nobody will learn anything from it. This is the one step that distinguishes a guard from a decoration, and it is the one most often skipped.
• Assert both directions: the attack input now yields the secure outcome, and a legitimate input still succeeds. A deny-only test passes equally well against an endpoint that is broken for everyone, so it will hold the line and tell you nothing when someone disables the feature entirely.
• Assert the effect, not only the status code. A 403 proves the request was refused; it does not prove the write did not land somewhere else, that the record was not returned in a different shape, or that a queue message was not emitted. Check the state the attack was trying to reach.
• Seed the fixtures the assertion depends on before writing it. A cross-tenant test needs two principals and two owned resources, deterministically created — if both test users end up in the same tenant, the deny assertion is untestable and passes for the wrong reason.
• Make it deterministic and offline. Freeze the clock for expiry and TTL assertions, stub the HTTP client, dialer or resolver rather than reaching the network, fix random seeds, and avoid sleep-based timing assertions and state shared between tests. Note that a canonical SSRF payload such as http://169.254.169.254/latest/meta-data/ is a live, reachable address on a cloud CI runner, so the naive test both leaves the sandbox and returns a different verdict on a laptop.
• Graduate a live proof-of-concept by keeping its shape and dropping its mechanism. dynamic-verification confirms some classes with a timing delta or an out-of-band callback, and neither survives in CI: replace the time-based payload with an assertion that the query was parameterized, and the callback with a stubbed dialer asserting no outbound request was attempted.
• Put the test in a job that can block the merge. A committed test in a job that is continue-on-error, not a required check, or nightly-only is documentation. cicd-security owns the gate configuration itself.
• Name it so a reviewer sees what it guards — test_idor_orders_cross_tenant_403 — and reference the finding in the test body or the commit, so the next person to see it fail knows what it is protecting rather than what it is blocking.

NEVER

• Close a confirmed finding without a regression test. "Fixed by inspection" rots: the next refactor reintroduces it, and there is nothing to notice.
• Skip, quarantine, xfail, or delete a security test to get a build green. A skipped security test is a removed control that still looks like a present one, and it is indistinguishable in a passing build from one that runs. A failing security test is either a real regression or a wrong test — both need a person, neither needs a decorator.
• Relax an assertion until it passes. Widening an expected status set, dropping the effect check, or accepting either outcome converts the guard into a formality.
• Put a live-target probe in CI as the verification. Attack traffic from a build agent is non-deterministic, sometimes illegal against infrastructure you do not own, and the reason the committed test exists.
• Put a real secret in a fixture to exercise a secret check. Use the documented sentinels that scanners already exclude by name — secret-detection owns them.

KNOWN FALSE POSITIVES

• A regression test necessarily commits attack payloads: ' OR 1=1--, <script>alert(1)</script>, {{7*7}}, ../../etc/passwd, a fake token. The project's own SAST and secret scanners will re-flag them on the next run. That is the test doing its job — exclude the fixture path, do not weaken the payload.
• A refuted candidate never triggered the rule: there is no fix, so there is nothing to pin. This is not an exemption, it is the precondition not being met.
• A finding fixed in configuration rather than code — a base image, a workflow permission, a cloud setting, a dependency bump — is pinned by the scanner gate rather than a unit test, because there is no application behaviour to assert. The gate then needs the same both-directions proof: a fixture that fails it and one that passes, or nobody can tell a working gate from a misconfigured one.
• A test asserting a narrower outcome than the original report — a 404 where the report said 403, because the application hides existence — is correct if that is the intended behaviour. Match the application's contract, not the report's wording.

Context (for humans)

The failure this skill exists to prevent is not the missing test; it is the test that exists and proves nothing. There are three common ways to get one, and the rules above are ordered by how often each occurs: the test never fails because it does not actually reach the vulnerable path, the test only checks the deny direction so it stays green against a completely broken endpoint, and the test checks a status code while the dangerous effect still happens.

The second theme is that a security test's value decays through ordinary maintenance. Nobody deletes a control deliberately; someone adds @pytest.mark.skip to unblock a release, and the skip outlives the release. A build with a skipped security test and a build with a passing one look identical from the outside, which is why the rule against skipping is stated as flatly as it is.

Where a finding is confirmed live rather than by reading code, the proof usually cannot be committed as-is — a timing delta and an out-of-band callback are both properties of a running system. What transfers is the claim being made, not the way it was demonstrated.

References

• references/where-verification-runs.md — live probe versus CI regression test, and why CI must never send attack traffic
• references/test-by-class.md — attack input, secure outcome, the effect to assert and the paired control case, per finding class
• OWASP Web Security Testing Guide.