agentic-security:add-scan-rule

Adding a new scan rule

When the user wants the scanner to detect a new vulnerability pattern, follow this workflow. Loaded on-demand, not into every session — see scanner/src/sast/CLAUDE.md for the same content baked into the SAST tree.

Decision: where does this rule live?

Shape of the rule	Module pattern
Language-specific (e.g. Python sink, Kotlin force-unwrap)	Add to or create scanner/src/sast/<language>.js
Framework-specific hardening (e.g. Express auth, FastAPI defaults)	scanner/src/sast/<framework>-hardening.js
Cross-cutting vuln class (CSRF, prototype pollution, mass assignment)	New top-level scanner/src/sast/<topic>.js
Posture annotator (mutates existing findings)	scanner/src/posture/ — see scanner/src/posture/CLAUDE.md

If the rule is "X but with light cross-file context," still emit from sast/ and let the cross-file annotators in posture/cross-lang-*.js enrich it. Don't put cross-file logic into the SAST module.

Steps

1. Write the detector. Export a scan<Name>(fileContents, opts) function returning Finding[]. Required fields: id, severity, file, line, vuln, cwe, description, remediation. Set family and parser when you know them; posture/finding-defaults.js backfills, but detector-set wins.

2. Wire into the engine. Open scanner/src/engine.js. Add the import to the existing block (alphabetical) and call it inside runFullScan next to similar rules. Append the results to finalFindings.

3. Add the fixture pair. Create scanner/test/fixtures/<rule-name>/vulnerable/ and scanner/test/fixtures/<rule-name>/clean/. Each holds one small file demonstrating the vulnerable / fixed shape. The vulnerable file must produce a finding; the clean file must not.

4. Add the test. Either extend an existing topical test (e.g. test/python-sinks.test.js) or create test/<rule-name>.test.js modelled on the smallest existing test. Wire the file into the matching scoped script in scanner/package.json (test:sast, test:dataflow, etc.).

5. Verify. Use the scoped script that covers what you touched (npm run test:smoke, npm run test:sast, npm run test:dataflow, …). Always include npm run test:lifecycle — it catches a rule that ships without an engine.js wire-up. The full npm test is the CI gate but is usually overkill for a single new rule.

6. Rebuild the bundle if anything outside src/ will consume it. npm run build. Then npm run smoke to check the CLI path. If bundle smoke disagrees with unit-test smoke, you have an engine.js wiring miss — unit tests run against src/ directly, but the bundle re-binds at build time.

Common pitfalls

• Comments confuse regex detectors. Always go through blankComments() from scanner/src/sast/_comment-strip.js before scanning a file body. Otherwise a vuln pattern inside a // example: block fires.
• Snippet attribution. For multi-line sinks (exec('ping ' + req.body.host, …)), the match offset and the readable sink line can diverge. Use the actual sink expression for the finding's snippet, not lines[regex.lastIndex].
• Severity floor. Don't emit critical without strong evidence. annotateExploitability will lift high to critical when production context warrants. A flood of false criticals drowns real ones.
• Family field. If you can't pick a family, the rule probably covers too much. Split it.
• Bench-shape isolation. If your rule needs answer-key signals to fire correctly (file naming, label markers), it goes under scanner/src/sast/bench-shape/ and stays gated by AGENTIC_SECURITY_BENCH_SHAPE=1. Never make a production rule depend on bench shape.

Verify the wiring

After step 5 the lifecycle test should pass with the new file. If it fails with "exported X has no external call site," you forgot step 2 — go back and add the import + call in engine.js.

If npm run smoke reports the new rule fires in clean/ (false positive on the clean fixture), tighten the regex/AST match. If it doesn't fire in vulnerable/, your match shape is wrong — look at the actual fixture content with the actual detector and trace where it diverges.

When to stop

You're done when:

• Both fixtures behave as expected
• The scoped test passes
• npm run test:lifecycle passes
• npm run build && npm run smoke produces the same finding count via the CLI as the unit test reported