security(auth): reject unsupported JWT crit headers

[seonghobae]

Security outcome

Reject every JWT whose JOSE header declares crit, because this verifier implements no critical JWS extensions. The validation occurs before JWKS lookup or signature verification.

Fail-closed boundary

• reject a present crit: null instead of confusing absence with a null value
• reject non-list, empty, non-string-member, and more-than-10-member forms as malformed
• reject well-formed lists of 1–10 strings as unsupported critical extensions
• preserve headers with no crit member and the existing configured asymmetric algorithm allowlist
• impose the list-size bound before iterating attacker-controlled members

Test-first evidence

• PyJWT contract repair: CI first failed on raw JWK/options types; the verifier now converts the selected JWK to a typed PyJWK, supplies typed claim requirements, and passes leeway through PyJWT's dedicated argument. Existing claim and revocation tests were re-isolated at the JWK boundary.
• RED: the new boundary suite produced 7 expected failures against protected-main behavior
• GREEN: focused critical-header and auth-security suites passed (38 passed)
• exact-head CI, Security Scan, and SAST Semgrep: passed
• mypy for app/auth.py: passed

Exact head: b40f5efc9b0f4ce219d4236f50fb56ee3033482c. Automated review and independent approval remain required. This branch also contains the PyJWT dependency migration and lockfile updates already present on its predecessor head; it contains no database migration or workflow change.

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: ffdf8824-1214-4b18-8a3c-93c07df6f365

📥 Commits

Reviewing files that changed from the base of the PR and between 72afe6d and b40f5ef.

⛔ Files ignored due to path filters (1)

• frontend/package-lock.json is excluded by !**/package-lock.json

📒 Files selected for processing (6)

• .jules/sentinel.md
• backend/app/auth.py
• backend/pyproject.toml
• backend/tests/test_auth_crit_rfc7515.py
• backend/tests/test_auth_security.py
• frontend/package.json

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[seonghobae]

@coderabbitai review

Please review exact head b40f5efc9b0f4ce219d4236f50fb56ee3033482c; CI, Security Scan, and SAST Semgrep have passed on this head.

[coderabbitai]

@seonghobae I will review pull request #789 at exact head b40f5efc9b0f4ce219d4236f50fb56ee3033482c.

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