jwt-security-review

Imported from securityskills/skills (web-appsec/auth/jwt-security-review/SKILL.md). Install upstream with npx skills add securityskills/skills --skill jwt-security-review. Copyright stays with the author.

JWT Security Review

Review JWT-based authentication end to end.

Review Areas

Algorithm Handling

• Pin the expected algorithm server-side; reject alg: none outright
• RS256→HS256 confusion: if the server verifies HMAC using the public key as the secret, an attacker can forge tokens. Confirm verification uses the correct key type per algorithm
• Watch for jku/x5u header injection where the server fetches keys from an attacker-controllable URL
• kid injection: path traversal or SQL/Command injection in key lookup

Key and Secret Strength

• HS256 secrets: ≥32 bytes of entropy, not from wordlists (jwt-tools crack mode)
• Keys rotated; old keys revoked rather than just unused

Claims Validation

• exp and nbf enforced, clock skew bounded
• iss and aud validated against expected values
• Reject tokens with unexpected/extra privileged claims (e.g., admin: true) — parse then validate allowlist
• Don't trust client-controlled role/uid claims if a server-side lookup exists

Token Lifecycle

• Refresh tokens: rotation with reuse detection; stored hashed server-side
• Revocation path exists for logout/password change/admin force-logout
• Access tokens short-lived (≤15 min); no long-lived bearer tokens
• Tokens not persisted in localStorage if XSS surface exists; prefer httpOnly cookies with CSRF protection
• sub is stable and unambiguous (user ID, not email that can change)

Common Code Smells

jwt.decode(token)            // decode ≠ verify: signature never checked
jwt.verify(token, key)       // no algorithms option pinned
jwt.verify(token, key, { algorithms: ['*'] })

Test Cases

1. Remove the signature; send alg: none token
2. Flip alg to HS256 signed with the public key
3. Expired/missing exp, wrong iss/aud
4. Tampered claims with valid signature (e.g., escalate role)
5. Replay a rotated refresh token — should invalidate the family

Output

Findings with token samples (redacted), affected code paths, and remediation per issue.