GHSA-752w-5fwx-jx9f PyJWT accepts unknown `crit` header extensions

• PyPI PyJWT: < 2.12.0 → fixed in 2.12.0

Summary PyJWT does not validate the `crit` (Critical) Header Parameter defined in RFC 7515 §4.1.11. When a JWS token contains a `crit` array listing extensions that PyJWT does not understand, the library accepts the token instead of rejecting it. This violates the MUST requirement in the RFC. This is the same class of vulnerability as CVE-2025-59420 (Authlib), which received CVSS 7.5 (HIGH). --- RFC Requirement RFC 7515 §4.1.11: > The "crit" (Critical) Header Parameter indicates that extensions to this > specification and/or [JWA] are being used that MUST be understood and > processed. [...] If any of the listed extension Header Parameters are > not understood and supported by the recipient, then the JWS is invalid. --- Proof of Concept ```python import jwt # PyJWT 2.8.0 import hmac, hashlib, base64, json Construct token with unknown critical extension header = {"alg": "HS256", "crit": ["x-custom-policy"], "x-custom-policy": "require-mfa"} payload = {"sub": "attacker", "role": "admin"} def b64url(data): return base64.urlsafe_b64encode(data).rstrip(b"=").decode() h = b64url(json.dumps(header, separators=(",", ":")).encode()) p = b64url(json.dumps(payload, separators=(",", ":")).encode()) sig = b64url(hmac.new(b"secret", f"{h}.{p}".encode(), hashlib.sha256).digest()) token = f"{h}.{p}.{sig}" Should REJECT — x-custom-policy is not understood by PyJWT try: result = jwt.decode(token, "secret", algorithms=["HS256"]) print(f"ACCEPTED: {result}") # Output: ACCEPTED: {'sub': 'attacker', 'role': 'admin'} except Exception as e: print(f"REJECTED: {e}") ``` Expected: `jwt.exceptions.InvalidTokenError: Unsupported critical extension: x-custom-policy` Actual: Token accepted, payload returned. Comparison with RFC-compliant library ```python jwcrypto — correctly rejects from jwcrypto import jwt as jw_jwt, jwk key = jwk.JWK(kty="oct", k=b64url(b"secret")) jw_jwt.JWT(jwt=token, key=key, algs=["HS256"]) raises: InvalidJWSObject('Unknown critical header: "x-custom-policy"') ``` --- Impact • Split-brain verification in mixed-library deployments (e.g., API gateway using jwcrypto rejects, backend using PyJWT accepts) • Security policy bypass when `crit` carries enforcement semantics (MFA, token binding, scope restrictions) • Token binding bypass — RFC 7800 `cnf` (Proof-of-Possession) can be silently ignored • See CVE-2025-59420 for full impact analysis --- Suggested Fix In `jwt/api_jwt.py`, add validation in `_validate_headers()` or `decode()`: ```python _SUPPORTED_CRIT = {"b64"} # Add extensions PyJWT actually supports def _validate_crit(self, headers: dict) -> None: crit = headers.get("crit") if crit is None: return if not isinstance(crit, list) or len(crit) == 0: raise InvalidTokenError("crit must be a non-empty array") for ext in crit: if ext not in self._SUPPORTED_CRIT: raise InvalidTokenError(f"Unsupported critical extension: {ext}") if ext not in headers: raise InvalidTokenError(f"Critical extension {ext} not in header") ``` --- CWE • CWE-345: Insufficient Verification of Data Authenticity • CWE-863: Incorrect Authorization References • RFC 7515 §4.1.11 • CVE-2025-59420 — Authlib crit bypass (CVSS 7.5) • RFC 7800 — Proof-of-Possession Key Semantics