CVE-2026-32597: PyJWT accepts unknown `crit` header extensions (RFC 7515 §4.1.11 MUST violation)

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

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.urlsafeb64encode(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 jwjwt, jwk key = jwk.JWK(kty="oct", k=b64url(b"secret")) jwjwt.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/apijwt.py, add validation in validateheaders() or decode():

python SUPPORTEDCRIT = {"b64"} # Add extensions PyJWT actually supports

def validatecrit(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.SUPPORTEDCRIT: 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