GHSA-752w-5fwx-jx9f

Опубликовано: 13 мар. 2026

Источник: github

Github: Прошло ревью

CVSS3: 7.5

Описание

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).

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

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"')

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():

_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

Ссылки

Пакеты

Наименование

PyJWT

pip

Затронутые версииВерсия исправления

<= 2.11.0

2.12.0

EPSS

Процентиль: 1%

0.00009

Низкий

7.5 High

CVSS3

CVE ID

CVE-2026-32597

Дефекты

CWE-345

CWE-863

Связанные уязвимости

CVE-2026-32597

CVSS3: 7.5

ubuntu

14 дней назад

PyJWT is a JSON Web Token implementation in Python. Prior to 2.12.0, 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 vulnerability is fixed in 2.12.0.

CVE-2026-32597

CVSS3: 7.5

redhat

14 дней назад

A missing verification step has been discovered in PyJWT. 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.

CVE-2026-32597

CVSS3: 7.5

nvd

14 дней назад

CVE-2026-32597

CVSS3: 7.5

debian

14 дней назад

PyJWT is a JSON Web Token implementation in Python. Prior to 2.12.0, P ...

EPSS

Процентиль: 1%

0.00009

Низкий

7.5 High

CVSS3

CVE ID

CVE-2026-32597

Дефекты

CWE-345

CWE-863