GHSA-752W-5FWX-JX9F

Vulnerability from github – Published: 2026-03-13 20:05 – Updated: 2026-03-16 17:07
VLAI?
Summary
PyJWT accepts unknown `crit` header extensions
Details

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

Severity ?
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 2.11.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "PyJWT"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.12.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-32597"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-345",
      "CWE-863"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-13T20:05:04Z",
    "nvd_published_at": "2026-03-13T19:55:09Z",
    "severity": "HIGH"
  },
  "details": "## Summary\n\nPyJWT does not validate the `crit` (Critical) Header Parameter defined in\nRFC 7515 \u00a74.1.11. When a JWS token contains a `crit` array listing\nextensions that PyJWT does not understand, the library accepts the token\ninstead of rejecting it. This violates the **MUST** requirement in the RFC.\n\nThis is the same class of vulnerability as CVE-2025-59420 (Authlib),\nwhich received CVSS 7.5 (HIGH).\n\n---\n\n## RFC Requirement\n\nRFC 7515 \u00a74.1.11:\n\n\u003e The \"crit\" (Critical) Header Parameter indicates that extensions to this\n\u003e specification and/or [JWA] are being used that **MUST** be understood and\n\u003e processed. [...] If any of the listed extension Header Parameters are\n\u003e **not understood and supported** by the recipient, then the **JWS is invalid**.\n\n---\n\n## Proof of Concept\n\n```python\nimport jwt  # PyJWT 2.8.0\nimport hmac, hashlib, base64, json\n\n# Construct token with unknown critical extension\nheader = {\"alg\": \"HS256\", \"crit\": [\"x-custom-policy\"], \"x-custom-policy\": \"require-mfa\"}\npayload = {\"sub\": \"attacker\", \"role\": \"admin\"}\n\ndef b64url(data):\n    return base64.urlsafe_b64encode(data).rstrip(b\"=\").decode()\n\nh = b64url(json.dumps(header, separators=(\",\", \":\")).encode())\np = b64url(json.dumps(payload, separators=(\",\", \":\")).encode())\nsig = b64url(hmac.new(b\"secret\", f\"{h}.{p}\".encode(), hashlib.sha256).digest())\ntoken = f\"{h}.{p}.{sig}\"\n\n# Should REJECT \u2014 x-custom-policy is not understood by PyJWT\ntry:\n    result = jwt.decode(token, \"secret\", algorithms=[\"HS256\"])\n    print(f\"ACCEPTED: {result}\")\n    # Output: ACCEPTED: {\u0027sub\u0027: \u0027attacker\u0027, \u0027role\u0027: \u0027admin\u0027}\nexcept Exception as e:\n    print(f\"REJECTED: {e}\")\n```\n\n**Expected:** `jwt.exceptions.InvalidTokenError: Unsupported critical extension: x-custom-policy`\n**Actual:** Token accepted, payload returned.\n\n### Comparison with RFC-compliant library\n\n```python\n# jwcrypto \u2014 correctly rejects\nfrom jwcrypto import jwt as jw_jwt, jwk\nkey = jwk.JWK(kty=\"oct\", k=b64url(b\"secret\"))\njw_jwt.JWT(jwt=token, key=key, algs=[\"HS256\"])\n# raises: InvalidJWSObject(\u0027Unknown critical header: \"x-custom-policy\"\u0027)\n```\n\n---\n\n## Impact\n\n- **Split-brain verification** in mixed-library deployments (e.g., API\n  gateway using jwcrypto rejects, backend using PyJWT accepts)\n- **Security policy bypass** when `crit` carries enforcement semantics\n  (MFA, token binding, scope restrictions)\n- **Token binding bypass** \u2014 RFC 7800 `cnf` (Proof-of-Possession) can be\n  silently ignored\n- See CVE-2025-59420 for full impact analysis\n\n---\n\n## Suggested Fix\n\nIn `jwt/api_jwt.py`, add validation in `_validate_headers()` or\n`decode()`:\n\n```python\n_SUPPORTED_CRIT = {\"b64\"}  # Add extensions PyJWT actually supports\n\ndef _validate_crit(self, headers: dict) -\u003e None:\n    crit = headers.get(\"crit\")\n    if crit is None:\n        return\n    if not isinstance(crit, list) or len(crit) == 0:\n        raise InvalidTokenError(\"crit must be a non-empty array\")\n    for ext in crit:\n        if ext not in self._SUPPORTED_CRIT:\n            raise InvalidTokenError(f\"Unsupported critical extension: {ext}\")\n        if ext not in headers:\n            raise InvalidTokenError(f\"Critical extension {ext} not in header\")\n```\n\n---\n\n## CWE\n\n- CWE-345: Insufficient Verification of Data Authenticity\n- CWE-863: Incorrect Authorization\n\n## References\n\n- [RFC 7515 \u00a74.1.11](https://www.rfc-editor.org/rfc/rfc7515.html#section-4.1.11)\n- [CVE-2025-59420 \u2014 Authlib crit bypass (CVSS 7.5)](https://osv.dev/vulnerability/GHSA-9ggr-2464-2j32)\n- [RFC 7800 \u2014 Proof-of-Possession Key Semantics](https://www.rfc-editor.org/rfc/rfc7800)",
  "id": "GHSA-752w-5fwx-jx9f",
  "modified": "2026-03-16T17:07:33Z",
  "published": "2026-03-13T20:05:04Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/jpadilla/pyjwt/security/advisories/GHSA-752w-5fwx-jx9f"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-32597"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/jpadilla/pyjwt"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "PyJWT accepts unknown `crit` header extensions"
}


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