jwt.decode() accepts unsupported critical header extensions #406

Summary

On python-jose commit 018b310ddb8b50dcfd09a0c152117835a21dd656,
jose.jwt.decode() accepts a valid HS256 JWT whose protected header declares
an unsupported critical extension:

{
  "alg": "HS256",
  "typ": "JWT",
  "crit": ["x-custom-policy"],
  "x-custom-policy": "require-mfa"
}

The same replay accepts the matched no-crit control token and returns the
expected claims. The witness token also verifies successfully and returns the
same claims instead of failing closed on the unsupported critical extension.

As a same-host control, PyJWT 2.12.1 rejects the witness token with
InvalidTokenError: Unsupported critical extension: x-custom-policy.

Environment

• Host OS: Ubuntu 24.04.4 LTS
• Architecture: x86_64
• Toolchain: Python 3.14.2
• Build mode: direct source-tree import via PYTHONPATH; no local source modifications

Affected Commit

• reproduced commit: 018b310ddb8b50dcfd09a0c152117835a21dd656

Reproduction

Prerequisites:

• Git
• Python 3
• python3 -m venv
• internet access to install python-jose runtime dependencies into a fresh virtualenv

Build:

mkdir python-jose-crit-repro
cd python-jose-crit-repro
git clone https://github.com/mpdavis/python-jose.git python-jose
git clone https://github.com/jpadilla/pyjwt.git pyjwt
cd python-jose
git checkout 018b310ddb8b50dcfd09a0c152117835a21dd656
cd ../pyjwt
git checkout dcc27a9d3182a2349c30b160758785c6ce7a6508
cd ..
python3 -m venv .venv
./.venv/bin/pip install cryptography ecdsa rsa pyasn1

Use these exact JWTs:

Control token:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjMiLCJpYXQiOjE3MDAwMDAwMDB9.bY5fWpkENiXrTv6SNLjfocPxGkSD3NSSFTg2SurXt8U

Witness token:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCIsImNyaXQiOlsieC1jdXN0b20tcG9saWN5Il0sIngtY3VzdG9tLXBvbGljeSI6InJlcXVpcmUtbWZhIn0.eyJzdWIiOiIxMjMiLCJpYXQiOjE3MDAwMDAwMDB9.izNblYLSgSQ1aJBq1iCtZ-xvMJd_HFnvWgj6Q_yM5eU

Run:

PYTHONPATH="$PWD/python-jose" ./.venv/bin/python -c 'from jose import jwt; control="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjMiLCJpYXQiOjE3MDAwMDAwMDB9.bY5fWpkENiXrTv6SNLjfocPxGkSD3NSSFTg2SurXt8U"; witness="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCIsImNyaXQiOlsieC1jdXN0b20tcG9saWN5Il0sIngtY3VzdG9tLXBvbGljeSI6InJlcXVpcmUtbWZhIn0.eyJzdWIiOiIxMjMiLCJpYXQiOjE3MDAwMDAwMDB9.izNblYLSgSQ1aJBq1iCtZ-xvMJd_HFnvWgj6Q_yM5eU"; print(jwt.decode(control, "secret", algorithms=["HS256"])); print(jwt.decode(witness, "secret", algorithms=["HS256"]))'
PYTHONPATH="$PWD/pyjwt" ./.venv/bin/python -c 'import jwt; witness="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCIsImNyaXQiOlsieC1jdXN0b20tcG9saWN5Il0sIngtY3VzdG9tLXBvbGljeSI6InJlcXVpcmUtbWZhIn0.eyJzdWIiOiIxMjMiLCJpYXQiOjE3MDAwMDAwMDB9.izNblYLSgSQ1aJBq1iCtZ-xvMJd_HFnvWgj6Q_yM5eU"; print(jwt.__version__); print(jwt.decode(witness, "secret", algorithms=["HS256"]))'

Observed Result

python-jose accepts both tokens and returns verified claims:

{'sub': '123', 'iat': 1700000000}
{'sub': '123', 'iat': 1700000000}

PyJWT rejects the witness token:

2.12.1
Traceback (most recent call last):
...
jwt.exceptions.InvalidTokenError: Unsupported critical extension: x-custom-policy

Controls:

• the no-crit control token still verifies successfully with python-jose
• the witness token differs only by the protected-header crit declaration and
  x-custom-policy value

Why This Looks Like A Real Bug

• The exercised surface is the public jose.jwt.decode() verification API
  documented in the current README.
• The witness token has a valid HS256 signature; the bug is not signature
  breakage but missing must-understand enforcement on the protected header.
• The current jose.jwt / jose.jws verification path never inspects crit
  before returning verified claims.
• A same-host PyJWT control fails closed on the same witness token, which
  strengthens the interpretation that the missing crit check is the relevant
  differential.

Duplicate Check / Prior Art

• I checked the current target history with
  git log --oneline --decorate --grep='crit' --all and found no matching fix
  commit in python-jose.
• I checked public GitHub issues and PRs in mpdavis/python-jose for
  crit, critical extension, x-custom-policy, and
  Unsupported critical extension and did not find an exact prior public
  match for this witness.

Suggested Fix Direction

I do not have a tested patch yet.

• Inspect the protected header after parsing and before returning verified
  claims.
• Reject malformed crit values, such as non-array or non-string entries.
• Reject any declared critical extension that the library does not explicitly
  support.

Scope Note

This report is limited to the fresh current-head witness above: a public JWT
verification path accepts a token whose protected header declares an
unsupported critical extension and still returns verified claims.