GHSA-993g-76c3-p5m4: PyJWKClient: missing scheme allowlist enables CVE-2024-21643-class SSRF + token forgery via file://, ftp://, data: schemes

[!NOTE] The library does not directly return non-HTTP(S) URI contents to the attacker; the chained "plant a JWKS to forge tokens" scenario described in the original report requires additional application-layer flaws (attacker write access to a filesystem path, untrusted jku derivation) that this fix does not address. Severity is scored for the scheme-acceptance bug in isolation. Summary PyJWKClient passes its uri argument directly to urllib.request.urlopen() which uses Python stdlib's default OpenerDirector registering HTTPHandler, HTTPSHandler, FTPHandler, FileHandler, and DataHandler. There is currently no documented option to restrict which schemes PyJWKClient will fetch. If an application's jku URL ingestion path accepts attacker-influenced URLs (e.g., from JWT header, configuration file, OAuth flow parameter), the attacker can: 1. Cause PyJWKClient to read arbitrary local files via file:// (SSRF on local filesystem) — the file's contents are passed to json.load. 2. Cause PyJWKClient to attempt FTP / data-URI fetches (broader SSRF surface). 3. Forge tokens that PyJWT verifies as valid — if the attacker can write to any path the JKU URL points at AND influences the URL, they can plant a JWK Set containing their own public key, sign tokens with the matching private key, and jwt.decode() accepts. Affected versions Tested and reproducible on PyJWT 2.11.0 and 2.12.1. Likely all versions back to PyJWKClient introduction. Reproducer (full attack chain — verified empirically) 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