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CVE-2026-114618 — Authlib: Default JWT bearer grant accepts assertions for the wrong audience

Affected: authlib

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to 1.9.0, the default RFC 7523 JWTBearerGrant.get_audiences() returns an empty list, causing JWTBearerGrant.verify_claims() to require only that an aud claim exists without matching the current authorization server. An attacker holding a legitimate assertion for another service can present it to a server that shares the signing key and uses the default grant, obtaining an access token across a trust boundary. Implementations that override get_audiences() with the server's valid audience identifiers are not affected. This issue is fixed in version 1.9.0.

GHSA-fch4-3c6x-j4gc — Authlib: Default JWT bearer grant accepts assertions for the wrong audience

Affected: PyPI: authlib

## Description

### Summary

Authlib's default RFC 7523 JWT bearer grant leaves `get_audiences()` empty, so `verify_claims()` 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 `get_audiences()` returns empty list - `authlib.jose.jwt.decode()` with `claims_options` for audience validation - `authlib.integrations.flask_oauth2.ResourceProtector.validate_request()` 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 (get_audiences() = []) 5. verify_claims() 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 doe…

PYSEC-2026-4358

Affected: PyPI: authlib