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BREW-fastmcp-CVE-2026-114617 — Authlib: ID token can disable its own nonce validation via the nonce_supported claim

Affected: Homebrew: fastmcp

### Summary

`OpenIDMixin.parse_id_token()` reads the `nonce_supported` claim from the ID token it is validating. When that claim is boolean `false`, the expected nonce is set to `None` and nonce validation is skipped for that token.

An ID token that carries this claim is therefore accepted in a session whose nonce does not match. A captured, validly signed ID token belonging to a victim can be replayed into an attacker-controlled session, authenticating the attacker as the victim at the relying party.

The code is identical in `authlib/integrations/base_client/sync_openid.py:82-83` and `authlib/integrations/base_client/async_openid.py:84-85`, so all framework integrations are affected. Fixed versions: none; reproduced on 1.8.0 (commit c529a61a). Preconditions: the ID token must carry a valid signature and pass `iss` and `aud` validation, and the attacker must be able to supply it to `parse_id_token()`. Authorization code flow is not affected. Implicit and hybrid relying parties, and applications calling `parse_id_token()` directly, are affected.

### Details

## Affected code

`authlib/integrations/base_client/sync_openid.py:80-85`, in `parse_id_token()`:

```python claims = claims_cls(token.claims, token.header, claims_options, claims_params) # https://github.com/authlib/authlib/issues/259 if claims.get("nonce_supported") is False: claims.params["nonce"] = None

claims.validate(leeway=leeway) ```

`authlib/integrations/base_client/async_openid.py:82-86` contains th…

CVE-2026-114617 — Authlib: ID token can disable its own nonce validation via the nonce_supported claim

Affected: authlib

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to 1.9.0, OpenIDMixin.parse_id_token() reads the nonce_supported decision from the untrusted ID token being validated and sets the expected nonce to null when that claim is the JSON boolean false. In implicit and hybrid relying parties, or applications that directly accept externally supplied ID tokens, an attacker with a captured validly signed token can bypass the nonce-to-session binding and replay the victim's token into an attacker-controlled session. Plain authorization code flow is not affected, and the token must still pass signature, issuer, and audience validation. OpenIDMixin.parse_id_token() trusts nonce_supported in both authlib/integrations/base_client/sync_openid.py and authlib/integrations/base_client/async_openid.py to disable the expected nonce. This issue is fixed in version 1.9.0.

PYSEC-2026-4357

Affected: PyPI: authlib