Authlib: ID token can disable its own nonce validation via the nonce_supported claim
- Published
- Last modified
- Status
- published
- Known exploitation
- Not listed in CISA KEV
- Public exploit
- None on Exploit-DB
Summary
Authlib, a Python library for building OAuth and OpenID Connect clients and servers, is affected, specifically its OpenID token validation logic in relying party applications. An attacker holding a captured, validly signed ID token could replay it into their own session and bypass nonce checks, which weakens protection against token replay in implicit, hybrid, and externally-supplied-token scenarios.
Summarized by anthropic/claude-haiku-5.5. Verify details against the original sources.
Read the original description
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.
Affected versions
- authlib authlib
fixed in 1.9.0