Authlib's OpenIDImplicitGrant checks the client‑controlled raw scope before validating the allowed scope, allowing a malicious client to append the openid scope to the request. The server then issues an ID token containing the user’s subject and any profile claims the client is permitted to see. This flaw effectively grants the client access to user identity information it is not authorized to obtain.

Authlib library, any deployment using Authlib version prior to 1.9.0, specifically the OpenID implicit grant implementation.

The CVSS base score is 4.6, indicating moderate impact. EPSS is not available, so the likelihood of exploitation is uncertain but the vulnerability is not listed in CISA KEV. The attack requires control over a client application that uses the implicit grant flow to add the openid scope; no privileged access or additional conditions are needed. Once exploited, the attacker can acquire an ID token that includes user identity and profile data beyond what the client should be able to access.