A Missing Authorization vulnerability was identified in Keycloak Google Identity Provider implementation. The flaw exists in the external access-token exchange code path, which is used when the Token Exchange V1 feature is enabled.
While the standard Google ID-token login flow correctly validates the hd hosted domain claim against the configured hostedDomain setting, the access-token exchange flow skips this validation. When an external access token is provided, Keycloak retrieves the user profile from the Google user-info endpoint but fails to verify that the domain associated with the user matches the restricted domain configured in the Identity Provider settings.
An attacker with a valid Google access token for any domain can exploit this flaw if they have access to a confidential client authorized to perform token exchange. Successful exploitation allows the attacker to bypass domain restrictions, obtain a Keycloak access token, and potentially create or link a brokered identity that should have been restricted.