Improper Authentication vulnerability in team-alembic AshAuthentication allows an attacker to be signed in as another user by linking an OAuth2 identity to an account that is not theirs.

AshAuthentication.Strategy.OAuth2.UserResolver.resolve/3 matches an existing account using the register action's upsertidentity keys, then gates linking the incoming provider identity to it on emailtrusted?/2, which reads only the provider's emailverified boolean and never compares the provider's email value with the matched account's email. That gate assumes the account was matched by its email field, so under any other upsertidentity it is vacuous and an attacker presenting their own verified email is attached to, and issued a session for, an account matched on some other attribute. The same unguarded gate applies in OAuth2.SignInPreparation on the registrationenabled? false path, where the account is matched by the sign-in action's read filter instead. The upsert also rewrites the matched account's email to the attacker's address, so later account recovery reaches the attacker rather than the owner.

This issue affects ashauthentication: from 4.14.0 before 4.15.0 and from 5.0.0-rc.10 before 5.0.0-rc.14.