The Sign In With Google plugin for WordPress is vulnerable to authentication bypass in all versions up to, and including, 1.8.0. This is due to the 'authenticate_user' user function not implementing sufficient null value checks when setting the access token and user information. This makes it possible for unauthenticated attackers to log in as the first user who has signed in using Google OAuth, which could be the site administrator.
When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.