The Nextend Social Login Pro plugin for WordPress is vulnerable to authentication bypass in versions up to, and including, 3.1.16. This is due to insufficient verification on the user being supplied during the Apple OAuth authenticate request through the plugin. This makes it possible for unauthenticated attackers to log in as any existing user on the site, such as an administrator, if they have access to the email.
A product requires authentication, but the product has an alternate path or channel that does not require authentication.