Firebase Support & Chat Management <= 3.1.1 - Missing Authorization to Authenticated (Subscriber+) Privilege Escalation

The Firebase Support & Chat Management plugin for WordPress is vulnerable to privilege escalation in all versions up to, and including, 3.1.1. This is due to the `firebase_auth()` function authenticating the request as the WordPress user whose email is supplied in the `user_email` POST parameter without verifying ownership of that email (no Firebase ID token signature/issuer/audience verification). This makes it possible for authenticated attackers, with Subscriber-level access and above, to log in as an arbitrary existing user — including an Administrator — by submitting that user's email address to the `acb_firebase_auth` AJAX action, resulting in full account takeover.