The JAY Login & Register plugin for WordPress is vulnerable to authentication bypass in versions up to, and including, 2.4.01. This is due to incorrect authentication checking in the 'jay_login_register_process_switch_back' function with the 'jay_login_register_process_switch_back' cookie value. 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 user id.