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.
The product relies on the existence or values of cookies when performing security-critical operations, but it does not properly ensure that the setting is valid for the associated user.