The Single-user-chat plugin for WordPress is vulnerable to unauthorized modification of data that can lead to a denial of service due to insufficient validation on the 'single_user_chat_update_login' function in all versions up to, and including, 0.5. This makes it possible for authenticated attackers, with subscriber-level access and above, to update option values to 'login' on the WordPress site. This may be leveraged to update an option that would create an error on the site and deny service to legitimate users or be used to set some values to true such as registration.