The WP User Switch plugin for WordPress is vulnerable to authentication bypass in versions up to, and including, 1.0.2. This is due to incorrect authentication checking in the 'wpus_allow_user_to_admin_bar_menu' function with the 'wpus_who_switch' cookie value. This makes it possible for authenticated attackers, with subscriber-level permissions and above, to log in as any existing user on the site, such as an administrator, if they have access to the username.
