The Really Simple Security (Free, Pro, and Pro Multisite) plugins for WordPress are vulnerable to authentication bypass in versions 9.0.0 to 9.1.1.1. This is due to improper user check error handling in the two-factor REST API actions with the 'check_login_and_get_user' function. This makes it possible for unauthenticated attackers to log in as any existing user on the site, such as an administrator, when the "Two-Factor Authentication" setting is enabled (disabled by default).