OpenProject is an open-source, web-based project management software. For OpenProject versions from 11.2.1 to before 16.6.2, when sending a POST request to the /account/change_password endpoint with an arbitrary User ID as the password_change_user_id parameter, the resulting error page would show the username for the requested user. Since this endpoint is intended to be called without being authenticated, this allows to enumerate the user names of all accounts registered in an OpenProject instance. This issue has been patched in version 16.6.2.