PRTG Network Monitor before 18.2.40.1683 allows remote unauthenticated attackers to create users with read-write privileges (including administrator). A remote unauthenticated user can craft an HTTP request and override attributes of the \'include\' directive in /public/login.htm and perform a Local File Inclusion attack, by including /api/addusers and executing it. By providing the \'id\' and \'users\' parameters, an unauthenticated attacker can create a user with read-write privileges (including administrator).