vBulletin through 5.3.x on Windows allows remote PHP code execution because a require_once call is reachable with an unauthenticated request that can include directory traversal sequences to specify an arbitrary pathname and because ../ traversal is blocked but ..\\ traversal is not blocked. For example an attacker can make an invalid HTTP request containing PHP code and then make an index.php?routestring= request with enough instances of \..\ to reach an Apache HTTP Server log file.