Stefan Esser discovered that Horde, a web application framework providing classes for dealing with preferences, compression, browser detection, connection tracking, MIME, and more, is insufficiently validating and escaping user provided input. The Horde_Form_Type_image form element allows to reuse a temporary filename on reuploads which are stored in a hidden HTML field and then trusted without prior validation. An attacker can use this to overwrite arbitrary files on the system or to upload PHP code and thus execute arbitrary code with the rights of the webserver.