Admidio is a free, open source user management system for websites of organizations and groups. In Admidio before version 4.0.4, there is an authenticated RCE via .phar file upload. A php web shell can be uploaded via the Documents & Files upload feature. Someone with upload permissions could rename the php shell with a .phar extension, visit the file, triggering the payload for a reverse/bind shell. This can be mitigated by excluding a .phar file extension to be uploaded (like you did with .php .phtml .php5 etc). The vulnerability is patched in version 4.0.4.