My XenForo forum was hacked today using a PHP shell script. The website is ran on Linux (Ubuntu 18).
I'm not too experienced with backend stuff so I apologize if I use the wrong technical names.
Basically someone has uploaded a PHP script to the root folder of the XenForo website. I use an addon called XenForo Resource Manager to let users share files. I believe this is how it was done but I'm not 100% sure which is why I am asking for help here.
Below are the two files that were uploaded
My original index.php file was overwritten with this one: https://hastebin.com/iqatujedux.xml
There is also this file called Yanishshll.php https://hastebin.com/ejupozarix.php
Any advice/help on securing my linux box would really be appreciated. Thank you!