To quote:
The exploit allows an attacker to upload and execute arbitrary code.
Typically, in cases like this the hackers upload a PHP script that lets them execute unix commands in the browser.
There's still the possibility that they used a weakness in some other software on your server, but that was the first thing that came to mind.
To make sure, check your webserver's logfiles for requests directly accessing the file manager. Also check the file manager's directories for any suspicious files, as explained in the above article.
bye, Dirk