7
If you are on a shared service - then the problem is probably bigger than the really good suggestions for just locking down XOOPS.
If files were actually modified, then look in your httpd access_log file or equivalent on a windows server. Inside, do a search for any
wget
type commands. Those might shed light on what really happened. You can also look for any redirects to ftp sites.
The real problem on large shared systems is that unless most of the obvious ports are locked down, and any firewall or BFD (brute force detection) system is in place, you are vulnerable. It's not XOOPS - but your server.
Another place to check is your secure log. Find out what attempts to gain access happened. You'll probably see lots of ftp or ssh access against accounts that may or may not exist on your server. Again, not much you can do if on a shared server, but you can forward to the provider for support.
If possible, try to get the provider to install mod_security with the latest rules. This will take care of a lot of the access issues. Along with that, see if you can get them to force SSH only with version 2 DSA keys. These are easy to create and manage, and will at least allow you to lock out any password attempts. You could also try getting them to create a whitelist for ssh.
As you can see, there are many options. But I wouldn't assume off the bat that it's just because of XOOPS php code that you got hacked.
Good luck!