[bigredseo]

Along with the usual FTP changes, also check if you have other scripts on your site that are outdated. In particular, scripts that allow users to upload things. We often see people exploited through forums, blogs and gallery scripts that allow users to upload images or files to the site, than then exploit them.

Check for any outdated scripts on your site and patch them. Also, contact your host. If it was a server side exploit, other users could be affected too.