[problem] How did they do it?

[kevcj]

Are you sure its the forums fault? Maybe its a hole in the server allowing them to upload files? Is the server and all of the server services up to date? Meaning, are all of the latest security patches installed on the server? Is apache up to date, is the ftp server up to date, is the database server up to date,,,,.

Have you asked the host to do a security audit? Have you changed the root password and the admin passwords? Do you have brute force protection installed?

If you do not have something to stop a brute force attack on the server level, they can throw tens of thousands of passwords at the server until the finally break the root password.

Is this a shared hosting plan, VPS or a dedicated server?

There are a lot of questions that need to be looked at outside of vbulletin.