I'd look at upgrading Drupal first as theres a few known issues with 5.5
(may or may not be applicable in your situation):
http://www.cvedetails.com/cve/CVE-2008-3742/ (Unrestricted file upload)
http://www.cvedetails.com/cve/CVE-2008-3222/ (Session fixation)
If you own the server then restrict SSH access to the IP you usually manage
the site from, remove root SSH access and restrict down to only the user
you connect as using AllowUsers. Change the default listening port. Put
on fail2ban etc. Restrict all outbound access as a lot of exploits require
outbound access to work. Ideally you'd do this from a managed firewall
instead of IPTables but make use of whatever you have. Remove services
you're not using, especially ones that listen on the outside interface.
Make sure your Apache is patched and look into mod_security. Pretty much
anything you can do to limit the potential attack surface, and therefore
make it more effort than it's worth.
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