vBulletin has announced a work around to help secure installations of vBulletin version 4.1+ and 5+. We do not know if this is related to the recent increase in compromised vBulletin sites, however the advice they give is sound and should be followed. Failure to remove the installation directory could allow a malicious visitor to create an additional admin user and take over control of your installation.
A potential exploit vector has been found in the vBulletin 4.1+ and 5+ installation directories. Our developers are investigating this issue at this time. If deemed necessary we will release the necessary patches. In order to prevent this issue on your vBulletin sites, it is recommended that you delete the install directory for your installation. The directories that should be deleted are:
4.X - /install/
5.X - /core/install
After deleting these directories your sites can not be affected by the issues that we’re currently investigating.
vBulletin 3.X and pre-4.1 would not be affected by these issues. However if you want the best security precautions, you can delete your install directory as well.