The remote host is vulnerable to an HTML injection attack.
The version of vBulletin installed on the remote host fails to properly sanitize user-supplied input. Specifically, the 'title' parameter of the calendar.php script can be used by an attacker to inject script code into a public section of the bulletin board. An attacker exploiting this flaw would only need the ability to post data to the calendar.php script. When an unsuspecting user browses the site, the script code would be executed within the client browser.
Upgrade to version 3.6.7 or higher.