An undocumented behaviour in all Windows versions of Internet Explorer has rendered vBulletin vulnerable to a potential cross-site scripting flaw (XSS).
Therefore, Jelsoft have decided to put out a preventative security releases in order to work-around the Internet Explorer problem before it is exploited.
The official announcement threads for these releases can be found here.
vB 3.6.3
vB 3.5.6
vB 3.0.16
vB 2.3.11
In addition, vB 3.6.3 also includes fixes for approximately 50 bugs that were discovered in 3.6.2. For this reason, Jelsoft recommend all customers upgrade to 3.6.3 as soon as possible.