We have been trying to validate a possible 0day in vBulletin. While we do not have definitive proof that one exists at this time, we decided to send out a alert to be on the safe side.
Various forums have taken their sites down until further notice, if you are going to keep your forum online make sure you have great backups.
More information can be found here: http://thehackernews.com/2013/11/vBulletin-hacked-Zero-Day-vulnerability.html?m=1
Ongoing Discussion via WHT:
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