Last week Naked Security warned of a Facebook worm that was spreading on the social network, tricking users into believing that they were clicking on a link to an image.

The bad news is that the attack appears to still appears to be spreading via Facebook's chat system, exploiting compromised users' accounts.

An analysis by SophosLabs has identified that malware designed to install the Dorkbot worm onto users' computers is being spread via Facebook chat. And, for now at least, Facebook's built-in security systems are not preventing it.