The URL of the "video" is designed to look like it leads to Google or YouTube. In fact, it leads to a Trojan downloader on a server associated with a .cn (China) domain. If the link is clicked, it installs Trojan software and downloads an image of a jester.
In a blog post, Max Kelly, head of security at Facebook, said that the company is taking steps to remediate the problem.
"[W]e spent most of last night working on a fix for a worm, which was targeting people on Facebook and placing messages on Walls urging users to view a video that pretends to be hosted on a Google or YouTube Web site," Kelly said. "We've identified and blocked the ability to link to the malicious Web sites from anywhere on Facebook. Less than .002% of people on Facebook have been affected, all of whom we notified and suggested steps to remove the malware."
Graham Cluley, senior technology consultant at computer security company Sophos, in a blog post observed that less .002% of Facebook's claimed user base of 90 million means less than 1,800 people. And presumably, more than 900 people were affected or Facebook would have said less than .001%, he suggested.
Facebook didn't immediately respond to a request for comment.
Cluley also questioned whether companies are continuing to block access to Facebook, as if to imply the social networking site isn't safe for a business environment. Last year, a Sophos study of 600 workers found that Facebook was blocked at 50% of workplaces.
At the Black Hat conference in Las Vegas on Thursday, Shawn Moyer, CISO for Agura Digital Security, and Nathan Hamiel, senior consultant for Idea Information Security, demonstrated how easily social networking site mechanics could be abused to conduct social engineering attacks and to spread malware.