Koobface - the computer worm that targets users of the social networking websites such as Facebook, MySpace, Twitter and others - has stopped using Facebook to spread.

It's somewhat ironic that the worm whose name is an anagram of "Facebook" has stopped propagating via that particular social network, but according to FireEye researchers, the last time Koobface tried to infect users was around February 13th and the link offered int he message redirected the victims to a fake YouTube video that required a specific codec to be watched. The offered codec, of course, was a Koobface malware binary.

"Our first impression was that it's just a temporarily move but a continued silence for about two months is not something that can be ignored," says FireEye's Atif Mushtaq. But, he points out that Koobface C&C servers are very much alive. "We observed around 153 live C&Cs during the last 7 days. It's just that Koobface is no longer using Facebook to spread itself."

What brought about this change? Mushtaq speculates that infecting Facebook users turned out to bring too much unwanted attention to the malware and its propagators. He believes that the actions taken by Facebook's security team - blocking malicious URLs, attempts to shutdown the C&Cs - have begun to require too much effort for the criminals behind the worm.

"By not using Facebook as its primary infection vector, Koobface will make Facebook lose interest in it, one less enemy," says Mushtaq. "I have no doubt that the guys behind Koobface are using other channels to spread their creations - like pay per install, exploit kits and most recently torrents."