A worm that's been tricking Facebook users into downloading malicious software since July has resurfaced.

Criminals have released a new variation of the Koobface worm, Facebook said on Friday. Koobface is spreading via Facebook messages that look as if they're videos. Often they say something like "you look funny". When the user clicks to see the video, he is taken to a new website and asked to download special software in order to see the video. That software is malicious.

"Only a very small percentage of Facebook users have been affected and we're working quickly to update our security systems to minimise any further impact, including resetting passwords on infected accounts, removing the spam messages, and coordinating with third parties to remove redirects to malicious content elsewhere on the web," Facebook said. "Users with up-to-date anti-virus software are generally well protected from this and similar viruses."

Facebook would not say exactly how many of its users have been hit with the worm.

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The new variant, first reported by McAfee last Wednesday, uses new techniques to get around filtering software that Facebook is using to block it, said Guillaume Lovet, threat response team manager with Fortinet.

In fact, Koobface is now using one of Facebook's own features against it, Lovet said. The latest variant uses Facebook's ability to redirect web links to drive users to malicious websites, often hosted on Geocities.com, Lovet said.

Facebook can redirect visitors to outside Web sites. For example, the URL http://www.facebook.com/l.php?u=http://www.pcadvisor.co.uk would take a visitor to PC Advisor's website.

Facebook warns users who are being redirected in this fashion that they are leaving Facebook's website, but the feature makes it easier for the worm to avoid filtering software, Lovet said.

Once installed, the worm installs a Trojan horse downloading program and keylogging software, he said.

"Users should always have up-to-date anti-virus and not click on links in messages that seem strange," Facebook advises.