Users of social networking websites have been warned to be on their guard after Facebook was targeted by a virus dubbed "Koobface".

The Koobface video virus uses the social network's messaging system to infect computers and then tries to gather sensitive information such as credit card numbers.

It is the latest attack by hackers looking to cash in on the popularity of social networking sites where e-mails and messages supposedly from friends are more likely to be opened, exposing the user to the virus. Facebook has more than 120 million users.

"A few other viruses have tried to use Facebook in similar ways to propagate themselves," Barry Schnitt, a Facebook spokesman, said in an e-mail. He said a "very small percentage of users" had been affected by these viruses.

Koobface video virus spreads by sending notes to friends of someone whose PC has been infected. The messages, with subject headers like, "You look just awesome in this new movie," direct recipients to a website where they are asked to download what it claims is an update of Adobe Systems Flash player.

If they download the software, users end up with an infected computer, which then takes users to contaminated sites when they try to use search engines from Google, Yahoo, MSN and Live.com, according to security experts. Dozens of versions of the virus have been around since August. Users of MySpace were infected users at one point.

"Unfortunately, users are very trusting of messages left by 'friends' on social networking sites. So the likelihood of a user clicking on a link like this is very high," said Alexander Gostev, a security analyst at Kaspersky Lab. "At the beginning of 2008 we predicted that we'd see an increase in cyber-criminals exploiting MySpace, Facebook and similar sites, and we're now seeing evidence of this."

The Koobface variant on MySpace was eliminated after new security measures were put in place. Computer security experts McAfee warned in a blog entry this week that its researchers had discovered that Koobface was making the rounds on Facebook.

Facebook has told members to delete contaminated e-mails and has posted directions at facebook.com/security on how to clean infected computers. Infected users should install one of several available anti-virus programs, and be sure to change their Facebook password.

Richard Larmer, chief executive of RLM Public Relations in New York, said he threw out his PC after it became infected by Koobface, which downloaded malicious software onto his PC. "It was really bad. It destroyed my computer," he said.

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