Sky News UK reports Facebook users are most at risk from a variant of the Koobface virus, which has been disguised as a Festive greetings video, hosted on a YouTube site.
The virus works by posting malicious links on Facebook wall pages, inviting users to click on the Christmas card videos. However, any attempt to play the video turns over control of the PC to the attackers.
The victim then sees a warning message, requiring them to solve a Captcha (Completely Automated Public Turing Test To Tell Computers And Humans Apart) puzzle within three minutes. If the puzzle goes unsolved, the PC freezes up.
Experts warn that rebooting the computer will not help and the only way to end the loop is to solve the puzzle. The victim would then be able to use their PC as normal, but the attacker would still have control
Internet security company Panda Security recommends that users do not reply to or follow links included in unsolicited Facebook messages and users should always carefully check that the URL they are entering is really that of the site they want to access