Hackers are exploiting social networkers in the run up to Christmas with a new variant of the Koobface virus, says Panda Security.
According to the security firm, Koobface GK posts a link to a Christmas message video hosted on YouTube on Facebook profiles.
Social networkers that attempt to watch the video clip will have a worm installed on their PC.
As soon as the worm is installed, PC users are presented with a Windows warning message that requires them to enter a CAPTCHA (completely automated public turing test to tell computers and humans apart) code.
It then threatens to reboot the computer within three minutes. After the three minutes are up, nothing happens, but the computer is rendered unusable.
Panda Security says every time the CAPTCHA text is entered, the worm registers a new domain where the video will be hosted in order to continue being distributed.
Luis Corrons, technical director of PandaLabs, said: "Social networks have become one of the methods most frequently used by hackers to spread their creations, due to the false sense of security many users have regarding the content published on these networks".
"Users generally trust the messages and content they receive, and consequently hackers get a high level of response through these channels".