PandaLabs, the research arm of anti-malware company Panda Security, says that a variant of the Koobface worm, Koobface.GK, is being spread through the posting of malicious links of Facebook wall pages.
The links take Facebook users to a fake embedded video player that offers a Christmas greeting, in keeping with the tendency of cybercriminals to try to exploit current or seasonal events.
Attempting to play the video or to click on the links on the page leads to an infection attempt, which will compromise the victim's computer if successful.
Once installed, the malware presents a CAPTCHA image that threatens to reboot the computer within three minutes.
Solving the CAPTCHA puzzle at the worm's behest creates new Facebook accounts to help the worm spread.
PandaLabs says that while the infected computer does not shut down if the victim does not solve the CAPTCHA image within three minutes, it is rendered inoperable.
The computer's operating system isn't damaged so badly that it requires a clean system installation, but Sean-Paul Correll, threat researcher at PandaLabs, says that the typical user will not be able to escape the CAPTCHA screen presented by the malware.
Luis Corrons, technical director of PandaLabs, says that social networks have become a popular attack vector because users tend to trust messages that appear to come from friends.
Cisco's 2000 Annual Security Report also sees risk in social networks and notes that the Koobface worm has affected approximately 3 million computers.
"New attacks rely on social media users' willingness to respond to messages that supposedly originate from people they know and trust," the report says. "It is easier -- and often, more lucrative -- to fool a social media user in order to launch an attack or exploit or steal personal information."
To prove that point, Sophos, another security firm, set up two fake Facebook accounts to see how much personal information users would reveal upon receiving a friend request from an unknown person. After sending 200 friend requests from the two fake accounts, Sophos won 95 "friends" who revealed personal information about themselves and about others they knew.
The impact of attacks that arrive through social networks affects businesses as well as consumers. Cisco says that as much as 2% of Web traffic on corporate networks comes from social media sites.
Correll says that business users should worry about attacks through social networks, just like consumers.