Computer security pros Kaspersky are warning of a rampant virus that is hitting Facebook and MySpace.
The worm is capable of turning victims' computers into zombies on a botnet, says Kaspersky Lab.
And, more worryingly, the problem is quickly spreading because infected machines are propagating the worm by sending messages via the social networking websites to their friends in the network.
The messages look like they contain links to video clips.
However, when you click on the link, they prompt the recipient to download an executable file that purports to be the latest version of Flash Player.
It is, in fact, the worm itself.
When owners of the infected machines next log onto the social networks, their machine automatically sends the malicious messages out to new victims grabbed from the friend list, said Ryan Naraine, security evangelist at Kaspersky.
"We've seen these types of worms before, typically around MySpace", he said.
"People are more trusting of things they receive from a friend."
He added that many people simply don't realise that the file that they are downloading is not a legitimate Flash Player file.
Naraine is urging Facebook and MySpace users to be wary when downloading anything onto their computers, even if it appears to come from a friend.