The pop-up window, used for person-to-person chat, pops up with a message from a 'friend' which links to an innnocent looking website.
Clicking the link instantly infects your PC with the virus, labelled Steckt.Evl by discoverers Trend Micro.
The virus instantly disables and removes anti-virus software, then spreads itself by opening chat windows on the Facebook friends of the user.
'The worm propagates via instant messaging applications and social networking sites,' says security experts Trend Micro which uncovered the new threat.
'The instant message it sends is a link that downloads a copy of itself.'
Like many other new internet threats, the first thing the worm does is disable programs that might detect and destroy it.
It then actually deletes them, before going on to download further software which compromises the infected machine further.
'It terminates processes or services that are mostly related to anti-malware programs,' says Trend Micro.
'It also deletes the files that are related to the processes or services it terminates. It does the said routine to keep itself running on the affected system.'
It connects to a remote website to send and receive information.
Like most similar threats, the goal is to 'control' the PC - both to steal sensitive information such as bank details and to use the 'zombie' machine as part of a network of attackers in large-scale cyber attacks.
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