“It has the potential to infect about 30% of Windows systems online, a potential 300 to 350 million PCs,” says Don Jackson, director of threat intelligence in the counter threat unit at SecureWorks. The worm, first identified in November and suspected to have originated in the Ukraine, is quickly ramping up, and while Downadup today is not malicious in the sense of destroying files — its main trick is to block users from accessing antivirus sites to obtain updates to protect against it — the worm is capable of downloading second-stage code for darker purposes. Many experts anticipate that could occur soon.

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Early Friday, the Finnish firm revised its estimate of the number of computers that had fallen victim to the worm, and explained how it came to the figure. “The number of Downadup infections [is] skyrocketing,” Toni Koivunen, an F-Secure researcher, said in an entry to the company’s Security Lab blog . “From an estimated 2.4 million infected machines to over 8.9 million during the last four days. That’s just amazing.”

On Tuesday, Koivunen put the number of infected systems at 2.4 million, then updated the estimate Wednesday to 3.5 million, an increase of 1.1 million in just 24 hours .

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