January 16, 2009 (Computerworld) Calling the scope of the attack "amazing," security researchers at F-Secure Corp. today said that 6.5 million Windows PCs have been infected by the "Downadup" worm in the last four days, and that nearly 9 million have been compromised in just over two weeks.

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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