The worms had propagated through several banner advertising companies, including Falk in Germany, which provides banner ads to a number of news and IT websites throughout Europe.
The worms affect all Windows platforms except XP Service Pack 2. Microsoft said it would shortly release a patch for other operating systems.
The worms use buffer overflow style attacks targeting Internet Explorer browsers, a common method of delivering for spyware programs, said enterprise security firm NetSec, which identified eight new worms used in attacks.
The flaw can allow hackers to redirect vulnerable browsers to external sites and download malicious code, said NetSec. It recommended that users perform a full system scan as soon as possible.
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