January 14, 2009 (Computerworld) The computer worm that exploits a months-old Windows bug has infected more than a million PCs in the past 24 hours, a security company said today.

Early Wednesday, Helsinki, Finland-based security firm F-Secure Corp. estimated that 3.5 million PCs have been compromised by the "Downadup" worm, an increase of more than 1.1 million since Tuesday.

"[And] we still consider this to be a conservative estimate," said Sean Sullivan, a researcher at F-Secure, in an entry to the company's Security Lab blog. Yesterday, F-Secure said the worm had infected an estimated 2.4 million machines.

The worm, which several security companies have described as surging dramatically during the past few days, exploits a bug in the Windows Server service used by all supported versions of Microsoft Corp.'s operating system, including Windows 2000, XP, Vista, Server 2003 and Server 2008.

Microsoft issued an emergency patch in late October, fixing the flaw with one of its rare "out of cycle" updates.

The soaring number of infections by Downadup -- also called "Conficker" by some security companies -- prompted Microsoft to add detection for the worm to its Malicious Software Removal Tool (MSRT), the anti-malware utility that the company updates and redistributes each month to Windows machines on Patch Tuesday. The MSRT scans for known malware, then scrubs the system of any it finds.

Like researchers at firms such as Symantec Corp. and Panda Security, Microsoft blamed lackadaisical patching for the infections. "A number of our customers have contacted our support team for assistance with containment in environments that were, largely, not patched when the worm was released," said Cristian Craioveanu and Ziv Mador, two researchers at Microsoft's Malware Protection Center, in a Tuesday blog entry. "Either Security Update MS08-067 was not installed at all or was not installed on all the computers."

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