More than a million Windows PCs have been infected in the past 24 hours by the computer worm that exploits a months-old Windows bug, a security company has warned.
Finnish security firm F-Secure estimated on Wednesday 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. On Tuesday F-Secure said the worm had infected an estimated 2.4 million machines.
The worm, which several security companies have reported surging dramatically during the last few days, exploits a bug in the Windows Server service used by all supported versions of Microsoft'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 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."
Craioveanu and Mador said that the highest number of infection reports had come from the US, Canada, Mexico, Korea and several European countries, including the UK, France and Germany.
F-Secure also reported that it was spying on Downadup's command and control process by registering domains it thought the worm would try to use to download additional malware to infected PCs. The worm, said Mikko Hypponen, F-Secure's chief research officer, generates hundreds of possible domain names daily using a complex algorithm.
"This makes it impossible and/or impractical for us good guys to shut them all down," acknowledged Hypponen in a blog entry. "The bad guys only need to predetermine one possible domain for tomorrow, register it, and set up a website, and they then gain access to all of the infected machines. Pretty clever." Even so, F-Secure has registered some of the possible hosting domains so that it can eavesdrop on the attackers and get an idea of the number of infected PCs.
Microsoft recommended that Windows users install the October update, then run the January edition of the MSRT to clean up compromised computers.