Windows XP PCs breed rootkit infections

Three-fourths of all rootkits on decade-old OS, says antivirus firm

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Vlcek urged users running legal copies to upgrade to XP SP3. "Moving to SP3 is the most basic thing that should be done," he said.

Also in play, said Vlcek, is Windows 7's stronger security, especially the 64-bit version.

"The 64-bit version [of Windows 7] has some technologies that really make it much more difficult for rootkits to infect the computer," said Vlcek, calling out that version's kernel driver-signing feature as key to keeping rootkits off machines.

But that hasn't completely protected Windows 7 64-bit, as Vlcek acknowledged.

"The surprising part to me was that I thought the Windows 7 [number] would be even smaller," Vlcek said.

Rootkits able to infect 64-bit copies of Windows 7 remain relatively rare, but they're certainly not unknown: The first popped up in August 2010, and a massive botnet some have called "practically indestructible" last month used a variant of the same malware to install a 64-bit rootkit on Windows 7.

That malware, which goes by a number of names -- Alureon, TDL, Tidserv and most recently, TDL-4 -- is especially devious, as it installs the rootkit into the Master Boot Record (MBR). The MBR is the first sector -- sector 0 -- of the hard drive, where code is stored to bootstrap the operating system after the computer's BIOS does its start-up checks.

By subverting the MBR, the rootkit is even tougher to detect, since it's already in place by the time the OS and security software are loaded into memory.

Avast found that rootkits which infected the MBR were responsible for 62% all rootkit infections.

Users who suspect that their PC is infected with an MBR-based rootkit can scrub their machine with one of several free rootkit detectors, including Avast's "aswMBR" and Sophos' "Anti-Rootkit."

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His e-mail address is

See more articles by Gregg Keizer.

Copyright © 2011 IDG Communications, Inc.

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