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Excuse me sir: there's a rootkit in your master boot record

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Security mavens have uncovered a new class of attacks that attach malware to the bowels of a hard drive, making it extremely hard to detect and even harder to remove.

The rootkit modifies a PC's master boot record (MBR), which is the first sector of a storage device and is used to help a PC locate an operating system to boot after it is turned on. The result: the rootkit is running even before Windows loads. There have been more than 5,000 infections in less than a month, researchers say.

"Master boot record rootkits are able to subvert the Windows kernel before it loads, which gives it a distinct stealth advantage over rootkits that load while Windows is running," said Matthew Richard, director of the rapid response team for iDefense, a security provider owned by VeriSign. "It gives it a great stealth mechanism that allows it to persist even after removal." Such rootkits can even survive reinstallation of the operating system, he said.

Because the rootkit lurks deep within the hard drive, well below the operating system, most antivirus programs don't detect the malware. Symantec's antivirus program is an exception, however. It labels the pest Trojan.Mebroot, according to Javier Santoyo, a senior manager for emerging technologies at Symantec.

The new rootkit is part of the arms race between security vendors and malware writers, he said. "We're definitely making it harder and harder for the bad guys to do stuff to the operating system," he said. They respond by attacking new parts of a PC.

Every version of Windows, including Vista, is vulnerable to the rootkit.

About 30,000 websites, mostly located in Europe, are actively trying to install the rootkit by exploiting users who have failed to install Windows updates, Richard says. There were 5,000 infections from December 12 to January 7. The rootkit is being spread by the same group responsible for distributing the Torpig banking Trojans, which are used to steal online banking credentials.

While the number of infections is relatively low, the number could rise quickly if the group expanded the number of exploits it used.

MBR-based attacks date back to the days of MS-DOS, when viruses such as Brain, Stoned, Tequila caused mischief by hiding themselves in a hard drive's primary partition. In 2005, researchers from eEye Digital Security demonstrated a proof of concept that showed how the technique could be extended to the Windows NT domain.

The rootkit comes as a wake-up call, demonstrating that new measures are needed to protect PC security. Many motherboard makers still don't provide functionality that prevents the overwriting of an MBR. But even when such capabilities do exist, the average user has no idea how to enable them. And even when they're enabled, malware writers are likely to target even deeper recesses within a PC.

"We will never win the battle with malware, especially rootkits, without a help from hardware and changes in the *design* of the OSes," Joanna Rutkowska, a researcher specializing in rootkits, wrote in an email. ®

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