Microsoft clarifies MBR rootkit removal advice

Now says users don't have to reinstall Windows to remove super-stealthy malware, but botnet expert disagrees

Microsoft yesterday clarified the advice it gave users whose Windows PCs are infected with a new, sophisticated rootkit that buries itself on the hard drive's boot sector.

Several security researchers agreed with Microsoft's revisions, but a noted botnet expert doubted that the advice guaranteed a clean PC.

Last week, the Microsoft Malware Protection Center (MMPC) highlighted a new Trojan, dubbed "Popureb," and said that the only way to eradicate the malware was to use a recovery disc.

Because a recovery disc returns Windows to its factory settings, Microsoft was essentially telling users that they needed to reinstall Windows to completely clean an infected PC.

That recommendation was similar to what Microsoft had offered more than a year ago, when another Trojan buried rootkit code into the master boot record (MBR) of the PC's hard drive.

On Wednesday, MMPC engineer Chun Feng clarified Microsoft's advice.

"If your system is infected with Trojan:Win32/Popureb.E, we advise fixing the MBR using the Windows Recovery Console to return the MBR to a clean state," Feng wrote on an updated blog yesterday.

Feng provided links to instructions on how to use the Recovery Console for Windows XP, Vista and Windows 7.

Once the MBR has been scrubbed, users can run antivirus software to scan the PC for additional malware for removal, Feng added.

Malware like Popureb is especially difficult to detect and delete once it's on a system because it overwrites the hard drive's MBR, the first sector -- sector 0 -- where code is stored to bootstrap the operating system after the computer's BIOS does its start-up checks. Because it hides on the MBR, the rootkit installed by Popureb makes not only itself, but any follow-on malware installed by it later, invisible to both the operating system and security software.

MBR rootkit malware is among the most advanced of all threats, researchers said yesterday during interviews about a different family, called "TDL-4," a bot whose collection of compromised computers they called "practically indestructible."

Several security firms have also weighed in on the debate about whether users need to reinstall Windows.

"Reinstalling is definitely overkill for this malware problem," said Vikram Thakur, principal security response manager with Symantec, in an interview today. "It can be resolved simply by fixing the MBR via an external disk."

Symantec offers a tool to help users do that.

Named "Norton Bootable Discovery Tool," the free download creates a boot disc for starting up the PC without accessing the hard drive -- and thus without loading the infected MBR. Once the Windows machine boots using the recovery disc, the tool downloads new malware signatures -- the digital "fingerprints" antivirus software uses to detect threats -- sniffs out signs of infection and if necessary, cleans the MBR.

"When you fix the MBR, you pretty much expose the threat itself to other applications, including antivirus applications," said Thakur. "They can then pick up on the threat, and delete it."

Other researchers with Webroot and CA agreed with Thakur that Popureb could be removed without reinstalling Windows.

But an internationally-known botnet expert disagreed.

Joe Stewart, director of malware research at Dell SecureWorks, said that reinstalling Windows was the only way to insure that MBR rootkits and the additional malware they install are completely removed.

"Once you're infected, the best advice is to [reinstall] Windows and start over," said Stewart. "[MBR rootkits] download any number of other malware. How much of that are you going to catch? This puts the user in a tough position."

Marco Giuliani, the Webroot threat research analyst who published his own analysis of Popureb, cautioned that users may end up having to reinstall Windows after all.

"What is really a nightmare is that [Popureb] looks like it has bugs and sometimes it hangs the system during the reboot stage," Giuliani wrote on the Webroot blog. "This could become a problem that would require you to perform a full system reinstall."

In a follow-up statement today, Microsoft seemed to acknowledge that users could encounter problems with the MMPC advice, and may need to restore their PC from a recent backup.

"Microsoft recommends that customers whose systems are infected with Trojan:Win32/Popureb.E, contact Microsoft PCSafety, who can help them identify and remove malware from their systems," said Jerry Bryant, general manager of with Microsoft's Trustworthy Computing group, in an e-mailed statement. "While using the recovery console to address Master Boot Record (MBR) issues is not designed to affect personal files, we continue to recommend customers practice reasonable back-up processes."

PCSafety is a toll-free telephone support line that Microsoft operates for customers with malware-infection problems. The number in the U.S. is: 866-727-2338.

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

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