Stealthy rootkit slides further under the radar

A known Master Boot Record rootkit gets an upgrade that makes it difficult to detect

Thousands of Web sites have been rigged to deliver a powerful piece of malicious software that many security products may be unprepared to handle.

The malicious software is a new variant of Mebroot, a program known as a "rootkit" for the stealthy way it hides deep in the Windows operating system, said Jacques Erasmus, director of research for the security company Prevx.

An earlier version of Mebroot, which is what Symantec named it, first appeared around December 2007 and used a well-known technique to stay hidden. It infects a computer's Master Boot Record (MBR). It's the first code a computer looks for when booting the operating system after the BIOS runs.

If the MBR is under a hacker's control, so is the entire computer and any data that's on it or transmitted via the Internet, Erasmus said.

Since Mebroot appeared, security vendors have refined their software to detect it. But the latest version uses much more sophisticated techniques to stay hidden, Erasmus said.

Mebroot inserts program hooks into various functions of the kernel, or the operating system's core code. Once Mebroot has taken hold, the malware then makes it appear that the MBR hasn't been tampered with.

"When something is trying to scan the MBR, it displays a perfectly good-looking MBR to any security software," Erasmus said.

Then, each time the computer is booted, Mebroot injects itself into a Windows process in memory, such as Since it's in memory, it means that nothing is written to the hard disk, another evasive technique, Erasmus said.

Mebroot can then steal any information it likes and send it to a remote server via HTTP. Network analysis tools such as Wireshark won't notice the data leaking out since Mebroot hides the traffic, Erasmus said.

Prevx saw the new variant of Mebroot after one of the company's consumer customers became infected. It took analysts a few days to nail down exactly how Mebroot was managing to embed itself in the operating system. "I think everyone at the moment is working on modifying their [antimalware] engines to find it," Erasmus said.

And those companies need to act fast. Erasmus said it appears that thousands of Web sites have been hacked to deliver Mebroot to vulnerable computers that don't have the proper patches for their Web browsers.

The infection mechanism is known as a drive-by download. It occurs when a person visits a legitimate Web site that's been hacked. Once on the site, an invisible iframe is loaded with an exploit framework that begins testing to see if the browser has a vulnerability. If so, Mebroot is delivered, and a user notices nothing.

"It's pretty wild out there now," Erasmus said. "Everywhere you go, you have a chance to be infected."

It's unknown who wrote Mebroot, but it appears that one aim of the hackers is to simply infect as many computers as possible, Erasmus said.

Prevx has a self-named specialized security product that works alongside antivirus software to detect drive-by browser exploits, password stealers, rootkits and rogue antivirus software.

Prevx released the 3.0 version of its product on Wednesday. The software will detect malware infections for free, but users must upgrade to get the full removal functionality. However, Prevx 3.0 will remove some of the more evil malicious software, including Mebroot, as well as any advertising software, known as adware, free of charge, Erasmus said.

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