Mebroot: The Stealthiest Rootkit in the Wild?

Security vendors are reporting a new version of Mebroot, aka Torpig and Sinowal, that has been armored with new functionality to avoid detection. The new variant hooks itself deep within Windows to thwart efforts at spotting it.

Malware writers have added new moves to the notorious Mebroot rootkit.

The malware also goes by the name Sinowal and Torpig, and made headlines late last year when EMC's RSA security division found a trove of financial data stolen by attackers. Now, security vendors say a new variant has been armed with functionality designed to cloak it as it spreads through drive-by downloads and by exploiting a recent Adobe Reader and Acrobat vulnerability.

"This is the stealthiest rootkit in the wild today," Jacques Erasmus, director of research at Prevx, told eWEEK. "There are proof-of-concept rootkits that are more stealthy, but in terms of them being able to be implemented for mainstream use, that is still a very long way off."

Once on your Windows PC, Mebroot delivers a payload that can record keystrokes, sniff HTTP and HTTPS Post requests, and inject arbitrary HTML into Websites, particularly banking sites. But according to security vendors, what's really new is how Mebroot infects a system. According to researchers at Prevx, Mebroot no longer hooks into the disk.sys driver, but checks to see what lower device \Device\Harddisk0\DR0 is attached to and then hooks the relative driver.

"If the lower driver to which the device is attached is atapi.sys, then atapi.sys will be hooked," a Prevx researcher explained in a blog post. "If the lower driver is acpi.sys, then that driver will be attacked. ... This is why you can get different results from pc to pc, and from a pc and a virtual machine like VMware."

In this new version, the malware authors also fixed a bug that had previously made it easier to detect anomalies with the master-boot-record, said Andreas Baumhof, CTO for TrustDefender.

He added that after the initial infection, Mebroot is never present as a file on the hard drive. It gets injected into various kernel drivers during the boot-up procedure and is finally injected into services.exe and svchost.exe. Then it will through IAT compromise all processes to get access to the internal data, he said.

"So altogether, Mebroot is not visible as a kernel driver, not visible as a usermode process, [and] if you scan your hard drive, nothing is found," he said, adding however that two executables are executed at the time of the initial infection and will be present on the hard drive for a short period in the temp directory.

Prevx has added functionality into its new tool, Prevx 3.0, to help organizations deal with the threat. TrustDefender has done the same with its own tool, and has also posted manual detection and removal instructions here. According to researchers at Symantec and TrustDefender, infections of the new variant do not appear to be extremely widespread at the moment.

"We have seen it 'popping up' on a small amount of servers, and now it almost disappeared again; however, we all know that it won't take too long before it will reappear again," Baumhof said. "In my opinion, these guys know the workings of the security industry in very much detail, and they don't want to run the risk of infecting too many computers, for example, to be included in an MSRT, or to face a global targeted alliance against them-as seen, for example, with conficker.c."