Over the past month, a new type of malicious software has emerged, using a decades-old technique to hide itself from anti-virus software.

The malware, called Trojan.Mebroot by Symantec, installs itself on the first part of the computer's hard drive to be read on startup, then makes changes to the Windows kernel, making it hard for security software to detect it.

Criminals have been installing Trojan.Mebroot, known as an MBR (master boot record) rootkit, since mid-December, and were able to infect nearly 5,000 users in two separate attacks, staged on Dec. 12 and Dec. 19, according to Verisign's iDefense Intelligence Team. In order to install the software on a victim's computer, attackers first lure them to a compromised Web site, which then launches a variety of attacks against the victim's computer in hopes of finding a way to run the rootkit code on the PC.

Once installed, the malware gives attackers control over the victim's machine.

The group behind this latest rootkit is the same one responsible for the Torpig Trojan, and it is believed to have already installed more than 250,000 Trojan programs, iDefense said in a report on the rootkit published Monday.

The interesting thing about Trojan.Mebroot is that it installs itself on the MBR. This is the first sector of the computer's hard drive, and it is the place the computer goes to first whenever it wants to boot up the operating system. "Basically, if you can control the MBR, you can control the operating system and therefore the computer it resides on," wrote Symantec researcher Elia Florio in a blog posting on Trojan.Mebroot.

The criminals are using several different versions of this attack code, some of which are not currently being detected by some anti-virus products, iDefense said.

"At the moment the AV detection is hit and miss across the board; however, in the last day, a number of vendors have added detection for it already," said Andrew Storms, director of security operations with nCircle Network Security. "As for penetration, so far many people are showing it as having a low overall distribution. The concern is that the group which may be preparing to distribute the rootkit is well-prepared."

Malicious software that infected the master boot record was common during the MS-DOS era, but it has not been used much in attacks in recent years.

In 2005, however, researchers at eEye Digital Security gave a talk at the Black Hat security conference, showing how a rootkit could hide itself on the MBR. This Trojan.Mebroot software is derived from that code, iDefense said.

Getting this kind of malicious software to work reliably is a technical challenge, and there have generally been easier ways for the bad guys to take over PCs in recent years, said Marc Maiffret, an independent security researcher who was chief technology officer at eEye when the code was first developed.

Attackers were given a hand last year, however, when researchers at NV Labs published a proof of concept MBR rootkit.

Maiffret said that while we may soon see more of these MBR rootkits, "it won't take long for all the anti-virus companies to react."

"It's not some new attack vector that's going to be hard to prevent," he said. "It's just something that people haven't really paid attention to."