For years, hackers have focused on finding bugs in computer software that give them unauthorized access to computer systems, but now there's another way to break in: Hack the microprocessor.

On Tuesday, researchers at the University of Illinois at Urbana-Champaign demonstrated how they altered a computer chip to grant attackers backdoor access to a computer. It would take a lot of work to make this attack succeed in the real world, but it would be virtually undetectable.

To launch its attack, the team used a special programmable processor running the Linux operating system. The chip was programmed to inject malicious firmware into the chip's memory, which then allows an attacker to log into the machine as if he were a legitimate user. To reprogram the chip, researchers needed to alter only a tiny fraction of the processor circuits. They changed 1,341 logic gates on a chip that has more than 1 million of these gates in total, said Samuel King, an assistant professor in the university's computer science department.

"This is like the ultimate back door," said King. "There were no software bugs exploited."

King demonstrated the attack on Tuesday at the Usenix Workshop on Large-Scale Exploits and Emergent Threats, a conference for security researchers held in San Francisco.

His team was able to add the back door by reprogramming a small number of the circuits on a LEON processor running the Linux operating system. These programmable chips are based on the same Sparc design that is used in Sun Microsystems' midrange and high-end servers. They are not widely used, but have been deployed in systems used by the International Space Station.

In order to hack into the system, King first sent it a specially crafted network packet that instructed the processor to launch the malicious firmware. Then, using a special login password, King was able to gain access to the Linux system. "From the software's perspective, the packet gets dropped ... and yet I have full and complete access to this underlying system that I just compromised," King said.

The researchers are now working on tools that could help detect such a malicious processor, but there's a big problem facing criminals who would try to reproduce this type of attack in the real world: How do you get a malicious CPU onto someone's machine?

This would not be easy, King said, but there are a few possible scenarios. For example, a "mole" developer could add the code while working on the chip's design, or someone at a computer assembly plant could be paid off to install malicious chips instead of legitimate processors. Finally, an attacker could create a counterfeit version of a PC or a router that contained the malicious chip.

"This is not a script kiddie attack," he said. "It's going to require an entity with resources."

Though such a scenario may seem far-fetched, the U.S. Department of Defense (DoD) is taking the issue seriously. In a February 2005 report, the DoD's Defense Science Board warned of the very attack that the University of Illinois researchers have developed, saying that a shift toward offshore integrated circuit manufacturing could present a security problem.

There are already several examples of products that have shipped with malicious software installed. In late 2006, for example, Apple shipped video iPods that contained the RavMonE.exe virus.

"We're seeing examples of the overall supply chain being compromised," King said. "Whether or not people will modify the overall processor designs remains to be seen."