It's a given that hackers will target software, and that's enough for many people to worry about. But now there's the possibility that hackers would hide malicious code in the hardware itself.

John Villasenor, a professor of electrical engineering at the University of California, Los Angeles, studies the way information moves and the way chips are built. He says the problem is that protecting hardware is neither that expensive nor difficult, but many companies don't give it the attention it deserves.

A hardware hack could be an annoyance, by stopping a mobile phone from functioning. Or it could be more dangerous, if it damages the way a critical system operates.

"The vulnerability is in the design process," he said. "The trouble can only be caused by someone with access to that." Traditionally, hardware hacking was almost impossible. Chip manufacturers did all the design in-house, and any problems would be quickly traced.

But in the last several years, hundreds of companies have become involved in hardware design as bigger manufacturers have started outsourcing parts of the design process.

Chips are divided into "blocks," each of which has a different function or set of functions. A chip maker will outsource some blocks if it doesn't have the expertise in certain fields. "A chip might need to do 15 different things," Villasenor said. "So the company outsourcing might say, 'I need a block that adds six and four.'"

A malicious designer could then build a block that functions innocently and well enough until a very specific function is called for. When that happens a certain program is initiated, which could be anything from simply freezing the machine by monopolizing the input and output of data and paralyzing the rest of the system, to transmitting private data to someone else. Such a problem wouldn't be caught in the fabrication process, since by that time the factory is just following instructions.

"The system assumed that everybody was going to be nice," Villasenor said.

Villasenor says there are several types of attacks. Broadly they would fall into two categories: one is when a block stops a chip from functioning, while the other involves shipping data out.

In the first case, a block might execute a piece of code that makes it monopolize the transmission of data between the chip it is on and the rest of the system. That would simply stop the chips from functioning, possibly freezing a computer or cell phone. In the second, a block might be programmed to gather data and send it elsewhere.

Most of the defenses involve adding a kind of "policing" function to the chip's architecture. For example, one could design a block that would monitor the behavior of other blocks and make sure they fit certain patterns. If another block misbehaves, it would be "quarantined" and the monitoring hardware would take over the now-missing functions.

Intel Spokesman George Alfs said the company makes every effort to protect s platform and takes these issues seriously, but said the company was not going to speak about the problem directly. Other chip makers declined to comment.

While the additional hardware would cost, it's the very nature of hardware attacks that limits how much that cost can go up, Villasenor says. Chip sizes would also rise a little -- on the order of a few percent -- but the added protection would be well worth it. He added that the Department of Defense takes the problem seriously enough that it has a program of only buying chips from reliable companies, who in turn only outsource to other vendors vetted by the government. This minimizes the number of companies involved in any particular design.