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A Web page distributing what it claimed was an Xbox game emulator has been yanked from cyberspace after it was found to contain a Trojan horse program designed to generate money based on online ad hits.

The program was presented as an emulator that would enable users to play Microsoft's Xbox games on their PCs. Thousands of copies of the ersatz emulator may still be circulating on the Internet.

A Trojan horse program is a piece of code or a small application that misrepresents what it does. A worm or virus may travel as a Trojan horse, which purports to be a useful utility or graphic but actually contains harmful or invasive software. Most antivirus programs can protect against common Trojan horses.

The emulator had been distributed from a free Web site account hosted by Angelfire, a company owned by Lycos. Angelfire has removed the site from the Web for violating the hosting firm's terms-of-service policy, according to a notice posted on the site.

Piggyback File

The fake emulator had a file name of EMU_xbox.exe. Hidden inside the download was a program called NetBUIE.exe. That program appears to have sent "a massive burst of data to Web sites" designed to increase the hit totals of online ads, said Roger Thompson, director of malware research at TruSecure, based in Herndon, Virginia. Online ads make money by the number of people who view them and the number of people who click on them.

The Trojan horse was "probably generating click-throughs for someone," Thompson said. It's unclear whether the program did anything else to victims' PCs, he said.

"It's needle-in-a-haystack stuff to see what else it might be doing," he said, adding that it "didn't seem to drop any other programs."

Caution Urged

Because nothing obvious happened when users launched the "emulator" program, they likely assumed that it had malfunctioned and forgot about it, he said. So the program could still be on many people's PCs and "may still be generating click-throughs," he said.

"The moral of the story," Thompson said, "is that people really do have to be wary."

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