Pressed by increasingly effective anti-spam efforts, senders of unsolicited commercial e-mail are resorting to outright criminality in their efforts to conceal the source of their ill-sent missives, using Trojan horses to turn the computers of innocent netizens into secret spam zombies.

"This is the newest delivery mechanism," says Margie Arbon, director of operations of anti-spam group MAPS. "I've been looking for it for a year, and in the last couple of months people have actually found Trojans that are doing it... They're carrying their own SMTP engines. Failing that, they install open proxy software."

One of those programs popped up last week. Named "Proxy-Guzu," when executed by an unwitting user the Trojan listens on a randomly-chosen port and uses its own built-in mail client to dash off a message to a Hotmail account, putting the port number and victim's IP address in the subject line. The spammer takes it from there, routing as much e-mail as he or she likes through the captured computer, knowing that any efforts to trace the source of the spam will end at the victim's Internet address.

Trojan horses generally rely on their wielder's ability to trick innocent people into executing them. Proxy-Guzu, naturally, arrives as spam -- in one sighting the program was offered as a naughty peek at an online webcam.

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