The rogue programs, known generically as "Trojan horses," have enabled pornographers and others to mask their identities by using unwitting people's computers as relay stations. It had been assumed that diligent investigators could ultimately shut down a system by identifying the server computer used as the initial launching pad. But now a researcher has determined that a new kind of Trojan horse could make the systems virtually unstoppable.

Joe Stewart, a computer expert at Lurhq, a security company based in Chicago, said that he discovered this new phase in the evolution of Trojan horse programs while taking apart a program called Backdoor.Sinit, which has been circulating on the Internet since late September.

Sinit, Stewart said, does something unexpected: It uses the commandeered machines to form a peer-to-peer network like the popular Kazaa program used to trade music files. Each machine on the network can share resources and provide information to the others without being controlled by a central server machine.

[ Read more ]