A new Trojan horse program circulating around the Internet this week appears to be on a moral mission to stamp out adult Web sites, according to security research firm Sophos.

Instead of snooping for sensitive financial information or secretly taking control of an infected computer, the Trojan, called Yusufali-A, monitors Web surfing habits. When it spots an objectionable term such as "sex" or "exhibition," in the browser's title bar, it hides the Web site and instead pops up a message taken from the Koran, said Gregg Mastoras, a senior security analyst with Sophos.

"Allah knows how ye move about and how ye dwell in your homes," reads part of the message, Mastoras said.

If the user does not quit the offending Web site, the Trojan will eventually displays a message reading "Oh! NO i'm in the Cage" and force the computer to log-out.

Trojan horse programs are similar to viruses in that both contain malicious software that is installed on the user's computer. But Trojans, unlike viruses, do not try to spread to other computers once installed.

Other than chastising adult Web site surfers, Yusufali-A appears to cause no serious harm to infected systems, Mastoras said. Users must be running the Windows operating system and click on an e-mail attachment to become infected with the program, he added.

While Yusufali-A is unremarkable from a technical perspective, its moral tone sets it apart from other malicious software, Mastoras said. "It's remarkable to me because it's not really trying to steal any money or confidential information."

Sophos has not received many reports of users being affected by Yusufali-A, which is blocked by up-to-date antivirus software, Mastoras said. Infected users can removed the Trojan by following steps posted here .

Mastoras said he had no idea why the Trojan was written, or what the "i'm in the Cage" reference might mean. "One thing I never do is try to imagine what's in a virus writer's head," he said.

The IDG News Service is a Network World affiliate.