A new Trojan Horse program circulating around the internet this week appears to be on a moral mission to stamp out adult websites, according to security research firm Sophos.

Instead of snooping for sensitive financial information or secretly taking control of infected computers, 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 website and instead pops up a message taken from the Koran, according to Gregg Mastoras, a senior security analyst at Sophos.

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

If the user does not quit the offending website, the Trojan eventually displays a message reading "Oh! NO i'm in the Cage" and forces 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-website surfers, Yusufali-A appears to cause no serious harm to infected systems. Users must be running the Windows operating system and click on an email attachment to become infected with the program, Mastoras said.

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

Sophos has not received many reports of users being affected by Yusufali-A, which can be blocked by up-to-date antivirus software. 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 added.