A new Trojan circulating is on a moral mission against porn sites.

Instead of snooping for sensitive financial information or taking control of a 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 website and instead pops up a message from the Koran.

"Allah knows how ye move about and how ye dwell in your homes," reads part of the message, said Gregg Mastoras, a senior security analyst with Sophos.

If the user doesn't leave the offending website, 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 website surfers, Yusufali-A appears to cause no serious harm to infected systems, Mastoras said. Users must be running the Windows and click on an e-mail attachment to become infected with the program.

Sophos has not received many reports of users being affected by Yusufali-A, which is blocked by up-to-date anti-virus software, Mastoras said. Sophos has posted instructions on how to remove the Trojan.