Virus writers have created a Trojan horse which tries to disrupt visits the pornographic websites by displaying messages from the Koran.

The low-risk Yusufali-A Trojan horse monitors the websites Windows users are visiting. If the malware sees one of a set of trigger words (such as "teen", "sex" or "penis") in the url it minimises the window so the user cannot see its content and displays a message from the Koran instead. The message, partly written in Arabic, contains the following English text:

Yusufali: Know, therefore, that there is no god but Allah, and ask forgiveness for they fault, and for the men and women who believe: for Allah knows how ye move about and how ye dwell in your homes.

"Unlike other malware, it appears this Trojan horse isn't trying to steal money or confidential information, but acting as a moral guardian instead - blocking the viewing of websites it determines are unsavoury," said Graham Cluley, senior technology consultant for Sophos. "Of course, it's possible for the Trojan horse to make mistakes and block sites that are not pornographic - such as medical sites, or social sites designed for teenagers."

Once the message is displayed the malware performs a variety of other actions before forcing infected users to shutdown. All very disconcerting but there's no need for undue alarm since the Yusufali-A Trojan is not yet in the wild. It's unclear whether the malware was written as a joke, or as a serious attempt to clean up the habits of internet users.

Malware featuring an Islamic theme is rare but not unprecedented. Previous examples have include the Mawanella worm which highlighted the friction between Muslims and Buddhists in Sri Lanka and the Cycle worm which contained a message about life in Iran. ®