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

The Register reports (5 September) that 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. According to The Register, 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.

The publications quotes a senior technology consultant for Sophos as saying that 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.

The Sophos spokesman says that 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.

According to The Register, once the message is displayed the malware performs a variety of other actions before forcing infected users to shutdown.