Watch out! Melissa virus is back

The deadly Melissa virus is back, but this time it's disguised as a Mac document that can infect both PC and Mac users.

There have been 20 to 30 incidents of the virus infecting companies in Europe, according to antivirus company Network Associates, which has ranked the virus as a "medium risk" so far.

Called Melissa-X or Melissa 2001, the virus is a variant of the Melissa virus which caused millions of pounds worth of damage to companies worldwide when it struck in April 1999.

Melissa was considered a particularly serious virus due to the speed at which it spread, mailing itself to the first 50 email addresses listed in users' address books held on infected systems. This lead to a mass emailing of the virus around the world.

According to UK-based antivirus company Sophos, Melissa-X is an infected Microsoft Office 2001 file (the Mac version of Office 2000). The company said it appears this viurs variant came about when a Mac user who had a file infected with WM97/Melissa-X, saved it using Office 2001.

The file (ANNIV.DOC) was then sent to a colleague running Microsoft Office 97 or 2000. When the file was opened, the viral macro code ran (even though the file format was still Office 2001) and the mass-mailing part of the virus code executed.

Although the virus was originally created in a Mac version of Word, it can also infect Microsoft Word users running Windows.

The virus sends a message to the first 50 addresses in all of the address books accessible by Outlook. This message has the subject line: "Important message from " and the message text reads "Here is that document you asked for... don't show anyone else ;-)."

Accompanying this message is an attachment: a copy of the infected document from which the virus is launched. In most of the cases seen by Sophos, the company's researchers said this attached file is called ANNIV.DOC.

Graham Cluley, senior technology consultant at Sophos, said Macintosh computers can be infected by Word viruses: "Most Word macro viruses can infect the Mac versions of Word," he said.

"The reason why this is unusual is because it is probably the first virus we've seen which is distributing itself in Word 2001. Many antivirus products may need to have an engine update to scan inside the file format effectively."