A new version of the Zafi email worm is spreading Christmas wishes along with its malicious code, according to antivirus software companies.

Zafi.D is a mass-mailing worm that arrives in a .zip file attached to email messages with the subject "Merry Christmas". Instead of a gift, however, the email package delivers worm code that infects Microsoft Windows systems on which it is opened.

Leading antivirus companies, including McAfee, Sophos and Computer Associates, have issued warnings about the new worm and updated antivirus signatures to stop the new threat.

In addition to the Christmas well wishes in the subject line, Zafi-generated emails contain the message "Happy Hollydays" and are signed "Jaime".

CA researchers have collected almost 100 samples of Zafi.D since spotting the new worm variant early on Tuesday, says Stefana Ribaudo, manager of the company's eTrust Security Management division.

At McAfee, around 50 samples of the worm have been collected, mostly from Europe, says Vincent Gullotto, vice president of McAfee's Anti-Virus Emergency Response Team.

Both companies rated Zafi.D a "medium" threat, indicating that a number of samples have been spotted, and that the worm has a destructive payload.

Like most other mass-mailing email worms, Zafi.D modifies the configuration of Windows machines, shutting down other security software and harvesting email addresses from files on the infected computer. After it harvests email addresses, Zafi uses a built-in SMTP (simple mail transfer protocol) to send email to those addresses with copies of the worm code, antivirus companies say.

The worm has had more luck spreading than earlier Zafi variants, possibly because of its well-timed and appealing subject line and message, which are good examples of what antivirus researchers call "social engineering" – subtle tricks used to gain victims' confidence, Ribaudo says.

However, the increase in reports could be due to an initial spam distribution of the worm. The similarity of Zafi.D to its predecessors – and to other mass mailing worms – means that it's likely that few examples of the new worm are actually getting through to email inboxes, Gullotto says.

Antivirus experts advise email users to update their antivirus software to obtain the latest virus definitions for Zafi.D and to use extreme caution when handling unexpected e-mail attachments.