IT security experts have issued a 'severe risk' threat warning after detecting a virulent new worm spreading in the wild.

Kaspersky Lab warned that Win32.Warezov.at uses its own SMTP engine to send itself to email addresses harvested from the Windows address books on infected machines. 

The subject line, message body and attachment name vary, but mail system messages like 'Mail Server Report', 'Mail Delivery System' and 'test' are typical.

The worm runs when the user clicks on the attached file, a portable executable of around 117KB, packed using UPack. The worm copies itself to disk and modifies the registry to ensure that it loads automatically on start up.

David Emm, senior technology consultant at Kaspersky Lab, said: "It has been some time since we've seen an email worm outbreak. But email worms still have all the ingredients necessary to spread successfully, not least through social engineering.

"Users should be wary of emails received from unknown sources, and make sure that their antivirus protection is up to date."