The e-mails carryin an attachment which, when opened, infects the recipient's computer. The worm then attempts to disable antivirus programs and send copies of itself to any e-mail addresses found on the hard drive.
The Sober worm still accounts for close to 43 percent of all viruses being reported to the British antivirus firm Sophos. At its peak, it accounted for one out of every 13 e-mails relayed over the Internet, the group said on Wednesday.
As the most widespread variant since Sober first appeared about two years ago, the new offshootand slow message delivery, Sophos said. Postini, another computer security firm, estimates that the latest Sober outbreak is twice as large as the biggest previous attack.
Infected e-mails carry a variety of messages. One claims to be a message from the FBI or CIA. It informs recipients that they've visited illegal Web sites and instructs them to answer questions in the e-mail's attachment. Another promises video clips of socialites Paris Hilton and Nicole Richie, while a German version references that country's version of the TV show "Who Wants To Be A Millionare."
"Mocking the feds is a sure-fire way of goading the authorities, and you can't help but wonder whether the author is desperate to be caught," Carole Theriault, senior security consultant at Sophos, said in a statement.
Sophos also reported that close to 3 percent of all e-mails, or one in 38, contain viruses. The firm collects data from a global network of monitoring stations.