Security software company Sophos has warned that the new W32/Sober-K worm is "spreading widely".
The latest variant can masquerade as X-rated videos of US society heiress Paris Hilton. It is currently the third most common virus in the wild, amounting to over 10% of all viruses reported to Sophos in the last 24 hours.
The W32/Sober-K worm bulk-mails itself using a variety of different subject lines including "Paris Hilton, pure!" and "Paris Hilton SexVideos". It can send itself in the German or English language, depending on whether it believes the recipient's e-mail address to be owned by a German or English speaker.
The very first Sober worm appeared in October 2003, and is believed to have originated in Germany. Since then several variants have appeared.
The worm can slow an infected user's machine and greatly add to the amount of e-mail traffic over the internet, slowing overall web performance.
"This latest variant of the Sober worm may catch out the unwary as they open their e-mail inbox," said Graham Cluley, senior technology consultant at Sophos. "Although much publicised virus outbreaks in the past should have made users more nervous of double-clicking on unsolicited e-mail attachments, some still find it hard to resist."
The W32/Zafi-D virus, which emerged in December, is presently the most prevalent virus, representing 27.6% of all reports to Sophos in the last 24 hours, with the Harry Potter-inspired Netsky-P worm in second position at 22.4%.