A new version of the Sober worm has started attacking computers in Europe and the US, a security services company said. The worm is a mass-mailer, meaning it spreads itself via e-mail using contacts listed in the address books of computers it infects.

The first instance of the worm, called W32.Sober-K-mm, was intercepted by UK security company MessageLabs early yesterday morning. The company detectedmore than 2,200 instances between 11.00 and noon, prompting MessageLabs to give it a high-risk rating, said Maksym Schipka, a senior anti-virus researcher with the company. "Compared to other Sober worms it looks to me like this one is spreading itself more aggressively," he said.

The latest variant appears to have originated in Germany, and by mid-morning it had also been detected in France, the UK and the US. It may have been created by the same hacker that wrote the first version of the Sober worm, which appeared in October 2003 and also originated in Germany, Schipka said.

"I'm not aware that the source code for this worm was made public. It is written in Visual Basic which makes it more difficult to reverse engineer the virus than if it were written in C++ or Assembler. So it would be logical to assume it is the same virus writer," he said.

W32.Sober-K-mm spreads itself as an e-mail attachment and creates random subject lines and body texts in either English or German, depending on the e-mail addresses it gathers. Subject lines observed so far include "Alert! New Sober worm," "Paris Hilton Sex Videos," "You visit illegal websites" and "Your new Password," according to MessageLabs..

The worm can also generate fake messages that try to fool the recipient into opening the attached .zip file. Some e-mails purport to be from an anti-virus company offering a security patch against a new version of the Sober worm. Others pretend to be from the FBI and include an attachment labelled "indictment," Schipa said.

When a user opens the attachment the worm creates several executable files with the names csrss.exe, winlogon.exe and smss.exe. It then modifies the registry key Software\Microsoft\Windows\CurrentVersion\Run so that the files execute on startup.

The worm also displays the contents of part of the infected machine's file system in a notepad document. Schipka said it is not clear yet why this document appears and that it may be a bug in the worm. "Sober is known as being relatively buggy," he said.

The notepad file may be a sign that the virus writer is experimenting with new techniques, one observer speculated.

Such worms can make the computers they infect sluggish to operate and also clog e-mail servers and networks. Users are advised to update their anti-virus software to keep the definition files current.