Virus warning: Klez keeps on spreading

UK businesses are being urged to stay alert today as the Klez worm shows no sign of abating, with the malware having now plagued over seven per cent of computers worldwide.

The Klez - H variant does not destroy any files, but does install code which enables it to launch again.

On some unpatched systems it is also able to spread without victims having to open an attachment or an email.

It can also hide other damaging payloads, such as the Elkern virus, which it drops once the attachment is executed.

Alex Shipp, anti-virus technologist at MessageLabs, said the virus is persistent for several reasons.

"Most AV (anti-virus) people have the remedy already available but people who don't have any anti-virus software keep on spreading it by accident. It also has different email subject lines and body text, which makes it hard for people to recoginise and easy for the worm to spread."

In addition, an old bug in Microsoft's Outlook software means a user can inadvertently pass the worm on merely by highlighting an infected email in the inbox.

A survey published by Panda Software earlier today found out that 7.2 per cent computers worldwide were infected by variants of the Klez worm because of users' failure to update anti-virus software.

However, Shipp said the infection is nowhere near as severe as the Love Bug or Melissa.

"The Love Bug managed to infect one in 28 emails globally whereas Klez has only infected one in 200. We have graded Klez as a medium risk, but asked people to be alert because of its longevity," he said.