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Variants of the Klez worm were by far the most common viruses circulating on the Internet last month.

That's according to monthly statistics from managed services firm MessageLabs, which stopped more than 535,000 copies of the virus in August, up from 475,000 Klez-infected emails blocked in July. Klez has toppe the virus charts for five months in a row.

MessageLabs reports that virus infection rates are currently running at around one per 106 emails, which compares to one in 30 infected emails at the heights of the Goner and Love Bug epidemics.

In August MessageLabs blocked more than 520,720 emails infected with Klez-H alone, which became the worse virus ever in May, according to the company.

By comparison, the next most common virus, Yaha-E, was blocked only 161,353 times by MessageLabs during last month, with the infamous SirCam virus trailing in a distant third with 23,976 sightings.

Last month we received evidence from security experts that spammer's computer systems were infected with the Klez-H virus and spreading it along with their unsolicited emails.

Alex Shipp, Chief Anti-virus Technologist at MessageLabs, said its data couldn't confirm this theory, but spammers getting infected with the virus could be an important factor in explaining the prevalence of the virus.

The way the virus appears with random subject lines and spoofed email addresses of senders are also aiding to its spread, he says.

Klez is a mass-mailing worm, which searches the Windows address book for email addresses and sends messages to all recipients that it finds. The worm uses its own SMTP engine to send the messages. It can also spoof the 'from' field in messages, a factor which has resulted in widespread confusion about the bug.

The subject and attachment name of incoming emails is randomly chosen, making it harder for users to spot. The attachment will have one of the following extensions: .bat, .exe, .pif or .scr. Klez is capable of infecting files.

The worm exploits a vulnerability in Microsoft Outlook and Outlook Express in an attempt to execute itself when you open or even preview the message. Information and a patch for the vulnerability can be found here. ®

Top ten viruses blocked by MessageLabs in August


  1. Klez-H
  2. Yaha-E
  3. SirCam
  4. Klez-E
  5. Yaha-C
  6. Magistr-B
  7. Hybris-B
  8. Magistr-A
  9. Nimda-E
  10. Tettona-A

    External Links

    Analysis of the spread of the Klez-H worm by MessageLabs

    Related Stories

    Klez tops the virus charts
    Klez-H is the worst virus ever - official
    Reg gets Yaha treatment from top exec
    Multiple virus scanning needed, says multiple scanning firm

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