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Klez.H was first released in April and, in spite of the release of anti-virus signatures to prevent infection, it has continued to spread at an astonishing rate, according to MessageLabs. It has overtaken SirCam which, until the start of this week, was the most rampant virus MessageLabs had tracked so far.
According to MessageLabs virus specialist Alex Shipp, the reason for Klez.H's success lies in its ability to cover its tracks and deceive recipients.
"Klez.H is able to select random names from address books to use as the sender address, and also creates a large range of subject, text and attachment names, making it difficult to identify and track," said Shipp.
MessageLabs said Klez.H represents a disturbing trend in virus writing, first seen in SirCam last year, where the viruses continually modify until they foil enough anti-virus scanners to spread successfully to e-mail users worldwide.
SirCam shook the security community and e-mail users worldwide last July when it maintained a rate of growth never seen before.
Shipp said, "When SirCam broke out last summer we thought that we were seeing one of a kind. However, Klez.H has shown us that highly sustainable viruses will continue to appear."
MessageLabs estimates that one in every 300 e-mails is infected with Klez.H.