According to MessageLabs, one in every 759 email messages sent contains the virus. On 25 June the company's anti-virus gateway stopped 26,142 SoBig infected messages.
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Alex Shipp senior anti-virus technologist at MessageLabs said the virus' author was using the same technique as the previous attacks. "He has probably tweaked the executable code a little to avoid detection [by anti-virus] software," Shipp said.
According to Shipp the difference in SoBig.e is the type of file being transmitted.
SoBig.e is sent as a compressed zip file. Shipp thought the rationale behind using a zip rather than the more traditional executable file route was probable to avoid detection from company AV gateways.
He said that many users still need to send EXE files, so to prevent them being blocked at the gateway they compress the files.
Significantly, Shipp said users that have been infected have gone through a series of processes that should have alerted them to potential problems.
Unlike an EXE file, which can be run simply by dragging it onto the Windows desktop, users first need to save the attachment. They then have to run the UnZip program to uncompress the EXE file and then double click on this EXE file, before they can catch the virus.
Even so, according to MessageLabs on data, the virus has now spread in over 84 countries.
This is the fifth variant of the virus. SoBig.e uses e-mail and shared network folders to infect machines running Microsoft's Windows operating system and is distributed as an email attachment.
In early June, one version purporting to coming from Microsoft chairman Bill Gates, spread across the Net.