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The Sobig email worm that clogged in-boxes in August was the most prolific virus of 2003, according to a top 10 list of viruses published by antivirus software vendor Sophos.
The Abingdon, England company's list of the top viruses revealed that the Sobig-F variant accounted for almost 20% of virus reports to Sophos in 2003, easily besting its next closest rival, the Blaster worm, with 15%.
A Blaster derivative, Nachi (also known as Welchia) took the number three spot on the list, followed by Gibe at number four and Dumaru in the fifth spot.
Sophos ranks viruses by the number of infection reports it receives from customers, which may have tipped the scales in Sobig's favour, according to Carole Theriault, a security analyst at Sophos.
Unlike Blaster, Sobig used email to spread and generated massive volumes of email traffic once it infected machines. All those email messages caught the attention of companies, some of which reported receiving hundreds of thousands of infected messages a day, Sophos says.
"Blaster was an internet worm, so if people applied the necessary Microsoft patches, they didn't get infected. With Sobig, the flow of email was there regardless of whether you had the proper software patches and antivirus updates," Theriault says.
Sophos itself received more than 400,000 Sobig email messages within the first 24 hours after the worm appeared in the wild, she says.
The company was also flooded by calls from customers in the days after the worm began to spread, which vaulted Sobig-F, the sixth version of the worm to appear on the internet, to the list's number one spot, she says.
However, translating the number of phone calls into information on the number of systems infected with Sobig is difficult, if not impossible, she says.
"It's difficult to say 'this many people were infected,' and that can be misleading. There are a lot of people infected [with Sobig] now who don't know it and are continuing to spread the virus," Theriault says.
Other prominent virus outbreaks did not make it to the list, including the Slammer worm, which appeared in January and targeted Microsoft's SQL Server.
While that worm ranked highly, reports of infections dropped off quickly after the worm initially appeared, as organisations disinfected and patched vulnerable SQL Server installations, she says.
All of the top 10 viruses targeted Microsoft's Windows operating system, a trend that Sophos predicts will continue in 2004.
Federal legislation aside, spam will also continue to plague email users next year, with spammers adopting new techniques to get their messages past antispam products and filters, Sophos says.
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