LONDON, England (Reuters) -- Security experts warned on Thursday the fast-spreading MyDoom virus would plague e-mail users for some time as it counts down to a mammoth digital attack next week on Microsoft and software firm SCO Group Inc.
For a fourth consecutive day, Internet service providers and corporations were bogged down by a crush of infected e-mails.
Security experts said as many as one in three e-mails in circulation was triggered by MyDoom.A, making it the fastest spreading Internet contagion ever.
"We are seeing companies struggling with this as they cannot clear the viruses quickly enough," said Graham Cluley, technology consultant for anti-virus and anti-spam firm Sophos Plc. "This one will be with us for a while."
Meanwhile, sleep-deprived security experts said they were largely powerless to stop the virus's coordinated digital attacks, timed to hit Web sites for SCO on Sunday and Microsoft on Tuesday, security officials said.
"It's very difficult for anti-virus firms to react in these scenarios. We're always going to be on the back foot," said Paul Wood, chief information analyst for British-based e-mail security firm MessageLabs.
Machines turned into zombies
Since appearing this week, the MyDoom.A worm, also dubbed Novarg or Shimgapi, has infected computers across the globe by enticing users to open a file attachment that releases a program capable of taking over a victim's computer.
Once hit, the program scours the Web for more computers to infect. MyDoom.A is programmed to send spam e-mails to spread the infection further and marshal an army of infected machines to knock SCO's Web site offline on Sunday.
On Wednesday, a second variant dubbed MyDoom.B, appeared. It spread less quickly, but carried a program timed to unleash attacks on SCO and Microsoft. Also, it prevented access to anti-virus sites where patches for the bug are available.
Computer security companies continued to warn people not to open any suspicious attachments in e-mail messages.
Since the worms often appear as error messages from "Mail Administrators" and other official-looking addresses, many people inevitably open the attachment after finding minimal information in the message.
Computers running any of the latest versions of Microsoft's Windows operating system are at risk of being infected, although the worm doesn't exploit any flaws in Windows or software.
Instead, MyDoom is designed to entice the recipient of an e-mail to open an attachment with an .exe, .scr, .zip or .pif extension.
In the firing line
The financial damage from the outbreak -- from network slowdown to lost productivity -- is difficult to measure, but is assumed to be billions of dollars, according to experts.
For the ordinary computer user, MyDoom's toll will be measured in bounced e-mails and an inability at times to enter your inbox as ISPs seek to filter out bogus traffic.
For Microsoft and SCO, their Web sites are once again in the firing line.
SCO, a small Utah-based software maker suing International Business Machines Corp. over the use of code for the Linux operating system, has been the target of denial-of-service attacks in the past by apparent pro-Linux protesters.
Last year, Microsoft's site for software upgrades was permanently moved to a new Web address to avert a similar onslaught triggered by the Blaster worm.
SCO this week issued a $250,000 bounty for information leading to the arrests of the authors of MyDoom. In November, Microsoft offered two $250,000 rewards for tips leading to the arrest of the Blaster and SoBig virus writers.
Some security experts theorized that the MyDoom variants were written by the same individual or group, but had no solid clues on their whereabouts.
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