A variant of the MyDoom worm has emerged as the most devastating computer virus since last summer, and is likely to target Microsoft's website, security experts said.
Since appearing earlier this week, the worm has infected computers across the globe by enticing users to open a file attachment that releases a program that potentially allows other attackers to gain access.
The financial damage from the virus-like program - from network slowdown to lost productivity - is already being measured in the billions of dollars, according to anti-virus vendors.
The latest version of the worm is designed to flood Microsoft's website with requests for information in an attempt to bring it down, experts said. This strategy is similar to that of the first version, which targeted the website of the SCO Group, the small software maker suing IBM over the use of code for the Linux operating system.
The MyDoom variant appeared to have other similar aspects to the first version, in that it exempts email addresses for government agencies and other computer security companies, including Symantec.
Computers running any of the latest versions of Microsoft's Windows operating system email program 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 email to open an attachment with an .exe, .scr, .zip or .pif extension.
Since the worms often appear as error messages from "Mail Administrators" and other official-looking addresses, many inevitably open an attachment after finding minimal information in the message. Users who receive the worm and simply ignore or delete it will be able to avoid any damage.
In response to the worm targeting its website, SCO offered a $250,000 reward for "information leading to the arrest and conviction of those responsible for this crime". SCO has drawn the ire of many Linux advocates for its claims that Linux software includes copyrighted code from the Unix operating system.
The attacks from infected computers on SCO and Microsoft are due to begin on February 1st .
