The two opportunistic programs -- dubbed Doomjuice and Deadhat -- threatened only those users still infected with a version of the MyDoom virus, and didn't pose a major problem for businesses that had previously cleaned systems infected with the virus, the companies said.
"There are only about 50,000 or 75,000 machines left that are infected," said Vincent Gullotto, vice president for antivirus and vulnerability emergency response team at Network Associates.
Doomjuice, whose spread has been moderate, attempts to direct any re-infected PC to attack Microsoft's Web site, Gullotto said. The re-invigorated attack may be responsible for making Microsoft's site inaccessible on Sunday night and early Monday morning, according to Internet performance measurement firm Netcraft.
The first version of MyDoom spread through email two weeks ago, infecting a new computer every time an unwary user opened the attached file that contained the program. As many as 2 million PCs may have been infected, according to some estimates, while others put the number at 1 million or a few hundred thousand computers.
The original virus was programmed to attack the SCO Group's Web site on 1 February, while the variant MyDoom.B was programmed to target Microsoft's site from 3 February until 1 March.
The original attack succeeded in making the SCO Web site inaccessible when PCs infected with the original version of the MyDoom virus started sending mock Web requests to the company's main server. However, Microsoft appears to have suffered less from its MyDoom strike, benefiting from the slow spread of the second virus and a bug in the code that limits the attack to only 7 percent of all infected computers.
However, Microsoft had some Web site problems early on Monday, according to Netcraft. It's unknown if the latest worms caused the issues. Microsoft couldn't immediately comment on the issue.
Doomjuice, which scans for PCs infected with MyDoom, has spread to enough computers that customers have submitted samples to Network Associates' Gullotto.
"The Doomjuice has had some success," Gullotto said. "It only infects machines that [have been compromised], so obviously some people didn't know they were infected."
Network Associates still hasn't received any samples of the other worm, Deadhat. While some antivirus companies, including Network Associates and Symantec, believe the virus spreads by scanning for vulnerable computers that have already been infected with the MyDoom viruses, the worm hasn't spread as far as Doomjuice. Deadhat also spreads through the peer-to-peer file sharing program SoulSeek.
