The worm, Doomjuice, spreads to computers that have already been infected by either the original MyDoom virus or the MyDoom.B variant, and among other actions, places several copies of the source code for MyDoom.A on a victim's computer.
The author may be using the tactic to create a crowd of PC users in which to hide, or the author could be spreading the code in hopes that other virus writers will create variations on MyDoom, said Graham Cluley, senior technology consultant for antivirus company Sophos.
"If he has spread his code around the Net onto innocent computers in an attempt to hide in the crowd, then he's more sneaky than the average virus writer," Cluley said in a statement.
Doomjuice is one of two opportunistic programs -- the other dubbed Deadhat -- that started spreading this week. Both viruses infect computers that have already succumbed to either of the two MyDoom viruses. Doomjuice also attempts to direct any re-infected PCs to attack Microsoft's Web site.
Doomjuice's possession of the source code for the original MyDoom virus suggests that the creator of the worm is also the writer of the original virus. A word in both MyDoom viruses -- the name "andy" -- has already suggested to some researchers that the original MyDoom and the MyDoom.B variant were created by the same person or group.
Other antivirus researchers agree that the latest hostile program could be intended to confuse investigations into who created the viruses.
"It stands to reason that the author might be hiding his tracks," said Craig Schmugar, virus research manager for Network Associates. "He might be trying not to get caught."
The SCO Group and Microsoft have made separate offers of $250,000 (£134,091) for information leading to the arrest and conviction of the person or group that started spreading the MyDoom.A and MyDoom.B viruses, respectively. If the viruses were created and released by the same person or group, it could result in a $500,000 payoff.
