UPDATED: MyDoom.b has a slightly larger payload than MyDoom.a and targets Microsoft for a DOS attack to be launched Feb. 1.
A variant of the MyDoom worm discovered on Wednesday does more than launch a denial-of-service attack; it also locks out infected users from contacting Web sites that can come to their assistance, including most antivirus companies and Microsoft.
The MyDoom worm has been spreading through the Internet at a furious pace
since Monday. The variant, which Russian anti-virus specialist Kaspersky Labs has labelled MyDoom.b, has a slightly larger payload compared with MyDoom.a and targets Microsoft Corp. for a denial-of-service attack to be launched starting on Feb. 1, instead of The SCO Group Inc.
Kaspersky said the worm features minor modifications to the text of the e-mail that carries it, but is otherwise identical to the original.
It is understood that MyDoom.b may have been distributed using the network of PCs infected with MyDoom.a, something that a spokesman for Kaspersky claimed means the Internet "may be facing a much more serious outbreak than the one caused by Mydoom.a."
Microsoft, for its part, says that its researchers have yet to confirm that MyDoom.b contains code to launch a DDoS against its Web sites.
"Our investigation at this point has not confirmed that aspect of the attack code," said Christopher Budd, a Microsoft security program manager. "Were still looking at it to completely understand what the malicious code does and what people can do to protect themselves from it."
Microsoft has launched a Web site to provide updates on MyDoom and its worm variants
Denis Zenkin, head of corporate affairs for Kasperky, said it was likely the author of MyDoom.b was the same person as was responsible for MyDoom.a.
"To develop a new version of the worm would require source code of the program and time to learn how it works. The source code was not published, and a virus writer had no time to learn the code to create a revamped version," Zenkin said.
In addition, security firms on Wednesday noted that MyDoom.b overwrites the Windows HOSTS file on infected machines. This file defines domain name-IP address relationships much in the way that DNS does, but it takes precendence.
The worm adds to the users HOST file a series of domain names for Microsoft, antivirus companies and some other companies the author wants to target and sets them to 0.0.0.0, in an attempt to make them inaccessible to the user.
Antivirus companies reported differences in exactly which HOST file was overwritten. For example, Sophos Plc. pointed to the Windows directory, while SoftWin Bitdefender offered the %Systemroot%\system32\drivers\etc directory. Symantec Corp.s site suggested it overwrites "the local hosts file."
According to antivirus vendor Sophos
, a number of domains are set to 0.0.0.0 by MyDoom.b. The Heres the current list from Sophos:
The appearance of a variant of MyDoom is an unwelcome twist in the saga of one of the most rapidly spreading worms in Internet history. By Monday, MyDoom was infecting one in 12 e-mails sent, according to MessageLabs Inc, a New York-based e-mail security company. The worm has also been blamed for slow performance from Web servers over the past few days, as corporate firewalls and filters struggle to cope with increased traffic.
Editors Note: This story was updated to include information about the HOST-file changes made by the worm. Matt Hicks, and Larry Seltzer contributed to this story.