The latest variant of the mass-mailing Sober worm masquerades as an official Microsoft patch for the MyDoom worm.
Sober.D, discovered on Monday, is technically similar to its previous incarnation as Sober.C, where it used its own SMTP engine to send copies of itself to e-mail addresses found on infected systems. But the latest version displays fake Microsoft warnings and error messages.
"It arrives in an e-mail that pretends to be a patch to protect against a version of MyDoom," said Graham Cluley, a senior technology consultant at antivirus company Sophos. "The e-mail appears to be a Microsoft patch, so people will of course double-click on that attachment."
According to Finnish antivirus company F-Secure, Sober.D spreads either as an executable attachment or inside a password-protected Zip archive attached to an e-mail. Once a person clicks on the file, the worm scans the PC to see if it has already been infected.
If the system is clean, a small box appears with the message: "This patch has been successfully installed." If the system is already infected with Sober.D, the message says: "This patch does not need to be installed on this system."
Sober.D also changes its language depending on where it is being sent. If the recipient's e-mail address has a "de," "ch," "at," "li," "nl" or "be" extension, the text will be in German and the subject will read: "Microsoft Alarm: Bitte Lesen." Otherwise the subject line is in English and reads: "Microsoft Alert: Please Read!" Previous versions of Sober have also been bilingual, Cluley said.
This is not the first time that a worm has disguised itself as a Microsoft update. In January, the Xombe, or Trojan.Xombe, worm posed as a critical patch for Windows XP. This was believed to be a copycat of 2003's most successful worm, Swen, which is thought to be the first known worm to masquerade as a security warning from Microsoft.
Microsoft has always maintained that it does not e-mail patches to people, so they should ignore any such messages.