Antivirus software companies have warned of another variant of the Sober e-mail worm, which appeared on the internet masquerading as a Microsoft software update.
Sober.D is the latest version of a worm that first appeared in October. The worm poses as a software patch which will remove the MyDoom virus from infected Windows systems, said antivirus company F-Secure.
F-Secure first detected the worm variant in Germany and rated the virus a "Level 2" threat, indicating large infections.
Like its predecessors, Sober.D spreads by skimming e-mail addresses from victims' computers, then mailing copies of itself to those addresses.
Sober.D also adapts its message for German speaking audiences, inserts a German-language version of its pitch message into e-mail addresses belonging to German domains, such as those ending in .de, F-Secure said.
Copies of the Sober.D worm arrive in e-mail messages with the subject "Microsoft Alert: Please Read!" or "Microsoft Alarm: Bitte Lesen!".
The worm file is embedded in file attachments with the EXE or Zip file extension and names such as "Patch", "MS-Security" and "UpDate", F-Secure said.
Once the worm file has been opened, the virus places a copy of itself on the infected machine's hard drive and modifies the Windows configuration so that the worm file is launched each time Windows starts.
When run, the worm mimics a legitimate software patch, displaying a pop-up message indicating that the "patch has been successfully installed".
For machines that are already infected, the worm displays a message saying that the "patch does not need to be installed on this system".
It is common for worm authors to dress up their creations as software updates from Microsoft.
Microsoft has said that it never distributes software updates using e-mail and has published guidelines for identifying legitimate e-mail from the company. ( www.microsoft.com/security/antivirus/authenticate_mail.asp)
Antivirus companies have recommended that customers update their software as soon as possible to prevent Sober.D infection.
Paul Roberts writes for IDG News Service