Antivirus software companies warned customers about a new variant of the Sober e-mail worm, which began spreading on the Internet Monday and masquerades as a Microsoft Corp. software update.

Sober.D is the latest version of a worm that first appeared in October. The new worm poses as a software patch that will remove the MyDoom virus from infected Windows systems, said antivirus company F-Secure Corp. of Helsinki.

F-Secure first detected the new worm variant in Germany early Monday. The company rated the virus a “Level 2” threat, indicating “large infections,” the company said.

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, inserting 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!” said the antivirus company. 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, antivirus companies said.

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,” F-Secure said.

It is common for worm authors to dress up their creations as software updates from Microsoft. Recent worms including “Swen” and “Gruel” have posed as official correspondences from the Redmond, Wash., company.

In response, Microsoft publicly declared that it never distributes software updates using e-mail and has published guidelines for identifying legitimate e-mail from the company. Antivirus companies posted software updates Monday to detect the new worm and recommended that customers update their software as soon as possible to prevent against Sober.D infection.

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