Antivirus software companies warned customers about a new variant of the Sober e-mail worm, which has begun spreading on the Internet this week masquerading as a Microsoft software update. It is common for worm authors to dress up their creations as Microsoft software updates. Recent worms including “Swen” and “Gruel” have posed as official correspondence from Microsoft.

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