The hacker, using the pseudonym "Dimitri," gained access last week to a backup Web server and posted the message "Hack the planet," a Microsoft spokesman said.
The incident comes just days after a major intrusion on Microsoft's internal networks. Although hackers did not swipe Microsoft's precious software code, the break-in at one of world's largest and most recognized companies again raised important security questions for many corporations.
The latest breach was minor by comparison and was fixed almost immediately, the company said.
Known as a redirect server, the infiltrated computer served only to point Web surfers to a new URL, or Web site address, for a conference and events registration Web site, said Microsoft spokesman Adam Sohn.
The server, which was nearing its scheduled retirement age, suffered from not having received a new software update, or "patch," that was issued Oct. 17, Sohn said. Microsoft has corrected the problem, he added.
"It's a challenge when you run a major network with many servers. Even though this server was near retirement, we would have preferred that it had the patch. It's certainly the exception and not the rule; this one fell through the cracks," Sohn said.