Microsoft has filed a federal lawsuit against an alleged hacker who broke through its copy protection technology, charging that the mystery developer somehow gained access to its copyrighted source code.

For more than a month, the Redmond, Wash., company has been combating a program released online called FairUse4WM, which successfully stripped anticopying guards from songs downloaded through subscription media services such as Napster or Yahoo Music.

Microsoft has released two successive patches aimed at disabling the tool. The first worked--but the hacker, known only by the pseudonym "Viodentia," quickly found a way around the update, the company alleges. Now the company says this was because the hacker had apparently gained access to copyrighted source code unavailable to previous generations of would-be crackers.

"Our own intellectual property was stolen from us and used to create this tool," said Bonnie MacNaughton, a senior attorney in Microsoft's legal and corporate affairs division. "They obviously had a leg up on any of the other hackers that might be creating circumvention tools from scratch."