Source code is the intellectual property and lifeblood of any software company, since it is the basic language used to create software programs. Microsoft has only shared its source code with close partners and carefully chosen organizations, with legal agreements that threaten litigation in the event of that any of is leaked. The level of secrecy around the code is akin to that around the formula used in Coca-Cola Co.'s soft drinks.
"It's illegal for third parties to post Microsoft source code, and we take such activity very seriously," Microsoft spokesman Tom Pilla said in a statement.
"We are currently investigating these postings and are working with the appropriate law-enforcement authorities."
The code appeared to come from an unidentified software developer with access to Microsoft's code. Software companies that create programs running on Windows need access to source code to build their own products.
One main risk in having source code exposed to the public is the possibility that hackers could break into computers running Windows NT or Windows 2000 and destroy or steal data.
Although the company said that was unlikely, given the relatively small portion of code that had been circulating, a greater risk could come from others using the code as a base for developing software that competes with Windows.
Redmond, Washington-based Microsoft is the world's biggest software company. More than 9 out of 10 personal computers, runs on its software.
Microsoft said that it was working with the Federal Bureau of Investigation and legal authorities to try and track the origin of the source code leak.
"At this time there is no known impact on customers," Pilla said, "We will continue to monitor the situation."