Computer software giant Microsoft says parts of the tightly guarded blueprints of its Windows operating system have been leaked over the internet.
It is the second security worry for Bill Gates' company this week
Microsoft spokesman Tom Pilla said the company did not know how much of the source code had been leaked, or how many people may have access to it.
Hackers with the code could exploit the operating system and access machines running Windows.
It is the second major security headache for Microsoft in a week.
On Tuesday, it said there was a flaw in its latest Windows operating systems that could allow hackers access to many of the hundreds of millions of computers worldwide using those systems.
Hundreds of millions of computers use Microsoft software
The company could not immediately pinpoint the source of leak, and has contacted law enforcement authorities.
It is thought to comprise parts of the source code for Windows 2000 and Windows NT.
Mr Pilla said there was no indication the code leak was a result of a breach of Microsoft's corporate network.
There was no known immediate impact to Microsoft customers, he said.
But the other threat to Microsoft is the fact that such access could provide a competitive edge to its rivals, who would gain a much better understanding of the inner workings of Microsoft's technology.