Windows source code

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In February 2004, the source codes for the Windows NT 4.0 and Windows 2000 Service Pack 1 products were released and made available for illegal download on the Internet. This event received widespread news coverage by many technology journals since it was not only a breach of Microsoft's security but also in violation of intellectual property laws.


Mainsoft was a software company based in Israel which was responsible for the development of computer software which was designed to allow users to port Windows applications to the UNIX platform. They entered into an agreement with Microsoft in March 1994 to receive access to the Microsoft Windows source code to aid in the development of their products.

Source code leaks

NT 4.0 and 2000 SP1

On February 12th, 2004, the source codes of Microsoft Windows NT 4.0 and Microsoft Windows 2000 Service Pack 1 were leaked onto the Internet and subsequently made available for illegal download.

While the leak originated from Mainsoft, the company never authorized the leak at all, and in any sense was still standing behind Microsoft's efforts to see to it that the leaked source code was removed and the users responsible dealt with accordingly.

In response to this, Microsoft released the following statement:

REDMOND, Wash., Updated, Feb. 20, 2004 — On Thursday, February 12, Microsoft became aware that portions of the Microsoft Windows 2000 and Windows NT 4.0 source code were illegally made available on the Internet. Subsequent investigation has shown this was not the result of any breach of Microsofts corporate network or internal security, nor is it related to Microsofts Shared Source Initiative or its Government Security Program, which enable our customers and partners, as well as governments, to legally access Microsoft source code. Microsoft reaffirms its support for both the Shared Source Initiative and the Government Security Program.

Microsoft continues to work closely with the U.S. Federal Bureau of Investigation and other law enforcement authorities on this matter. Microsoft source code is both copyrighted and protected as a trade secret. As such, it is illegal to post it, make it available to others, download it or use it. Microsoft will take all appropriate legal actions to protect its intellectual property. These actions include communicating both directly and indirectly with those who possess or seek to possess, post, download or share the illegally disclosed source code.

Specifically, Microsoft is sending letters explaining to individuals who have already downloaded the source code that such actions are in violation of the law. Additionally, Microsoft has instituted the use of alerts on several peer-to-peer clients where such illegal sharing of the source code has taken place. These alerts are designed to inform any user who conducts specific searches on these networks to locate and download the source code that such activity is illegal.

Questions about the ongoing investigation should be referred to the FBI.


Shortly following the source code leaks, the Federal Bureau of Investigation (FBI) was also on the lookout for the individual responsible for the source code leaks.[2]

While the products mentioned above have since gone out of support, the source code contained within is still protected as a trade secret and is still not permitted to be republished in any way, shape, or form. As such, Microsoft has continued to issue requests to sites such as GitHub to remove the source code as recently as April 2015.[3][4][5]

XP SP1 and Server 2003

On September 23rd, 2020, the Windows XP Service Pack 1 and Windows Server 2003 source code has been leaked to 4chan.[6] The source code contained the Candy visual style, which was based on the macOS Aqua theme.[7]