Windows source code

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.

Background
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.

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:

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.

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.

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. The source code contained the Candy visual style, which was based on the macOS Aqua theme.