The source code of the original Xbox contains the system kernel (a customized version of Windows 2000) as well as the Xbox Development Kit and testing emulator software required for development, and internal materials.
Developers of unofficial emulators have for a long time been endeavoring to replicate the original Xbox kernel. However, of the roughly 900 Xbox titles that were released, only around 40 titles run on unofficial emulators (and even then, not perfectly). Microsoft has configured the current Xbox One to play original Xbox games.
But according to The Verge, it has been confirmed that this kernel and source code were already circulating privately within the fan community. The Verge therefore suggests that this new leak is not that significant in terms of its potential impact on emulation and homemade consoles. But it could attract the interest of people who are interested in developing homebrew titles for the original Xbox.
Along with the original Xbox source code, there's also a confirmed leak of Windows NT 3.5. This too includes everything from the source code to development tools. However, support for Windows NT 3.5 ended in December 2001, and cases of this OS being used now are surely few and far between. So it seems unlikely that the leak of this source code would pose any serious security problem.
Actually, some of the Windows NT 4.0 and Windows 2000 source code was leaked back in 2004. And in 2017 there was also a partial leak of the Windows 10 source code.
This article was originally written in Japanese. All images and content are directly from the Japanese version at the time of publication.