In a nutshell: The source code for Microsoft's original Xbox console has found its way to the Internet along with code for an early version of Windows. The significance of the leaks is negligible, however, given the software's age but still, Microsoft doesn't want its source out in the open like this.
According to The Verge, the leaked Xbox material includes the kernel for the OS on the original console which was based on Windows 2000 as well as some build environments, emulators used for testing, the Xbox Development and internal Microsoft documents.
A spokesperson for Microsoft told the site that they are aware of the reports and are investigating.
The Verge claims the Xbox source code has previously been shared between enthusiasts privately and thus, likely isn't going to be helpful in furthering homebrew and emulation efforts.
Also among the leaked material is a near-final version of Windows NT 3.5. The publication said this dump includes all of the necessary build tools to allow enthusiasts to dig deep into the legacy operating system.
Originally launched way back in 1994, Windows NT 3.5 is the second release of Windows NT. Codenamed "Daytona" due to its focus on performance, the OS was supported for several years before being discontinued at the end of 2001. The leak of this material likely isn't a major concern for Microsoft given its age.
Microsoft isn't the only company to find itself in this sort of situation recently. Nintendo suffered a major data leak earlier this month when more than two terabytes of IP including the source code for the Nintendo 64 and GameCube leaked online.