It was revealed that the Microsoft Windows operating system contains a cryptographic key used by the NSA intelligence agency, likely used to allow access to data on any copy of the operating system used worldwide.
source: https://www.heise.de/tp/features/How-NSA-access-was-built-into-Windows-3444341.html
archived source: https://wellthissucks.xyz/1999/09/04/microsoft-windows-contains-a-nsa-backdoor-access-archived-source/