YellowKey can be triggered simply by merely copying some files to a USB stick and rebooting to the Windows Recovery Environment. We tested this ourselves, and sure enough, not only does it work, it bears all the hallmarks of a backdoor, down to the exploit’s files disappearing from the USB stick after it’s used once.
100% certainty of backdoor. Is bitlocker developed outside of MSFT? Would seem to need MSFT cooperation to implement.