“As it turns out, data after the signature block isnt checked at all… and it can even override data that came before it.”
Microsoft’s Store DRM signs every license and then ignores whatever comes after the signature. MASSGRAVE found it, kept it quiet as a free Windows activation method, and only published once Cisco TALOS reported the same bug on April 8 and Microsoft patched it. A valid license signing key also sits unobfuscated in clipup.exe. Microsoft built a lock and left the key in the door.