Reading into the history of Secure Boot. Discovered Intel and AMD processors have back doors via Intel Management Engine [1] and AMD Platform Security Processor [2]. Both are closed source and have had a number of vulnerabilities over the years. They are essentially backdoors.
Seems disabling these "features" is nearly impossible as well.
[1] https://en.m.wikipedia.org/wiki/Intel_Management_Engine
[2] https://en.m.wikipedia.org/wiki/AMD_Platform_Security_Proces...