Summary: An Intel CPU in "Red Unlock" mode allows any user-mode code to read and write its microcode. The paper teaches security researchers how to do it. They can use it to discover undocumented Intel CPU internals and functionality.

I don't know if these instructions will be useful in exploits. They require the CPU to be in Red Unlock mode. One known way to enable Red Unlock requires connecting a special cable to the motherboard's USB port and exploiting the Intel Trusted Execution Engine core [0].

There are probably remote exploits via ethernet and the Intel Management Engine.

Could there be some motherboards that shipped with Red Unlock mode permanently enabled? User-mode code running on such machines could trivially root the machine and even escape a hypervisor.

[0] https://github.com/ptresearch/IntelTXE-PoC

thanks for linking to the paper, that contains considerably more information