two undocumented x86 instructions in Intel CPUs which completely control microarchitectural state (yes, they can modify microcode)
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EDIT 1: After doing some research myself, when Mark says "[microcode] in MSROM throws #UD if not in Red Unlocked state," this effectively amounts to the fact that your CPU has to be in debug mode, which itself usually requires a compromised Management Engine (in which case, these new opcodes aren't even your biggest worry). #UD is an error on Intel CPUs that means "undefined instruction," and Red Unlocked state usually requires a hacked ME. So, this discovery will likely cause in influx in related security research, but it in itself should not be too detrimental.