Joel Rees:
Just for the record, if microcode can play with the CPU internal system state, say, the bits that specify whether the CPU is in supervisor or user state, there's not much to stop, for instance, a microcode update from providing an undocumented trap to a hidden routine in the BIOS that does deep packet inspection and arbitrary forwarding.
http://security.stackexchange.com/questions/29730/processor-microcode-manipulation-to-change-opcodes
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