It's also conceivable that a state-level adversary could have hidden arbitrary DMA instructions in a NIC firmware, that only activate with a signed request embedded in a random packet. Some of the largest firmware blobs on Linux systems are for NICs.
Most people aren't facing state-level targeted attacks, but without open firmware, it's nearly impossible to know for sure if one is vulnerable. And, with botnets and worms, it only takes one non-state-level attacker discovering the backdoor for everyone to be affected.
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This attack could be used indefinite times to compromise the Intel’s AMT remote provisioning process and subverts the security of the non configured PCs that include the AMT functionality even while it is disabled within the BIOS configuration as presented in section 3.7.6.
AMT is always active, even if you've set it to "Disabled" and can be remotely activated. Again, without your authorization.