Undocumented x86 instructions in Intel CPUs that can modify microcode
The followup tweet indicates that the CPU has to be in an unlocked state before this is possible, which on a typical system requires there to be a Management Engine vulnerability first. Given what we currently know, this is going to be interesting for people interested in researching the behaviour and security of Intel CPUs and might well lead to discovery of security issues in future, but in itself I don't think this is dangerous.Edit to add: https://www.intel.com/content/dam/www/public/us/en/security-... is a discussion of a previous Intel security issue which includes a description (on page 6) of the different unlock levels. This apparently requires that the CPU be in the Red unlock state, which (in the absence of ME vulnerabilities) should only be accessible by Intel.