the backdoors are super risky for 3-letters to use for reasons like what if people have offline network wire monitoring equipment.
https://www.reddit.com/r/programming/comments/makszo/two_undocumented_intel_x86_instructions/
But of course there are backdoors people have been finding "oddities" for a while now like the always online intel management engine, which is an embedded cpu that not even your cpu can see that had vulnerabilities in it.
