Zen and the art of microcode hacking

Now that we have examined the vulnerability that enables arbitrary microcode patches to be installed on all (un-patched) Zen 1 through Zen 4 CPUs, let's discuss how you can use and expand our tools to author your own patches. We have been working on developing a collection of tools combined into a single project we're calling zentool. The long-term goal is to provide a suite of capabilities similar to binutils, but targeting AMD microcode instead of CPU machine code. You can find the project source code here along with documentation on how to use the tools.

Google's Bug Hunters website

I just read a whole bunch of words, but I barely understand what's going on. The general, very simplified gist is that the researchers discovered a way for an attacker with local administrator privileges to load arbitrary microcode onto AMD Zen 1-4 processors.