The most useful quote to those familiar with the linux boot process:

“An attacker would need to be able to coerce a system into booting from HTTP if it’s not already doing so, and either be in a position to run the HTTP server in question or MITM traffic to it,” Matthew Garrett, a security developer and one of the original shim authors, wrote in an online interview. “An attacker (physically present or who has already compromised root on the system) could use this to subvert secure boot (add a new boot entry to a server they control, compromise shim, execute arbitrary code).”