The computer is on a private IP network. So the only way in, so far as I know, is via the connection used by Bitcoin Core. Although it does get access to the outside world through other services such as NTP and whatever the heck Apple uses for software update.

My security plan, if it can be called that, is if any BTC of significant quantity is successfully mined, it gets moved into a cold storage wallet.

Anyone getting shell access to that machine would have a field day as they are now on my private LAN.