Then you've already lost.
The BMC needs to be ideally on a physically isolated network, or at least a separate one that has no route from the outside nor on the machine itself.
So you'd definitely have to have it connected to the internet somehow, even if very indirectly, and in an independent manner (different network with no direct routes).
It is common to keep admin and backup functions on separate network interfaces, on a disconnected network. You have to physically connect to the network in a secure location to use it.