It's certainly possible to achieve serious security but probably not practical for most private entities. I've spent most of my development career making softwa

It's certainly possible to achieve serious security but probably not practical for most private entities. I've spent most of my development career making software for the US intelligence community and their systems were definitely not going to get broken into by a ransomware gang. Security measures include multilevel air gapping plus heavily armed physical security, six foot thick concrete walls set back from the street by other concrete barriers, locating facilities on military installations, disabling USB ports on most devices, banning anything radio enabled from being anywhere near your workstations, jamming radio signals anyway, severely punishing, possibly executing, anyone caught working as an intentional insider threat, requiring multiple persons in the custody and approval chains to move any files from one network to another via write-once media like DVDs, having the transfer media itself in a separate locked cabinet in a separate locked room inside the actual classified vault serving as an office. Installing and running everything in a separately sandboxed staging environment even after it gets through all the walls and air gaps and DVDs and running it through some fairly extensive testing and analysis before putting it anywhere near a production system. Clearly, you can never make it literally impossible, but to my knowledge, nobody has ever managed to get malicious software onto a classified production system. Information leaks are, of course, another story.