Why is the US industrial critical infrastructure vulnerable to cyber-attack?
The answer to that question depends on how you defined a cyber-attack. In Western military doctrine, we tend to define cyber-attack, cyber operations as limited narrowly to computer viruses and computer hacking. Our potential adversaries; Russia, China, North Korea, Iran; have a much broader definition of cyber-attack that includes not only computer viruses and hacking, but it includes kinetic attacks, sabotage, all the way up the spectrum to nuclear electromagnetic pulse attack. A high altitude detonation of a nuclear weapon that causes an electromagnetic pulse, that’s a super-energetic radio wave that would fry electronics and destroy critical infrastructures of all kinds.
The short answer to the question is, all of the above if we’re talking about a cyber-attack as defined by our enemies. A nuclear EMP attack would collapse all of those critical infrastructures. The most important of them and the most vulnerable to EMP would be the electric power grid which by anyone’s definition is the most important of the critical infrastructures because all of the other critical infrastructures require electricity in order to operate and survive. Within 72 hours of the loss of the electric power grid, basically all of the critical infrastructures would collapse; communication, transportation, band and finance, even food and water. You basically cease to be a modern society