How much commerce disruption equates an act of war, even if no shots are fired?
I'm pretty sure blockading ports (as in cities where ships go, not network ports) counts, so that's a starting point. But on the other hand, blockades do tend to be backed up by at least threats of actual physical violence, so.... ?
Should the target be a whole nation? Or maybe enough of it to affect their ability to function? How much commerce disruption equates an act of war, even if no shots are fired?
You'd need some pretty good proof and how would you respond? With another cyber attack? Or escalate into a shooting war? Or by disconnecting the attacking country from the internet?