The offensive cyber arms race makes it even more urgent to think about defenses. The United States is still seriously vulnerable, as are other nations. We have deeply embedded network technology in every facet of our economy and our lives, and it has been under constant assault in recent years. So far, the attacks have been largely aimed at theft, disruption and spying, but it will get worse. We live in a mammoth glass house and ought to be mindful of the dangers when we throw stones.