As far as we know, no one has ever deliberately hacked into the U.S. electrical grid and pulled the plug on millions or even thousands of people. Just as on Sept. 10, 2001, no one had ever deliberately crashed a jet airliner into a skyscraper. Is the power grid vulnerable to cyberattack? What about natural gas pipelines, nuclear plants, and water systems? Or refineries and other industrial facilities that run on similar Internet-enabled digital control systems? Could a terrorist or disgruntled employee cause lethal accidents and millions of dollars of damage? What about a bored 14-year-old? "Are we vulnerable?" asked...