CAMBRIDGE – Fear of a “cyber Pearl Harbor” first appeared in the 1990s, and for the past two decades, policymakers have worried that hackers could blow up oil pipelines, contaminate the water supply, open floodgates and send airplanes on collision courses by hacking air traffic control systems. In 2012, then-US Secretary of Defense Leon Panetta warned that hackers could “shut down the power grid across large parts of the country.”