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In contrast to state-of-the-art conventional weapons systems, cutting-edge cybercapabilities are easy and cheap to acquire. “A small number of highly trained programmers, using off-the-shelf equipment, can develop toxic tools and deploy them to great effect,” Mr. Lynn said.

Moreover, cybercapabilities are spreading, he said. Other countries could be deterred from attacking, even with deniable cyberweapons, by U.S. military power.

“So even though nation-states are the most capable actors, they are the least likely to initiate a destructive [cyber]-attack,” he said. “Terrorist groups, however, have no such hesitation.”

“So in cyber, we have a window of opportunity to act before the most malicious actors acquire the most destructive technologies,” he said.