Russian hackers infiltrated the Department of Defense's unclassified network earlier this year, Secretary Ashton B. Carter said in a speech at Stanford University Thursday.
The attack has not previously been reported. Carter said attackers discovered a vulnerability in an older network that hadn't been repaired. An elite Defense Department team identified the break-in and began looking for the culprits, he said.
"After learning valuable information about their tactics, we analyzed their network activity, associated it with Russia, and then quickly kicked them off the network, in a way that minimized their chances of returning," Carter said.