Ash Carter Pentagon

The United States on Thursday disclosed a cyber intrusion this year by Russian hackers who accessed an unclassified U.S. military network, in a episode Defense Secretary Ash Carter said showed the growing threat and the improving U.S. ability to respond.

Carter cited the newly declassified incident during an address at Stanford University, in which he also warned the Pentagon was ready to help defend America’s networks and to use cyber weaponry, if needed.

The doctrine was articulated in a new Pentagon cyber strategy unveiled on Thursday. Reuters obtained a copy on Wednesday.

“Adversaries should know that our preference for deterrence and our defensive posture don’t diminish our willingness to use cyber options if necessary,” Carter said in prepared remarks.

Carter said that sensors guarding the Pentagon’s unclassified networks detected the intrusion by Russian hackers, who discovered an old vulnerability that had not been patched.

“While it’s worrisome they achieved some unauthorized access to our unclassified network, we quickly identified the compromise and had a crack team of incident responders hunting the intruders within 24 hours,” Carter 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,” he added.