Russian hackers successfully breached US Department of Defence (DoD) systems by exploiting an unpatched flaw in one of the department's legacy systems.
US secretary of defence Ash Carter revealed the breach during a speec at Stanford University on Thursday.
"Earlier this year, the sensors that guard the DoD's unclassified networks detected Russian hackers accessing one of our networks. They'd discovered an old vulnerability in one of our legacy networks that hadn't been patched," he said.
Carter explained that, while the hackers did get in, the DoD was able to quickly shut them down and gain "valuable" intelligence on the attack.
"While it's worrisome they achieved some unauthorised access to our unclassified network, we quickly identified the compromise and had a team of incident responders hunting down the intruders within 24 hours," he said.
"After learning valuable information about their tactics, we analysed their network activity, associated it with Russia, and then quickly kicked them off the network in a way that minimised their chances of returning."
Disturbingly, Carter added that "this was only one attack that we found" indicating that there have been other security incidents on the DoD network.
The DoD has since been working to improve security and has launched a variety of initiatives, including replacing a fragmented series of networks with a "single security architecture that's more easily defendable", according to Carter.
He explained that the move is part of a wider directive to consolidate government IT systems.
"Just this week I directed that we consolidate all of our IT services in DoD and throughout the Washington capital region, which will not only help improve our overall cyber security, but save millions of dollars we can better spend elsewhere," he said.
Other initiatives included bolstering the nation's cyber reserves programme and launching a Defence Innovative Unit Experimental that will be staffed by the DoD and "key people from the reserves".
Creating a cyber reserves force is an ongoing goal of many nations' militaries. The UK Ministry of Defence pledged to recruit hundreds of computer experts for a special cyber reserve task force in September 2013.
The US move is part of a wider strategy to increase collaboration between the public and private sectors when combating cyber threats.
Carter explained that increased collaboration is needed as the sophistication and number of state-sponsored cyber attacks targeting the nation grows.
"Nations like Russia and China have been pursuing long-term and comprehensive military modernisation programmes to close the technology gap with the US," he said.
"They've been working on new counter-space, cyber, electronic warfare, undersea, and air attack capabilities that challenge our own."
He added that it is only a matter of time before terrorist groups also start developing offensive cyber capabilities.
"Low-cost and global proliferation of malware have lowered barriers to entry and made it easier for smaller malicious actors to strike in cyber space," he said.
"Just as Russia and China have advanced cyber capabilities and strategies ranging from stealthy network penetration to intellectual property theft, criminal and terrorist networks are also increasing their cyber operations."
The news follows concerns in industry about US government agencies' mass surveillance operations.
The concerns erupted after whistleblower Edward Snowden leaked documents to the press proving that departments, including the US National Security Agency, are siphoning vast amounts of data from technology companies including Facebook, Microsoft, Google, Yahoo and Apple.
Carter moved to allay businesses concerns, arguing that the US government has already taken action to stop blanket surveillance operations.
"We share many of the same underlying objectives and values," he said. "As our government has demonstrated in recent trade negotiations, diplomacy, and decisions on net neutrality, we are strong proponents of a free and open internet, and strong supporters of protecting intellectual property rights."
Carter is one of many US officials calling for increased collaboration between the public and private sector.
US Secretary of Homeland Security Jeh Johnson similarly moved to allay businesses' PRISM concerns while announcing the opening of a cyber security base of operations in Silicon Valley on 22 April.
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