The US Department of Defense (DoD) has admitted that hackers stole over 24,000 data files from a compromised computer network in a single attack carried out in March.
The attack was part of a larger campaign against the DoD systems, and hackers are also trying to exploit the department's communications and satellite systems, according to US deputy secretary of defense William Lynn.
The most skilled hacking attempts are coming from nation states, rather than hacking groups, he warned.
"We have a pretty good idea who did it," Lynn told Associated Press in an interview, but he refused to elaborate.
The news came as the US government has been outlining its online defence strategy. The country spends 90 per cent of its cyber security budget on defence and 10 per cent on offence, but this ratio needs to be reversed, according to Marine General James Cartwright, vice chairman of the Joint Chiefs of Staff.
Wasim Ahmad, vice president at Voltage Security, agreed that going on the offensive is essential if the US is to defefend itself effectively, but added that protecting vital data is crucial.
"Traditional passive security systems that focus exclusively on protecting the IT infrastructure must be augmented with security that focuses on protecting the data itself," he said.
"Even the Pentagon must assume that hackers will eventually get through to core data. And that data needs to be rendered useless to attackers by using data encryption solutions."
The new DoD plans suggest that the online world will need to be addressed in the same way as land, air, water and space combat.
The military will not rule out using physical force in response to hacking attacks if they cause "massive damage, massive human losses [or] significant economic damage", Lynn said.
Stewart Baker, a former assistant secretary at the Department of Homeland Security, said that the plan had merit but lacks key details.
"It's an incomplete description of a defensive strategy. If the Pentagon announced that our nuclear warfare strategy centred on fallout shelters, we'd all hope that they had a real strategy that was better than that," he said.
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