According to the Washington Post, which has seen a confidential section of a report prepared by the Defense Science Board for the Pentagon, the designs for more than two dozen advanced US weapons systems were accessed by Chinese hackers. The public version of the report, titled "Resilient Military Systems and the Advanced Cyber Threat," limited itself to detailing in increasingly alarming language the thread posed by lax cybersecurity. It argues that the "DoD is not prepared to defend against this threat" and that "with present capabilities and technology it is not possible to defend with confidence against the most sophisticated cyber attacks."
The confidential portion of the document is perhaps more alarming, as it contains a list of weapons systems that includes the F-35, information related to drones and "electronic warfare," and even US missile defense systems. It's not known exactly when the attacks occurred nor how much information about the weapons systems was taken, but the information could give China a jump start in developing advanced weapons systems of its own. The Defense Science Board report didn't specifically name China in connection with the cyber attacks, but the Washington Post sources senior defense officials who said it was part of a larger campaign sourced out of China.
Both the public version of the report and analysts say that much of the blame for lax security falls at the feet of defense contractors. "In many cases, they don’t know they’ve been hacked until the FBI comes knocking on their door," one unnamed senior military official told the Washington Post. Meanwhile, the US department of Homeland Security has reportedly had a difficult time keeping experts on staff as support for going on the offensive grows. The reports of the attacks put a finer point on Obama administration's response to Chinese hacking— as recently as last month a wide array of options were still on the table.