More than two dozen advanced weapons systems were accessed, although documents obtained by The Washington Post do not indicate whether the breaches occurred on government or contractor networks.
The designs for some of the most sensitive advanced U.S. weapons systems were reportedly accessed by Chinese hackers, according to a confidential report prepared by Defense Science Board for the Pentagon.
The report, a copy of which was obtained by The Washington Post, listed more than two dozen weapons systems, including missile defenses and combat aircraft. The security breaches, the latest linked to China, allows the country to accelerate development of its own systems and weakens the U.S. military position, experts told the Post.
The public version of the report, titled "Resilient Military Systems and the Advanced Cyber Threat," (PDF) called the cyber threat "serious," likening the potential situation to "the nuclear threat of the Cold War."
"[The Defense Department] is not prepared to defend against this threat," the report said. "With present capabilities and technology it is not possible to defend with confidence against the most sophisticated cyber attacks."
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Some of the designs accessed by hackers included an advanced Patriot missile system, known as PAC-3, as well as those for the F/A-18 fighter jet, the V-22 Osprey, and the Black Hawk helicopter, the Post reported.
The documents did not indicate when or where the intrusions occurred, whether they were via government or contractor computer networks.
"In many cases, they don't know they've been hacked until the FBI comes knocking on their door," an unidentified senior military official told the Post. "This is billions of dollars of combat advantage for China. They've just saved themselves 25 years of research and development."
The report emerges as the Pentagon steadily increases the tenor of allegations that the Chinese government and military have engaged in widespread cyberespionage targeting U.S. government and business computer networks. In its annual report to Congress on China's military earlier this month, the Pentagon accused China of masterminding a steady campaign of computer intrusions in 2012 that were designed to acquire information about the U.S. government's foreign policy and military plans.
In March, the Obama administration demanded that China end its "unprecedented" campaign of cyberespionage, warning that the hacking activity threatens to derail efforts to build stronger ties between the two countries.
A recent report alleged that an "overwhelming percentage " of cyberattacks on U.S. corporations, government agencies, and organizations originate from an office building on the outskirts of Shanghai that's connected to the People's Liberation Army.
China has denied any involvement and condemned the report for lack of hard evidence.
