The information targeted by the cyberspies could help adversaries mount defenses against the fighter jet. The most sensitive information, however, was not breached because it is stored on computers not attached to the Internet, according to the WSJ.

The article noted that it was likely that the intruders entered through vulnerabilities in the networks of the contractors involved in building the aircraft. The WSJ reported that Pentagon insiders are pointing the finger at China as in some way being responsible for the cyberattack.

Lockheed Martin Corp. (2008 VAR500 rank 7) is the lead contractor for the Joint Strike Fighter program. Northrop Grumman Corp. and BAE Systems PLC also have major roles in the project, also known as the F-35 Lightning II project. Lockheed's Web site describes the program as the "focal point for defining affordable next-generation strike aircraft weapon systems for the Navy, Air Force, Marines and our allies." Last month, the United Kingdom announced it would purchase the aircraft.

The U.S. is increasingly growing concerned about cyberattacks. In February, UN secretary-general Ban Ki-moon said that cyber-weapons will be included on the list of arms falling under the auspices of the UN's Advisory Board on Disarmament Matters. At the time, Ban was quoted as saying recent breaches of critical systems represent "a clear and present threat to international security."

In response, the Pentagon is developing the National Cyber Range program, which will not only have the hardware that might be used to inflict cyber-attacks, but also will imitate likely actions of the aggressors. The Cyber Range comprises part of the government's Comprehensive National Cybersecurity Initiative, launched last year.