According to Reuters and Japanese newspaper Yomiuri Shimbun, the breach was discovered on August 11, and the investigation mounted by the company revealed the existence of some 80 infected computers in the company headquarters in Tokyo and various R&D and manufacturing sites in Kobe (nuclear power station components), Nagasaki (escort ships) and Nagoya (guided missiles and rocket engines).
A variety of malware has been discovered on said computers, including an information-stealing Trojan. The company has admitted that it is possible that confidential information was stolen by the attackers.
"We've found out that some system information such as IP addresses have been leaked and that's creepy enough," added a company spokesman. "We can't rule out small possibilities of further information leakage but so far crucial data about our products or technologies have been kept safe."
The discovery comes a month after the belief that Japanese defense contractors might also be targeted by hacking attacks was presented in a white paper.
Mitsubishi Heavy Industries is Japan's biggest defense contractor and the consensus is that the information hidden in its systems is well worth going after.