A single virus-infected PC caused confidential security information relating to a power station to leak on to the Internet, Japanese media have reported.
It is believed that files detailing information on the Chubu Electric Power Companys security procedures and plant layout were compromised by an unauthorised file-sharing program, Share.
Other compromised data included the names and addresses of security staff, and the location of the plants control room.
The virus has not been named, but is believed to have inadvertently infected the PC of a single member of staff. Ironically, this staff member was working for an out-sourced security firm supposed to guard the plant.
The company also runs nuclear power stations in Japan, though the plant affected in the incident was not in this category of risk.
Last year, however, a more potentially serious breach occurred year when Mitsubishi Electric leaked 40 megabytes of data, some of which related to a nuclear power station in Tsuruga. Again, the culprit was a single PC using a P2P program that allowed a virus to sneak through conventional data defences.