The Japanese Defence Ministry and the Self-Defence Forces discovered in September that their shared communication network had suffered a cyberattack that enabled a hacker to penetrate the Ground Self-Defense Force’s computer system, ministry sources said on Sunday. Some information may have been leaked in the incident, with an organised attacker such as a nation state strongly suspected, but the full scope of the damage is not clear, the sources said. Acknowledging that the attack, which penetrated the system through a security loophole, was highly skilled, the ministry immediately raised its cybersecurity alert level. Masakazu Saito, a senior ministry official in charge of cybersecurity issues, declined to comment on the matter. A senior SDF official said, “It is a very serious situation. We must quickly take measures to prevent a recurrence.” According to the sources, the Defence Information Infrastructure, a high-speed large-capacity communication network connecting SDF bases and camps, was subject to the attack. They also said the hacker appears to have gained unauthorised access to computers at the National Defence Academy and the National Defence Medical College, using them as a gateway to enter the GSDF’s computer system. The Defence Information Infrastructure is formed by a system connected to the Internet and another that is used for information sharing by people inside the organisations. While the two systems are operated separately to safeguard computers against virus, they are not completely detached. The hacker is believed to have used a link between the two systems to carry out the attack, the sources said. One computer network expert said the attack must have been highly sophisticated considering the network’s high level of protection. Following the incident, the ministry and the SDF temporarily banned Internet use among their personnel. The latest incident has sent shock waves inside the ministry as it occurred despite efforts to step up measures against cyberattacks, such as the setting up of a Cyber Defence Group in March 2014 to monitor the ministry and SDF communication networks around the clock. It has also installed an advanced analysis device for cyber defence.