Japanese defence officials are investigating a reported penetration of the country's high-speed Defence Information Infrastructure (DII) network.
The attacks, which Bloomberg attributes to a possible state-based actor, took place in September but have only now come to light.
The DII network is shared by the country's Defence Ministry and its Self-Defence Forces, and according to the South China Morning Post, that allowed the intruders to also penetrate the Ground Self-Defence Force.
The SCMP story says the discovery was confirmed by unnamed ministry officials on Sunday morning.
The attacker first got access to a network shared between Japan's National Defence Academy and its National Defence Medical College, which provided access to the DII network.
The Defence Information Infrastructure comprises two networks, one connected to the Internet, and a second more-protected internal network.
Penetrating into the private network is why the reports believe it was sophisticated enough to justify the “state actor” tag.
The incident led to a temporary ban on personnel on Self-Defence Force personnel using the Internet. ®