News about the breach first came out back in September when South Korean lawmaker Kim Jin-pyo revealed that a malicious code exploited a vulnerability in the military's routing server. He said the chances of the infiltrators stealing confidential info are "very low," since the command's computers use an intranet that's not connected to that server. Based on this new info, that wasn't the case -- South Korean authorities still aren't sure which among its confidential documents the hackers got a hold of, though.
North Korea is known to have thousands of personnel ready to wage cyberwarfare. The NSA believes it was responsible attacking Sony Entertainment's computers in 2014, which led to a huge info and unreleased movie dump online. In June, reports came out that the country stole US fighter jet blueprints. SK also blamed a recent cyberattack that compromised the phones of security officials on the North.