North Korea was apparently behind a recent hacking attack that netted a secret U.S.-South Korean plan to defend the peninsula should war break out, South Korea's military intelligence agency reported.
South Korean soldiers during a drill at the town of Paju near the North Korea border.
The suspected hacking occurred late last month when a South Korean military officer failed to remove a USB device when he switched a military computer from a restricted-access intranet to the Internet.
The USB device contained a summary of OPLAN 5027, a highly sensitive war plan prepared by the U.S.-South Korean Combined Forces Command in case of an all-out war with the North. The plan calls for the dispatch of nearly 700,000 U.S. troops to the peninsula should a full-scale war break out with the North.
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The plan also sets wartime operational guidelines for the troops of the two countries. South Korea remains technically at war with the North because the 1950-53 war ended in a truce and not a peace treaty.
The breach occurred via an Internet Protocol (IP) address registered in China, a route North Korea is believed to use when collecting computer files from foreign networks, military officials said.
Using the China-registered IP address, the North staged a high-profile computer attack in July that overwhelmed key government and private Internet sites in South Korea and the United States.
Seoul's spy agency, whose Internet site was also targeted has identified North Korea's telecommunications ministry as the source of the "distributed denial of service" (DDoS) attack in which scores of computers simultaneously try to connect to a single site, paralyzing the server.
Won Sei-Hoon, chief of the National Intelligence Service, told lawmakers that North Korea is believed behind the latest cyber attack that netted the 5027 war plan.
Won said the stolen document was not the full text of the operational plans, but an 11-page file used to brief military officials. He said it did not contain critical information.
The spy agency said more assaults are expected by hackers linked to North Korea, which has engaged in a series of provocative actions. The North has warned it is "fully prepared to engage in any forms of high-level technological warfare."