SEOUL—North Korean hackers are suspected to have broken into a South Korean shipyard and stolen military secrets, in a breach made public less than a month after news of a similar attack in which hackers stole joint U.S.-South Korea war plans.

The attack, revealed by a Seoul lawmaker and corroborated by defense-industry sources, targeted Daewoo Shipbuilding & Marine Engineering Co. , which makes warships including submarines.

Hackers stole 60 classified documents including blueprints and technical data for submarines and vessels equipped with Aegis weapon systems, said Kyeong Dae-soo, a lawmaker with the opposition Liberty Korea Party.

The case came to light just weeks after the disclosure of another suspected North Korean hack into Seoul’s defense ministry database in September last year, in which attackers stole sensitive military secrets, including a joint U.S.-South Korean plan detailing how to eliminate the Pyongyang leadership in the event of war.

Some of the leaked information in the Daewoo attack pertains to South Korea’s submarine-launched missile technology and advanced Aegis weapons systems that track and destroy multiple targets, Mr. Kyeong said.


Mr. Kyeong, a key member of the South Korean Parliament’s National Defense Committee, revealed the suspected attack in a prepared written statement for the continuing parliamentary audit this week.

He said the findings had been verified by South Korean military investigators who had looked into the case for the six months through last October before concluding that North Korean hackers had been behind the Daewoo breach.

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Further details on exactly what documents were leaked and how classified they were remain undisclosed.

The South Korean defense ministry declined to comment, citing the sensitivity of the security issue. A spokesman for Daewoo in Seoul said the company had no immediate plan to issue a statement on the case.


Defense sources familiar with the case said Daewoo had been found vulnerable to outside hacking attacks because it had failed to properly insulate its business intranet from the public internet. The breach prompted the firm to beef up safeguards.

Daewoo, the world’s biggest shipyard, has so far signed contracts to build a total of 17 submarines and 44 warships, including Aegis-equipped destroyers, for the South Korean navy.

Write to Kwanwoo Jun at kwanwoo.jun@wsj.com