North Korean hackers have breached a South Korean military network and have reportedly stolen military “war plans,” a South Korean newspaper reports.

The hack occurred last September, and had originally been downplayed by South Korean defense officials, saying that no critical information had leaked.

Now an anonymous South Korean government source is telling Chosun Ilbo that an official war plan, dubbed OPLAN 5027, was stolen during that hack. The plan is said to contain the joint Republic of Korea-U.S. military plans should a war break out on the Korean peninsula.

The same source has said that “discussions are still taking place” between Republic of Korea (ROK) and U.S. military commanders with regard to changing the plan now that North Korea has access to the documents, either in its entirety or only certain parts.