Chinese hackers breached the White House Military Office (WHMO) computer system in early September. This system contains highly sensitive information on U.S. nuclear codes.
This network not only contains nuclear commands, but also presidential travel details, communications, and other sensitive information reserved for "senior policy and intelligence officials." The WHMO also contains the secret information contained in the "nuclear football," which stays with the president at all times in the event that nuclear launches are necessary.
Because no one knows how much information such a breach could uncover, it is unclear how much information might have been seen by the Chinese hackers.
Anonymous officials believe this was the work of Chinese Military Cyber Warfare Specialists.