More than 600GB of high-sensitive data belonging to the United States Navy has been compromised in an operation reportedly conducted by hackers working for the Chinese government.

The breach, first reported by The Washington Post, occurred in the months of January and February and the target was a contractor working for the Naval Undersea Warfare Center — US Navy’s full-spectrum group for the research, development, test, and evaluation of submarines, autonomous underwater systems, and offensive and defensive underwater weapon systems.

Though the unnamed U.S. officials, who reported the breach to The Post, did not reveal details of the contractor and said the data was hosted on an unclassified network, they stressed that when compiled, such a massive trove can be considered highly sensitive.

Read more