As reported by The Washington Post, US officials have claimed that up to 614 Gigabytes of information was stolen, including signal and sensor data, as well as submarine radio information relating to cryptographic systems.
Plans for supersonic missiles which are due to be utilized by US submarines by 2020 were also compromised during the attack.
However, the most critical datasets stolen relate to a mission called Sea Dragon. While little is known about the project, the US Defence Department has described Sea Dragon as research into "disruptive offensive capabilities" by "integrating an existing weapon system with an existing Navy platform."
However, no details on the project's status have been revealed since 2016 in budget documents beyond plans for a "sea-based tactical demonstration" by the end of the 2018 financial year.
An electronic warfare library was also reportedly compromised. If this is the case, hundreds of mechanical and software-based systems may have been placed at risk.
The cyberattack took place across January and February this year. The unnamed contractor that was targeted worked with the Naval Undersea Warfare Center, a research establishment in Newport, Rhode Island.
While the data -- once compiled -- could be considered classified, the information was reportedly stored on an unclassified network and would otherwise be considered unclassified, according to officials.
"There are measures in place that require companies to notify the government when a 'cyber incident' has occurred that has actual or potential adverse effects on their networks that contain controlled unclassified information," Navy spokesman Commander Bill Speaks told the Washington Post.
However, Speaks would not reveal any further details of the security incident.
The US Navy and FBI are believed to be investigating the espionage case.
We do not hear about every example of military espionage from all sides and states. However, with Chinese missiles reappearing in the South China Sea, it seems the country would take any advantage possible to secure the area and strengthen its dominion over the disputed territory.
The maritime industry, in any form, appears to be just as vulnerable to cyberattacks as any other. Last week, security researcher ken Munro warned that a commonly-used system for navigation, the Electronic Chart Display (Ecdis), is vulnerable to exploit.