A Chinese hacker remotely accessed U.S. Treasury Department (UST) work stations and unclassified documents, in what officials are describing as a "major incident." 

According to CNN, the UST first learned of the breach on December 8 from third-party software provider BeyondTrust, after the company discovered that a hacker had stolen a security key the Treasury Department typically uses for technical support. Using that key, the hacker was then able to override BeyondTrust's security, and remotely access documents from a handful of employee work stations. 

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In a letter to lawmakers alerting them to the incident, the UST attributed the hack to a "state-sponsored actor," which a spokesperson for China's Foreign Ministry denied in a December 31 briefing. The Treasury Department also said that it doesn't believe the hacker has continued to access its systems since the initial breach, or that any malicious code was inserted. 

The UST will next work with the FBI and the Cybersecurity and Infrastructure Security Agency to assess any other potential impacts from the breach. The systems that were hacked have been taken offline in the meantime, with the Treasury Department planning to release more details on the incident in a briefing to Congress sometime within the next month.