Russian government workers were able to steal information about America's foreign computer networks and cyberattack defense systems from a National Security Agency contractor's home computer after the employee transferred the highly classified material to his home computer, according to a report Thursday.

The hacking has yet to be formally disclosed but could be among the most significant security breaches in recent history. U.S. officials believe the hackers were able to go after the contractor because he or she used a Russian-made antivirus software by Kaspersky Lab.

The NSA would not confirm to the Wall Street Journal whether the incident took place.

Kaspersky, which claims to have more than 400 million users worldwide, said it "has not been provided any information or evidence substantiating this alleged incident, and as a result, we must assume that this is another example of a false accusation."

The incident took place in 2015 but was not discovered until early 2016. Hackers stole information about the NSA's ability to break into foreign computer networks, computer coding it uses for spying, and how it protects domestic networks.

It marks the third publicly reported breach of an NSA contractor and comes after multiple U.S. intelligence agencies have confirmed Russia tried to interfere technologically in the 2016 presidential election.