Russian government hackers stole information on how the US infiltrates foreign computers and defends itself against cyberattacks, The Wall Street Journal reported Thursday.

The hackers were able to steal the information after a National Security Agency contractor took highly classified information from the agency and put it on his personal computer, multiple sources with knowledge told the Journal.

The contractor was using antivirus software made by Russia-based Kaspersky Lab, the sources said, which was how the hackers were able to target the contractor.

The Journal reported that the incident occurred in 2015 but wasn’t uncovered until last year.