An NSA contractor with highly classified material had his computer “backdoored” after downloading a pirated copy of Microsoft Office, according to a Russian cybersecurity firm.

Kremlin-backed hackers were able to steal information on American cyber defenses from a worker who brought tools onto his personal computer, the Wall Street Journal first reported earlier this month.

Sourced told the newspaper that the attackers were able to find the files by going through the Kaspersky Lab anti-virus software, though the company has denied working with the Russian government and issued an explanation of its actions on Wednesday.

It said that the unidentified contractor had turned off his anti-virus protection in 2014 before downloading a version of Microsoft Office with an infected code to make a “full blown backdoor which may have allowed third parties access to the user’s machine.”

Kaspersky said that it detected the malware when the anti-virus protection was turned back on, but also found files from the Equation Group hackers attributed to the National Security Agency.

The NSA says that its policy is not to comment on “personnel matters.”

Kaspersky did not say whether it told American authorities about the detection of the sensitive hacking tools on the worker’s computer.

The lab maintained Wednesday that the files were deleted from its system after an analyst told the CEO, though the company did publish a report on the Equation Group in early 2015 calling it the “mothership.”

Kaspersky said Wednesday that it was not deliberately looking for U.S. security service files based on words such as “top secret” and “classified.”

It said added that it had not seen any intrusions into its network other than a program linked to Israel, a country that the New York Times reported told the U.S. that Russian hackers were using Kaspersky to search for American government material.

Kaspersky’s explanation come as the company has already come under scrutiny in the wake of deteriorating U.S.-Russian relations and allegations that Moscow meddled in the 2016 election to benefit President Trump.

The Department of Homeland Security last month banned all government departments and agencies from using Kaspersky software.

Kaspersky has denied working at the behest of the Russian government, though Russian newspaper Kommersant reported earlier this year that a leader in its cybercrime division, Ruslan Stoyanov, had been arrested in an investigation that reportedly centers on giving classified information to American intelligence.

The company told the Daily News in January that Stoyanov's case involves a period before he worked for Kaspersky.