Russian hackers stole "highly classified" NSA files laying out how the agency combats cyberattacks and spies on other countries' networks, according to a new report in The Wall Street Journal.

The hackers reportedly found the files via Kaspersky's antivirus software after an National Security Agency contractor put the files on his home computer.

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The attack, which happened in 2015 though it was only discovered last year, "is considered by experts to be one of the most significant security breaches in recent years." The files reportedly lay out key parts of the NSA's strategy for spying and defending itself against cyberattack.

As the WSJnotes, the stolen data could have big implications for Russia's ability to both attack U.S. networks and defend itself from the NSA.

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Having such information could give the Russian government information on how to protect its own networks, making it more difficult for the NSA to conduct its work. It also could give the Russians methods to infiltrate the networks of the U.S. and other nations, these people said.

The report also goes a long way toward explaining government officials' recent concerns over Kaspersky's software. The Russian company's software was banned from U.S. government agencies last month after a report in Bloomberg alleged the company had been working closely with the Russian government for years.

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In a statement, Kaspersky CEO Eugene Kaspersky said his company "has not been provided any evidence substantiating the company's involvement in the alleged incident."

"The only conclusion sees to be that Kaspersky Lab is caught in the middle of a geopolitical fight."

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Others in the cybersecurity community were quick to point out that the WSJ's report stops short of suggesting that Kaspersky worked directly with the Russian government on the hack. Instead, it's possible that the Russians exploited vulnerabilities within Kaspersky's software to get the data.

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Either way, the breach is the latest headache for the NSA, which has faced criticism over its handling of repeated leaks.

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