Russian-based Kaspersky software believed to been used to take classified NSA data

Russian-based Kaspersky Lab software was believed to have been used to take very sensitive and classified NSA data from an NSA contractor's personal computer, CBS News confirmed Thursday, resulting in a significant security breach.

First reported by the Wall Street Journal Thursday, the 2015 hack occurred when the contractor took the data with him from the NSA and then loaded it onto his personal computer, which had Kaspersky antivirus software on it. The software enabled Russian hackers to see his files. The hack has still not been disclosed by the government, according to the Wall Street Journal 

In a statement, Kaspersky said it had not been "provided any evidence substantiating the company's involvement in the alleged incident reported by the Wall Street Journal and it is unfortunate that news coverage of unproven claims continue to perpetuate accusations against the company.

Kaspersky also insisted that it had no "inappropriate" ties to any government, and said it would "make no apologies for being aggressive in the battle against malware and cybercriminals."

The Washington Post reported that the contractor, a U.S. citizen born in Vietnam, worked for the division of the NSA that builds hacking tools designed to break into computers overseas for the purpose of intelligence gathering. The employee is thought to have taken the material not for malicious reasons, but for the purpose of continuing his work at home. 

The data that was stolen reportedly dealt with U.S. hacking code. The contractor was, according to the Post, working on tools that would replace those that had been considered compromised by former NSA contractor Edward Snowden's leaking. The loss is tantamount to having your playbook stolen by the other team, as an outside cyber security expert described it to CBS News. The data could help the Russians understand better how to protect their networks, as well as suggest how they could infiltrate U.S. networks, not to mention those of other nations, the Wall Street Journal reported.

This summer, Florida Sen. Marco Rubio to ask intelligence chiefs whether they used and trusted Kaspersky software and earlier ththe Department of Homeland Security (DHS) instructed all government agencies to cease using Kaspersky software.

As CBS News reported last month, Kaspersky is said by U.S. intelligence and law enforcement to have ties to Russian intelligence. Kaspersky is registered with the Russian spy agency, the FSB, and is required to be willing to provide information to Russian authorities if requested.

Eugene Kaspersky, the founder of the company, has consistently denied working for any intelligence service. 

CBS News' Andres Triay contributed to this report.