The WSJ reported that the Russian hackers targeted the contractor after identifying the files through the contractor's use of a popular antivirus software made by Russia-based Kaspersky Lab, these people said.

The theft is considered to be one of the most significant security breaches in recent years. It offers a rare glimpse into how the intelligence community thinks Russian intelligence exploits widely available commercial software product to spy on the US.

The incident occurred in 2015 but wasn't discovered until spring of last year. 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 US and other nations, these people said.

Ahead of the publication of the WSJ report, Kaspersky founder Eugene Kaspersky tweeted that it was a conspiracy theory and being pushed by unnamed sources. Kaspersky has just seen his US government business trashed over fears he might have been working a little closely with Putin’s spooks.

However, he has a point. The reporting of the story is a little weird and seems to fit the NSA narrative that Kaspersky is an evil Russian puppet which smells of Nintendo.

The Journal reports that the hackers identified the documents through Kaspersky Lab antivirus software, which really could mean anything and might not actually be Kaspersky’s fault. There is no other information about the defence contractor’s computer contained.