The Israeli spyware firm NSO Group is alleging that Facebook tried to buy its software in 2017 in order to monitor the activities of Facebook users.

NSO Group made the allegation in a new court filing as part of an ongoing legal battle with Facebook. It claims it rejected Facebook's attempt to buy its software because the firm only takes government clients.

Facebook first sued NSO Group in October alleging that it hacked users of its WhatsApp messaging platform.

NSO Group has previously faced allegations that it helped Saudi Arabia hack its enemies, including Washington Post journalist Jamal Khashoggi and Jeff Bezos.

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NSO Group — a secretive Israeli spyware firm facing allegations that it hacked platforms like WhatsApp and helped authoritative governments spy on dissidents — now claims that Facebook tried to buy its surveillance technology in 2017 to monitor its users.

Facebook has sued NSO Group for allegedly hacking its WhatsApp messaging platform on behalf of its government clients. NSO Group has also faced allegations that it helped Saudi Arabia spy on its perceived opponents, including Amazon CEO Jeff Bezos and Washington Post journalist Jamal Khashoggi, who was murdered by Saudi agents in 2018.

The NSO Group accused Facebook of trying to buy its software in a new court filing this week. Facebook allegedly tried to buy Pegasus, an NSO Group technology that secretly skims information from unsuspecting users' smartphones, according to the filing.

"Facebook wanted to use purported capabilities of Pegasus to monitor users on Apple devices," NSO Group claimed in the filing.

A Facebook spokesperson said in a statement to Business Insider that NSO Group is "trying to distract from the facts." The Facebook spokesperson also said NSO Group's filing misrepresented a discussion between the two companies, but did not provide an alternate characterization of that discussion.

"NSO is trying to distract from the facts Facebook and WhatsApp filed in court nearly six months ago. Their attempt to avoid responsibility includes inaccurate representations about both their spyware and a discussion with people who work at Facebook," the spokesperson said.

In 2017, Facebook was developing an app called Onavo Protect that touted VPN services for users, but also quietly tracked app data on users' smartphones in order to keep tabs on which other apps they were using. The practice of deploying spyware in seemingly-unrelated apps is somewhat common — analytics firms like Sensor Tower and App Annie both track people's app use with software built into VPN and ad blocker apps.

Apple ultimately forced Facebook to remove Onavo Protect from the App Store over surveillance concerns in 2019. But before that happened, NSO Group claims, Facebook tried to buy NSO Group's Pegasus software to beef up Onavo Protect's data-gathering capabilities.

NSO Group claims that it rejected Facebook's offer in 2017 because it only sells its software to sovereign governments or government agencies.