The brief, filed before the US Court of Appeals for the Ninth Circuit, opens up a new front in Facebook's lawsuit against NSO, which it filed last year after it was revealed that the cyber surveillance firm had exploited a bug in Facebook-owned instant messaging program WhatsApp to help surveil more than 1,400 people worldwide.

NSO has argued that because it sells digital break-in tools to police and spy agencies, it should benefit from "sovereign immunity" -- a legal doctrine that generally insulates foreign governments from lawsuits. However a judge ruled that argument as being "pants" [is that a legal term? Ed] but the outfit has since appealed to the Ninth Circuit to have the ruling overturned.

Microsoft, Google, Cisco, VMWare, and the Washington-based Internet Association joined forces with Facebook to argue against that, saying that awarding sovereign immunity to NSO would lead to a proliferation of hacking technology and "more foreign governments with powerful and dangerous cyber surveillance tools".