WhatsApp, which is used by 1.5bn people worldwide, discovered in early May that attackers were able to install surveillance software on to both iPhones and Android phones by ringing up targets using the app’s phone call function.

The malicious code, developed by the secretive Israeli company NSO Group, could be transmitted even if users did not answer their phones, and the calls often disappeared from call logs, said the spyware dealer, who was recently briefed on the WhatsApp hack.

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NSO’s flagship product is Pegasus, a program that can turn on a phone’s microphone and camera, trawl through emails and messages and collect location data.

NSO advertises its products to Middle Eastern and Western intelligence agencies, and says Pegasus is intended for governments to fight terrorism and crime.

But mostly to spy on people said governments don't particularly like, of course.