It has been confirmed that hackers were able to remotely install surveillance software on phones and other devices using a major vulnerability in messaging app WhatsApp.
WhatsApp, which is owned by Facebook, said the attack targeted a "select number" of users, and was orchestrated by "an advanced cyber actor".
The attack was developed by Israeli security firm NSO Group, according to a report in the Financial Times.
BBC reports WhatsApp has urged all of its 1.5bn users to update their apps as an added precaution.
The attack was first discovered earlier this month.
It involved attackers using WhatsApp's voice calling function to ring a target's device.
Even if the call was not picked up, the surveillance software would be installed, and the call would often disappear from the device's call log.