It has been revealed 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 what it described as an advanced cyber actor.
A fix was implemented on Friday.
According to a report in the Financial Times, the attack was developed by Israeli security firm NSO Group.
On Monday, WhatsApp urged all of its 1.5 billion users to update their apps as an added precaution.
The attack was first discovered earlier this month.
The security flaw 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 Financial Times reported, the call would often disappear from the device's call log.
WhatsApp told the BBC its security team was the first to identify the flaw, and shared that information with human rights groups, selected security vendors and the US Department of Justice earlier this month.