Hackers were able to remotely install surveillance software on phones and other devices using a major vulnerability in messaging app WhatsApp, it has been confirmed.
WhatsApp, which is owned by Facebook, said the attack targeted a "select number" of users, and was orchestrated by "an advanced cyber actor".
A fix was rolled out on Friday.
The attack was developed by Israeli security firm NSO Group, according to a report in the Financial Times.
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.
WhatsApp promotes itself as a "secure" communications app because messages are end-to-end encrypted, meaning they should only be displayed in a legible form on the sender or recipient's device.
However, the surveillance software would have let an attacker read the unencrypted messages on the target's device.