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," according to the BBC.
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.
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 FT 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 U.S. Department of Justice earlier this month.
"The attack has all the hallmarks of a private company reportedly that works with governments to deliver spyware that takes over the functions of mobile phone operating systems," the company said on Monday.