WhatsApp has rushed to roll out a security fix after it emerged hackers could spy on phones by injecting surveillance software via the call function.
The app discovered a vulnerability that allowed attackers to install malicious code on iPhones and Android phones by ringing up a target device.
The code could be transmitted even if users did not answer their phones and a log of the call often disappeared, the Financial Times reported.
The messaging company, which is owned by Facebook, said the attack bore a resemblance to spyware developed for intelligence agencies.
There are concerns that the software was used in attempts to access the phones of human rights campaigners, including a UK-based lawyer.
"We believe a select number of users were targeted through this vulnerability by an advanced cyber actor," WhatsApp told the FT.
"This attack has all the hallmarks of a private company known to work with governments to deliver spyware that reportedly takes over the functions of mobile phone operating systems.
"We have briefed a number of human rights organisations to share the information we can, and to work with them to notify civil society."
HOW BAD WAS THE ATTACK?
The firm is said to have alerted officials at the US Department of Justice after discovering the vulnerability in early May.
WhatsApp claims to have 1.5 billion users around the world and it released a software update on Monday.
According to the Financial Times, the spyware was developed by NSO Group, an Israeli cybersecurity and intelligence company.
NSO told the paper: "Under no circumstances would NSO be involved in the operating or identifying of targets of its technology, which is solely operated by intelligence and law enforcement agencies.
"NSO would not, or could not, use its technology in its own right to target any person or organisation, including this individual (the UK lawyer)."
The vulnerability and suspected attacks have been investigated by Citizen Lab, a research group at the University of Toronto.
"We believe an attacker tried (and was blocked by WhatsApp) to exploit it as recently as yesterday to target a human rights lawyer," the lab said.
On Monday, Amnesty International said it was backing legal action against the Israeli Ministry of Defence demanding that it revokes NSO Group's export licence.
Danna Ingleton, deputy director of Amnesty Tech, said: "NSO Group sells its products to governments who are known for outrageous human rights abuses, giving them the tools to track activists and critics."
WHAT SHOULD YOU DO?
WhatsApp says it has rolled out a security fix and that users should update their app on their phones.
This can be done using the phone’s application store.
For Android users, they should go to Play Store, then tap Menu > My apps & games. Tap UPDATE next to WhatsApp Messenger.
Alternatively, they can go to Play Store and search for WhatsApp. Tap UPDATE under WhatsApp Messenger.
For iPhone users, go to the App Store, then tap Updates. Tap UPDATE next to WhatsApp Messenger.
Alternatively, they can go to the App Store and search for WhatsApp. Tap UPDATE next to WhatsApp Messenger.