Hackers have developed a software which allows them to listen into WhatsApp users - without the victims knowing they are being spied on.
The app found a vulnerability which allowed attackers to remotely install malicious code on both iPhone and Android deceives by calling the target phone.
Hackers are able to install the software on your phone - even if the target phone is not answered.
After the software is installed, hackers can control your phone's microphone and camera.
The company, owned by Facebook, said the malicious software resembled a spyware developed by intelligence agencies.
A new software update was released by the company, which has more than 1.5 billion users across 180 countries.
There are concerns the software was used to try and gain access to 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.”
The firm said it alerted officials at the US Department for Justice after discovering the flaw in early May.
The spyware was allegedly developed by the NSO Group, an Israeli cybersecurity and intelligence firm, according to the FT.
The company 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.”
How do I install the new WhatsApp update?
Some phones will receive a notification message from WhatsApp, urging you to update the software to the latest available version.
Not all customers will, however.
If you have an iPhone, go to the App Store and click updates. When you find Whatsapp, tap the update button next to the app. You phone should then update.
If you own an Android phone, click on Play Store, then on the menu button.
Under My apps and games, tap update next to WhatsApp Messenger.