The Facebook-owned messaging service issued a statement regarding the incident which they believe involved a "private company" affiliated with the government who managed to install spyware which could control devices via its phone calling function.
They said: "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.
"We have briefed a number of human rights organizations to share the information we can and to work with them to notify civil society."
A new fix was issued in an update on Monday (13.05.19).
The hackers were reportedly able to target users by calling them and infecting the call even if there was no response.
The alert was raised earlier this month and on Sunday (12.05.19), a lawyer in the UK experienced the hack.
The person, who has not been identified, is working on a lawsuit against Israeli cyber intelligence firm NSO Group filed by a number of people including a group of Mexican journalists.
The attack was blocked by WhatsApp and they are currently investigating the situation to found out just how many people were affected.