WhatsApp has urged its 1.5 billion users to upgrade the app after the Facebook-owned platform discovered a vulnerability that allowed a spyware to be installed on users’ phones via the app’s phone call function. 

The vulnerability leveraged a bug in WhatsApp’s audio call feature, facilitating the installation of spyware on the device being called whether the call was answered or not. 

According to WhatsApp, it suspects a relatively small number of users were targeted. 

“This is, as you can imagine, an extremely severe security hole, and it is difficult to fix the window during which it was open, or how many people were affected by it,” reports TechCrunch.