WhatsApp yesterday encouraged its users to upgrade the app to plug a security breach that allowed sophisticated attackers to sneak spyware into phones, in the latest trouble for its parent Facebook.

The vulnerability—first reported by the Financial Times, and fixed in the latest WhatsApp update—allowed hackers to insert malicious software on phones by calling the target using the app, which is used by 1.5 billion people around the world.

“WhatsApp encourages people to upgrade to the latest version of our app, as well as keep their mobile operating system up to date, to protect against potential targeted exploits designed to compromise information stored on mobile devices,” a spokesperson said in a statement to AFP.

The FT cited a spyware dealer as saying the tool was developed by a shadowy Israel-based firm called the NSO Group, which has been accused of helping governments from the Middle East to Mexico snoop on activists and journalists.