WhatsApp tends to pride itself on the security it offers users, but every app has its weaknesses. Recently, the Financial Times reported that a vulnerability in the messaging app allowed for it to be hacked.

In early May, Whatsapp discovered that hackers were able to install surveillance technology by calling someone through the app. That would give them access to vital information, including location data and private messages.

The spyware in question was developed by Israel’s NSO Group, according to the Financial Times. It could also be transmitted even if the target didn’t answer their calls. In addition, a spyware dealer told the Financial Times that the calls themselves often disappeared from call logs.

“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,” WhatsApp said, according to the Financial Times. “We have briefed a number of human rights organizations to share the information we can, and to work with them to notify civil society.

It’s unclear how many users exactly were targeted by this breach. The app has 1.5 billion active users in over 180 different countries, according to 99firms.com. So, it’s possible that quite a few people were impacted.

According to a Facebook notice, the breach impacted both Android, iPhones, and Windows phones. The hack itself was reported to the Department of Justice, the Financial Times said.

On Monday, WhatsApp released an update that should take care of the issue. Users are being strongly encouraged to update, even if they haven’t had any suspicious call activity.