The zero-day bug was found in WebKit, the browser engine powering Safari and other apps, and allowed hackers to break out of WebKit’s protective sandbox with “maliciously crafted web content,” per Apple. A sandbox is part of the operating system that, even if compromised, can keep hackers from accessing data in other parts of the system.

Apple has said it has addressed a security vulnerability in its Safari browser that may have been exploited by threat actors to launch an “extremely sophisticated attack against specific targeted individuals.

Apple is urging users to update their devices with an emergency security update to fix a zero-day flaw found in Safari’s WebKit browser engine.