https://arstechnica.com/information-technology/2019/08/ios-vulnerability-that-let-you-jailbreak-your-iphone-is-once-again-dead/

The iOS vulnerability that made it possible for users and hackers to jailbreak fully up-to-date iPhones and iPads is no more, following an update Apple released on Monday that patches the highly unusual bug.

The security advisory accompanying the release of iOS version 12.4.1 says it patches a kernel vulnerability that allows malicious apps to execute code that runs with the highest of privileges. (The use-after-free vulnerability was first fixed in iOS 12.3.) Then, last weekend, researchers noticed that version 12.4, released in June, was once again vulnerable. Jailbreak enthusiasts—who like the freedom that such vulnerabilities permit—quickly capitalized on the Apple developer mistake by releasing exploits that worked on fully patched devices.