Apple has just released an emergency patch for the iPhone.

The company was forced to release a patch after it mistakenly made it easier for hackers to jailbreak up to date iPhones with the previous iOS release. In July, Apple released iOS 12.4, an update that reintroduced a bug that had been previously patched. Last week, iPhone hackers and jailbreakers realized Apple’s mistake and published a jailbreak for up to date iPhones—the first time in years it was possible to jailbreak iPhones on the latest version of iOS.

On Monday, Apple released iOS 12.4.1, which fixes the bug once again.

Security researchers who specialize in iOS hacking had warned that it was possible for malicious hackers to use the old bug reintroduced by Apple and chain it with another exploit to hack iPhone users.

Have a tip about Apple or a Apple-focused security company? You can contact Lorenzo Franceschi-Bicchierai securely on Signal at +1 917 257 1382, OTR chat at lorenzofb@jabber.ccc.de, or email lorenzofb@vice.com

“People must be very careful what Apps they download from the Apple AppStore,” warned iOS security expert Stefan Esser on Twitter.

Another researcher said that anyone who can get a Safari bug could use the old vulnerability reintroduced by Apple to “hack any up to date iPhone.”

Apple did not immediately respond to a request for comment.

In its release, the company thanked a Google researcher who found the original bug, and thanked Pwn20wnd, who developed the jailbreak.