The Sock Puppet exploit, originally discovered by Ned Williamson, was accidentally unpatched in iOS 12.4. This led to the first jailbreak for currently signed firmware in quite some time.
The jailbreak of iOS 12.4 currently has full support for A7-A11 devices and partial support for A12 and A12X devices. Pwn20wnd has successfully achieved code injection for A12 and A12X and has nearly completed an update that will bring Cydia to A12(X) devices.
Apple is still signing iOS 12.4 so if you want to jailbreak and haven't yet updated, you should to so asap. It would be safer to install iOS 12.4 via iTunes.