Developer Pwn20wnd has released an unc0ver jailbreak tool with support for iOS 12.0 - 12.1.2. It is now bundled with Cydia and Cydia Substrate. However, this is a public beta and it does not support all devices that can run iOS 12, but as the jailbreak tool's work continues throughout the public beta phase, we should expect that it supports all devices that compatible with iOS 12.
According to Pwn20wnd, there are few things need to keep in mind: you will have to delete OTA file from Settings, and then reboot before attempting the jailbreak to prevent a possible boot loop, the jailbreak tool doesn't support A12 devices and A7-A8 devices that include iPhone 5s, 6, 6 Plus, XS, XS Max, and iPhone XR.
For those people who are running iOS 12.1.3 and iOS 12.1.4, there's no need to be upset, as Apple still signs iOS 12.1.1 b3 for some reason, which means it is still possible to restore or downgrade your device to iOS 12.1.1~b3 to use this jailbreak.
Like every other jailbreak process, you should download the IPA file Pwn20wnd's GitHub repository, then drag the file to Cydia Impactor, and make sure your device is connected to a computer, reboot your device, then tap on the un0ver, and click on the jailbreak icon, and enjoy it.
Image Via Wccftech