*** NOTE FROM 2/4/21: unc0ver works for iOS 12 and 13, but not iOS 14. The info below was posted in February 2019, and is still mostly relevant, but check later in the thread for important updates. ***
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The unc0ver jailbreak tool for iOS 12 was just released.
This has been long awaited. Many iOS 12 jailbreaks were theorized and worked on, but none made it to the public with full Cydia support until a few days ago.
At the time of this writing, it only supports devices up to the iPhone X, including 5S, 6, 6 Plus, 6S, 7, 7 Plus, 8, and X. XS, XS Max, and XR. It will never support iPhone 5 and below.
Preparation: Get rid of any auto-downloaded iOS update your phone may have grabbed without your knowledge. If you don't do this, it might screw everything up. Go to Settings -> General -> iPhone Storage, and go through the list of items. Find anything like "iOS 12.1.4" or whatever (it should have the Settings gear icon), and delete it. Should be something like 2.4GB. If it's not there, no problem.
Assuming you have iOS 12.0 through 12.1.2, and the proper device, follow the instructions at this link to install the jailbreak: https://www.redmondpie.com/jailbreak...-how-tutorial/
However, DON'T download the "ipa" file from the above link. Instead, get the latest one from the repository of the guy who made it (scroll all the way down and get the highest version): https://github.com/pwn20wndstuff/Undecimus/releases
That's it! You should have full access to Cydia, and the tweaks/packages you install should have iOS support soon enough, and many will work already.