iOS 12.1.2 (build 16C101) is an official Apple over-the-air update for the iPhone X (GSM), released on 2018-12-17T21:06:16Z. The update package is 1.54 GB. This is a delta (partial) update that requires 11.3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: efd18d3d6fdb73ada9785e38d7fd49c0d5dd99d3.zip.