iOS 11.2.1 (build 15C153) is an official Apple over-the-air update for the iPhone X (GSM), released on 2017-12-13T18:11:04Z. The update package is 451.01 MB. This is a delta (partial) update that requires 11.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 908afd53abe4a42a8903ad164207ba1ad695a921.zip.