iOS 11.2.1 (build 15C153) is an official Apple over-the-air update for the iPhone X (GSM), released on 2017-12-13T18:11:05Z. The update package is 410.94 MB. This is a delta (partial) update that requires 11.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 68ddf45852beca6632f3e80dc9ecc69adf5092d3.zip.