iOS 11.3 (build 15E216) is an official Apple over-the-air update for the iPhone X (GSM), released on 2018-03-29T17:11:34Z. The update package is 681.47 MB. This is a delta (partial) update that requires 11.2.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: bdfbdeff7f2a9432313e639929fe349f9fa0a32c.zip.