iOS 11.2.1 (build 15C153) is an official Apple over-the-air update for the iPhone 5s (GSM), released on 2017-12-13T18:11:36Z. The update package is 1.03 GB. This is a delta (partial) update that requires 10.3.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 5b03247dfb3f6b71cd9ca6ffd58f2b5899407775.zip.