iOS 12.1.3 (build 16D39) is an official Apple over-the-air update for the iPhone 8 (GSM), released on 2019-01-22T18:08:59Z. The update package is 1.48 GB. This is a delta (partial) update that requires 11.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 80ea23050a2baa0d279aa452b0351cea71d4d2c6.zip.