iOS 12.1.3 (build 16D5037a) is an official Apple over-the-air update for the iPhone 8 (GSM), released on 2019-01-07T18:07:08Z. The update package is 90.05 MB. This is a delta (partial) update that requires 12.1.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 8e0f010fded25dc1881cc6937bd3714e8429b72b.zip.