On 12 January 2022, Apple released what looks like a minor update for the iPhone, iPad and iPod touch, however, iOS 15.2.1 and iPad OS 15.2.1 include important bug and security fixes.
Among other things, problems with Messages and CarPlay are fixed, but the most crucial change addresses a serious security problem with HomeKit.
As we explained in this article: HomeKit bug can render iPhone unusable, a security gap in HomeKit was highlighted by security researcher Trevor Spiniolas. He noticed that HomeKit devices could be renamed in such a way that an iPhone or iPad could crash and the Home app would become unusable.
Apple explains the risk to devices thus: "Impact: Processing a maliciously crafted HomeKit accessory name may cause a denial of service." And says that "A resource exhaustion issue was addressed with improved input validation." More details the security content of iOS 15.2.1 here.
The Messages tweak fixes a problem where some photos sent via an iCloud link did not load. The CarPlay problem relates to some third-party apps that weren’t working properly.
To install the update, open the Settings app and go to General > Software Update. Your device will seek out the update, download it and then you can install. For a more in-depth guide to updating your iPhone read this: How to update iOS on your iPhone.