Do note that tvOS 11.2.1 is available on the Apple TV 4K over the air only. The first few days went well for the iOS 11.2, until users found out that the update introduced another issue which deletes the Apple Pay Cash permanently from their devices.
Earlier today, Apple released iOS and tvOS 11.2.1, fixing a critical HomeKit vulnerability.
The bug in iOS 11 was giving unauthorised access to the Home app, and according to Apple's Security update page, it could allow a "remote attacker" to unexpectedly alter the application state. This would then let them control smart home accessories within the framework such as locks and garage door openers. After applying this update, you will be able to once again use remote access in HomeKit, with no security-related downsides. Apple has now reportedly fixed the issue with iOS 11.2.1, although the fix has not been mentioned it explicitly in its update log. Meanwhile, over in beta land, Apple's beamed out the iOS 11.2.5, watchOS 4.2.2, and tvOS 11.2.5 developer beta, so watch out for those in the weeks ahead.
Once iOS 11.2.1 is shown as an available update, tap "Download and Install" and follow the on-screen instructions. After this update, the vulnerability is fully patched and remote access for shared users is secure. There's also the usual bug fixes. The new iOS 11.2.1 build weighs in at under 70MB for most devices. Also, you should back up your data, and ensure that your device has over 50 percent of charge or connected to a power source before beginning the update process.
As usual, to get started on the update, just head to Settings General Software Update.