Apple kept its promise and released an update that fixes a character bug that could cause crashes in Appliances iPhone, Apple Watch, Apple TV and Mac computers.
IOS 11 will remain in Apple's history as one of the most problematic releases released. Read:
So operating systems for iOS, macOS, watchOS and tvOS can be updated immediately.
The updates are: iOS 11.2.6 (15D100), MacOS High Sierra 10.13.3 Supplemental Update (17D102 / 17D2102), tvOS 11.2.6 (15K600) and watchOS 4.2.3 (15S600b) and are available for download.
According to Apple's release notes the new updates software, fix one problem where certain character sequences could cause some applications to crash.
In more technical terms, updates fix a memory corruption problem that Apple encountered by improving the input validation to CoreText.
Anyway, it would be a good idea to update your devices immediately.