Apple released iOS 11.4.1 on Monday, fixing multiple vulnerabilities, one of which is CVE-2018-4290, which is a denial of service vulnerability: when a user enters the word Taiwan in the messaging app or receives an emoji containing the Taiwanese blue sky banner, the bug will cause some devices to crash.
Security researcher Patrick Wardle analysed the bug in detail on his blog:
Patrick Wardle found that when you set [China] in the [Language and Region] area of the iPhone, then enter [Taiwan] in English input method wherever you can enter text. iOS will crash because the emoji is an invalid input. Patrick Wardle demonstrates this crash with the iPhone 7 with iOS 11.3 installed.
If your region located in China, Apple will go a step further and completely remove the emoji of the Taiwan flag from the list of emojis. If you set it to the US region, you can see it.
After Patrick Wardle reported the bug to Apple, Apple solved the problem in the iOS 11.4.1 update.