Apple has released iOS 11.2.6, watchOS 4.2.3, and tvOS 11.2.6 with fixes to CoreText wherein a Telugu character could crash text apps.

Apple addressed a memory corruption issue via improved input validation.

iOS 11.2.6, watchOS 4.2.3, and tvOS 11.2.6 are available via the OSes’ Software Update mechanisms.

Apple releases macOS 10.13.3 Supplemental Update with Telugu text bug fix – February 20, 2018
How to fix the Indian character bug on iPhone and iPad – February 16, 2018
Major new iOS bug can crash iPhones and disable access to Messages and other apps – February 15, 2018
ChaiOS ‘text bomb’ can crash your Apple iPhone or Mac with a single link – January 17, 2018
Three-character message prank crashes pre-iOS 10.2 Apple iPhones with a single text – January 18, 2017
Random string of Arabic characters can crash nearly any Mac, iPhone or iPad – September 3, 2013