In iOS before 11.2.5, macOS High Sierra before 10.13.3, Security Update 2018-001 Sierra, and Security Update 2018-001 El Capitan, watchOS before 4.2.2, and tvOS before 11.2.5, a memory corruption issue exists and was addressed with improved memory handling.
https://support.apple.com/HT208465
https://support.apple.com/HT208464