A memory corruption issue was addressed with improved validation. This issue affected versions prior to iOS 12, macOS Mojave 10.14, tvOS 12, watchOS 5.
https://support.apple.com/kb/HT209193
https://support.apple.com/kb/HT209139
https://support.apple.com/kb/HT209108
https://support.apple.com/kb/HT209107