A logic issue existed resulting in memory corruption. This was addressed with improved state management. This issue affected versions prior to iOS 12.1.1, tvOS 12.1.1, watchOS 5.1.2, Safari 12.0.2, iTunes 12.9.2 for Windows, iCloud for Windows 7.9.
https://support.apple.com/kb/HT209346
https://support.apple.com/kb/HT209345
https://support.apple.com/kb/HT209344
https://support.apple.com/kb/HT209343