A memory corruption issue was addressed with improved input validation. This issue affected versions prior to iOS 11.4.1, macOS High Sierra 10.13.6, tvOS 11.4.1, watchOS 4.3.2, iTunes 12.8 for Windows, iCloud for Windows 7.6.
https://support.apple.com/kb/HT208937
https://support.apple.com/kb/HT208936