A resource exhaustion issue was addressed with improved input validation. This issue affected versions prior to iOS 12.1, tvOS 12.1, Safari 12.0.1, iTunes 12.9.1, iCloud for Windows 7.8.
https://support.apple.com/kb/HT209192
https://support.apple.com/kb/HT209194
https://support.apple.com/kb/HT209196