An issue existed in the method for determining prime numbers. This issue was addressed by using pseudorandom bases for testing of primes. This issue affected versions prior to iOS 12.1, macOS Mojave 10.14.1, tvOS 12.1, watchOS 5.1, iTunes 12.9.1, iCloud for Windows 7.8.
https://support.apple.com/kb/HT209198
https://support.apple.com/kb/HT209197
https://support.apple.com/kb/HT209195
https://support.apple.com/kb/HT209194