In iOS before 11.3, Safari before 11.1, tvOS before 11.3, watchOS before 4.3, iTunes before 12.7.4 for Windows, an array indexing issue existed in the handling of a function in javascript core. This issue was addressed with improved checks.
https://usn.ubuntu.com/3781-1/
https://support.apple.com/HT208698
https://support.apple.com/HT208695%2C
https://support.apple.com/HT208694%2C