A logic issue was addressed with improved validation. This issue is fixed in iOS 12.2, tvOS 12.2, Safari 12.1, iTunes 12.9.4 for Windows, iCloud for Windows 7.11. A malicious website may be able to execute scripts in the context of another website.
https://support.apple.com/HT209605
https://support.apple.com/HT209604
https://support.apple.com/HT209603