A cross-origin issue existed with the fetch API. This was addressed with improved input 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. Processing maliciously crafted web content may disclose sensitive user information.
https://support.apple.com/HT209605
https://support.apple.com/HT209604
https://support.apple.com/HT209603