A cross-origin issue existed with "iframe" elements. This was addressed with improved tracking of security origins. This issue is fixed in Safari 18, visionOS 2, watchOS 11, macOS Sequoia 15, iOS 18 and iPadOS 18, tvOS 18. A malicious website may exfiltrate data cross-origin.
https://support.apple.com/en-us/121250
https://support.apple.com/en-us/121249
https://support.apple.com/en-us/121248
https://support.apple.com/en-us/121241