An issue was discovered in certain Apple products. iOS before 10.3 is affected. macOS before 10.12.4 is affected. tvOS before 10.2 is affected. The issue involves the "Keychain" component. It allows man-in-the-middle attackers to bypass an iCloud Keychain secret protection mechanism by leveraging lack of authentication for OTR packets.
https://support.apple.com/HT207617
https://support.apple.com/HT207615
https://support.apple.com/HT207601