An issue was discovered in certain Apple products. iOS before 11.3 is affected. macOS before 10.13.4 is affected. The issue involves the "Mail" component. It allows man-in-the-middle attackers to read S/MIME encrypted messages by leveraging an inconsistency in the user interface.
https://support.apple.com/HT208693
https://support.apple.com/HT208692
http://www.securitytracker.com/id/1040608