An issue was discovered in certain Apple products. iOS before 11.2 is affected. macOS before 10.13.2 is affected. The issue involves the "Mail Drafts" component. It allows man-in-the-middle attackers to read e-mail content by leveraging mishandling of S/MIME credential encryption.
https://support.apple.com/HT208331
https://support.apple.com/HT208334