Decrypted S/MIME parts, when included in HTML crafted for an attack, can leak plaintext when included in a a HTML reply/forward. This vulnerability affects Thunderbird < 52.9.
https://access.redhat.com/errata/RHSA-2018:2251
https://access.redhat.com/errata/RHSA-2018:2252
https://bugzilla.mozilla.org/show_bug.cgi?id=1419417
https://lists.debian.org/debian-lts-announce/2018/07/msg00013.html
https://security.gentoo.org/glsa/201811-13
https://usn.ubuntu.com/3714-1/