dDecrypted S/MIME parts hidden with CSS or the plaintext HTML tag can leak plaintext when included in a HTML reply/forward. This vulnerability affects Thunderbird < 52.9.
www.securityfocus.com/bid/104613
access.redhat.com/errata/RHSA-2018:2251
access.redhat.com/errata/RHSA-2018:2252
bugzilla.mozilla.org/show_bug.cgi?id=1464056
bugzilla.mozilla.org/show_bug.cgi?id=1464667
lists.debian.org/debian-lts-announce/2018/07/msg00013.html
security.gentoo.org/glsa/201811-13
usn.ubuntu.com/3714-1/
www.debian.org/security/2018/dsa-4244
www.mozilla.org/security/advisories/mfsa2018-18/