Plaintext of decrypted emails can leak through by user submitting an embedded form. This vulnerability affects Thunderbird ESR < 52.8 and Thunderbird < 52.8.
www.securityfocus.com/bid/104240
www.securitytracker.com/id/1040946
access.redhat.com/errata/RHSA-2018:1725
access.redhat.com/errata/RHSA-2018:1726
bugzilla.mozilla.org/show_bug.cgi?id=1450345
lists.debian.org/debian-lts-announce/2018/05/msg00013.html
security.gentoo.org/glsa/201811-13
usn.ubuntu.com/3660-1/
www.debian.org/security/2018/dsa-4209
www.mozilla.org/security/advisories/mfsa2018-13/