Plaintext of decrypted emails can leak through by user submitting an embedded form by pressing enter key within a text input field. 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=1462910
https://lists.debian.org/debian-lts-announce/2018/07/msg00013.html
https://security.gentoo.org/glsa/201811-13
https://usn.ubuntu.com/3714-1/