Encrypted S/MIME parts in a crafted multipart/alternative message can leak plaintext when included in a a HTML reply/forward. This vulnerability affects Thunderbird < 68.1 and Thunderbird < 60.9.
https://www.mozilla.org/security/advisories/mfsa2019-30/
https://www.mozilla.org/security/advisories/mfsa2019-29/
https://usn.ubuntu.com/4150-1/
https://bugzilla.mozilla.org/show_bug.cgi?id=1571481
http://lists.opensuse.org/opensuse-security-announce/2019-10/msg00010.html
http://lists.opensuse.org/opensuse-security-announce/2019-10/msg00009.html