thunderbird is vulnerable to S/MIME Message Takeover Attacks. A malicious S/MIME message consisting of an inner encryption layer and an outer SignedData layer was shown as having a valid digital signature despite the signer having no access to the contents of the encrypted message.
lists.opensuse.org/opensuse-security-announce/2019-10/msg00009.html
lists.opensuse.org/opensuse-security-announce/2019-10/msg00010.html
bugzilla.mozilla.org/show_bug.cgi?id=1240290
lists.debian.org/debian-lts-announce/2019/11/msg00017.html
seclists.org/bugtraq/2019/Nov/24
security-tracker.debian.org/tracker/CVE-2019-11755
usn.ubuntu.com/4202-1/
usn.ubuntu.com/4335-1/
www.debian.org/security/2019/dsa-4571
www.mozilla.org/security/advisories/mfsa2019-32/