Plaintext of decrypted emails can leak through the src attribute of remote images, or links. This vulnerability affects Thunderbird ESR < 52.8 and Thunderbird < 52.8.
https://www.mozilla.org/security/advisories/mfsa2018-13/
https://www.debian.org/security/2018/dsa-4209
https://usn.ubuntu.com/3660-1/
https://security.gentoo.org/glsa/201811-13
https://lists.debian.org/debian-lts-announce/2018/05/msg00013.html
https://bugzilla.mozilla.org/show_bug.cgi?id=1457721
https://access.redhat.com/errata/RHSA-2018:1726
https://access.redhat.com/errata/RHSA-2018:1725