Mutt before 1.14.4 and NeoMutt before 2020-06-19 have a STARTTLS buffering issue that affects IMAP, SMTP, and POP3. When a server sends a "begin TLS" response, the client reads additional data (e.g., from a man-in-the-middle attacker) and evaluates it in a TLS context, aka "response injection."
https://www.debian.org/security/2020/dsa-4708
https://www.debian.org/security/2020/dsa-4707
https://usn.ubuntu.com/4403-1/
https://security.gentoo.org/glsa/202007-57
https://lists.debian.org/debian-lts-announce/2020/06/msg00040.html
https://lists.debian.org/debian-lts-announce/2020/06/msg00039.html
https://gitlab.com/muttmua/mutt/-/issues/248
https://gitlab.com/muttmua/mutt/-/commit/c547433cdf2e79191b15c6932c57f1472bfb5ff4
https://github.com/neomutt/neomutt/releases/tag/20200619
https://github.com/neomutt/neomutt/commit/fb013ec666759cb8a9e294347c7b4c1f597639cc
http://lists.opensuse.org/opensuse-security-announce/2020-06/msg00070.html
http://lists.opensuse.org/opensuse-security-announce/2020-06/msg00064.html
http://lists.mutt.org/pipermail/mutt-announce/Week-of-Mon-20200615/000023.html