An issue was discovered in Mutt before 1.10.1 and NeoMutt before 2018-07-16. They allow remote IMAP servers to execute arbitrary commands via backquote characters, related to the mailboxes command associated with a manual subscription or unsubscription.
https://www.debian.org/security/2018/dsa-4277
https://usn.ubuntu.com/3719-3/
https://usn.ubuntu.com/3719-2/
https://usn.ubuntu.com/3719-1/
https://security.gentoo.org/glsa/201810-07
https://neomutt.org/2018/07/16/release
https://lists.debian.org/debian-lts-announce/2018/08/msg00001.html
https://gitlab.com/muttmua/mutt/commit/185152818541f5cdc059cbff3f3e8b654fc27c1d
https://github.com/neomutt/neomutt/commit/95e80bf9ff10f68cb6443f760b85df4117cb15eb
https://access.redhat.com/errata/RHSA-2018:2526