An issue was discovered in Irssi before 1.0.7 and 1.1.x before 1.1.1. Certain nick names could result in out-of-bounds access when printing theme strings.
https://www.debian.org/security/2018/dsa-4162
https://usn.ubuntu.com/3590-1/
https://lists.debian.org/debian-lts-announce/2018/03/msg00021.html