Irssi before 1.0.5, when installing themes with unterminated colour formatting sequences, may access data beyond the end of the string.
https://www.debian.org/security/2017/dsa-4016
https://lists.debian.org/debian-lts-announce/2017/12/msg00022.html
https://irssi.org/security/irssi_sa_2017_10.txt