The ReadSUNImage function in coders/sun.c in GraphicsMagick 1.3.26 has a colormap heap-based buffer over-read.
https://www.debian.org/security/2018/dsa-4321
https://usn.ubuntu.com/4222-1/
https://lists.debian.org/debian-lts-announce/2018/06/msg00009.html
http://www.securityfocus.com/bid/100442
http://hg.code.sf.net/p/graphicsmagick/code/rev/95d00d55e978