ReadCMYKImage in coders/cmyk.c in GraphicsMagick 1.3.26 has a magick/import.c ImportCMYKQuantumType heap-based buffer over-read via a crafted file.
https://www.debian.org/security/2018/dsa-4321
https://usn.ubuntu.com/4248-1/
https://sourceforge.net/p/graphicsmagick/bugs/521/
https://lists.debian.org/debian-lts-announce/2018/06/msg00009.html
https://lists.debian.org/debian-lts-announce/2018/01/msg00005.html
http://hg.code.sf.net/p/graphicsmagick/code/rev/a9c425688397