The ReadWPGImage function in coders/wpg.c in GraphicsMagick 1.3.26 does not properly validate colormapped images, which allows remote attackers to cause a denial of service (ImportIndexQuantumType invalid write and application crash) or possibly have unspecified other impact via a malformed WPG image.
https://www.debian.org/security/2018/dsa-4321
https://usn.ubuntu.com/4248-1/
https://sourceforge.net/p/graphicsmagick/bugs/519/
https://lists.debian.org/debian-lts-announce/2018/08/msg00002.html
http://hg.code.sf.net/p/graphicsmagick/code/rev/e8086faa52d0