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://lists.debian.org/debian-lts-announce/2018/08/msg00002.html
https://sourceforge.net/p/graphicsmagick/bugs/519/