In ReadOneJNGImage in coders/png.c in GraphicsMagick 1.3.26, a Null Pointer Dereference occurs while transferring JPEG scanlines, related to a PixelPacket pointer.
https://www.debian.org/security/2018/dsa-4321
https://sourceforge.net/p/graphicsmagick/bugs/518/
https://lists.debian.org/debian-lts-announce/2018/08/msg00002.html