In GraphicsMagick 1.3.27a, there is a heap-based buffer over-read in ReadOneJNGImage in coders/png.c, related to oFFs chunk allocation.
https://lists.debian.org/debian-lts-announce/2018/01/msg00005.html
https://lists.debian.org/debian-lts-announce/2018/06/msg00009.html
https://sourceforge.net/p/graphicsmagick/bugs/530/