WritePNMImage in coders/pnm.c in GraphicsMagick 1.3.26 allows remote attackers to cause a denial of service (bit_stream.c MagickBitStreamMSBWrite heap-based buffer overflow and application crash) or possibly have unspecified other impact via a crafted file.
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/525/