GraphicsMagick 1.3.26 has a NULL pointer dereference in the WritePCLImage() function in coders/pcl.c during writes of monochrome images.
https://www.debian.org/security/2018/dsa-4321
https://usn.ubuntu.com/4206-1/
https://lists.debian.org/debian-lts-announce/2018/08/msg00002.html
http://hg.code.sf.net/p/graphicsmagick/code/rev/f3ffc5541257