The ReadMNGImage function in coders/png.c in GraphicsMagick 1.3.26 mishandles large MNG images, leading to an invalid memory read in the SetImageColorCallBack function in magick/image.c.
https://www.debian.org/security/2018/dsa-4321
https://usn.ubuntu.com/4222-1/
https://lists.debian.org/debian-lts-announce/2018/08/msg00002.html
http://hg.code.sf.net/p/graphicsmagick/code/rev/cd699a44f188