The ReadMNGImage function in coders/png.c in GraphicsMagick 1.3.26 has an out-of-order CloseBlob call, resulting in a use-after-free via a crafted file.
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
https://blogs.gentoo.org/ago/2017/07/12/graphicsmagick-use-after-free-in-closeblob-blob-c/
http://hg.code.sf.net/p/graphicsmagick/code/rev/d0a76868ca37