GraphicsMagick 1.3.26 has double free vulnerabilities in the ReadOneJNGImage() function in coders/png.c.
https://www.debian.org/security/2018/dsa-4321
http://www.securityfocus.com/bid/99504
http://hg.code.sf.net/p/graphicsmagick/code/rev/4d0baa77245b