There is an invalid free in the MagickFree function in magick/memory.c in GraphicsMagick 1.3.26 that will lead to a remote denial of service attack.
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
https://bugzilla.redhat.com/show_bug.cgi?id=1484196
https://bugs.debian.org/878511
http://www.securityfocus.com/bid/100518
http://openwall.com/lists/oss-security/2017/08/29/4
http://hg.graphicsmagick.org/hg/GraphicsMagick/rev/3db9449e3d6a/