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.
http://openwall.com/lists/oss-security/2017/08/29/4
https://bugs.debian.org/878511
https://bugzilla.redhat.com/show_bug.cgi?id=1484196
https://lists.debian.org/debian-lts-announce/2018/08/msg00002.html