In GraphicsMagick 1.4 snapshot-20171217 Q8, there is a stack-based buffer over-read in WriteWEBPImage in coders/webp.c, related to an incompatibility with libwebp versions, 0.5.0 and later, that use a different structure type.
https://www.debian.org/security/2018/dsa-4321
https://usn.ubuntu.com/4266-1/
https://sourceforge.net/p/graphicsmagick/bugs/536/
http://hg.graphicsmagick.org/hg/GraphicsMagick/rev/6dda3c33f35f