This is an old issue that is not a problem with current Fedora releases (due to providing xcftools 1.0.7), however gnome-xcf-thumbnailer embedds what looks like xcftools 1.0.4 (vulnerable version) or earlier, as reported to Debian . A patch  exists in the original Debian bug report  to correct the flaw. The MITRE entry for this flaw follows:
Stack-based buffer overflow in the flattenIncrementally function in flatten.c in xcftools 1.0.4, as reachable from the (1) xcf2pnm and (2) xcf2png utilities, allows remote attackers to cause a denial of service (crash) and possibly execute arbitrary code via a crafted image that causes a conversion to a location "above or to the left of the canvas." NOTE: some of these details are obtained from third party information.
Created gnome-xcf-thumbnailer tracking bugs for this issue
Affects: fedora-all [bug 647908]
This issue was fixed and pushed into updates:
Thanks for this report.