An integer overflow vulnerability was foud in tiff_image_parse function leading to undefined behaviour.
This is an integer overflow leading to a heap-buffer overflow, which can lead to application crash or even arbitrary code execution in some certain cases.
However as per https://bugzilla.gnome.org/show_bug.cgi?id=780269#c8 it seems that gcc is not really impacted (At least versions of gcc shipped in Red Hat Enterprise Linux which is used to compile all applications shipped in Red Hat Enterprise Linux is not impacted). So chances that this security flaw affects Red Hat versions of gdk-pixbuf2 are remote.
However there is an integer overflow flaw, so marking this as moderate is fine.