A heap buffer overflow in image_set_mask function of HTMLDOC before 1.9.15 allows an attacker to write outside the buffer boundaries.
https://github.com/michaelrsweet/htmldoc/issues/461
https://github.com/michaelrsweet/htmldoc/commit/71fe87878c9cbc3db429f5e5c70f28e4b3d96e3b