A heap out-of-bounds write exists in the Photo CD (PCD) decoder of GraphicsMagick. In DecodeImage() (coders/pcd.c), the Huffman delta loop advances its output pointer with q++ after every decoded delta and never checks it against the end of the heap-allocated luma/chroma plane buffers. The pointer is repositioned only when a sync marker introduces a new plane/row; between sync markers the run length is bounded solely by the input.
A crafted PCD file that positions the pointer near the end of a plane and then supplies a long run of deltas with no intervening sync therefore walks the pointer past the end of the allocation and writes through it. Processing an untrusted PCD file — for example with gm convert or gm identify, or through any application linked against libGraphicsMagick — can corrupt heap memory beyond the buffers.
Where upgrading is not possible, prevent the PCD and PCDS coders from being used on untrusted input. GraphicsMagick detects PCD by file content, so filtering on filename extension is not sufficient; the coder module must be removed or the format rejected before decoding.
Upgrade to GraphicsMagick 1.3.48 or later, which bounds the decode output pointer against the end of each plane buffer and over-provisions the buffers.