GHSA-MRFP-P49P-3HF2 Pillow PDF append accepts malformed shared page-tree DAG causing resource exhaustion

Summary Pillow's documented PDF append path can spend disproportionate CPU time on a compact malformed PDF page tree. An attacker who can supply an existing PDF to an application that appends an image with Image.save..., format="PDF", append=True can provide a malformed /Pages tree where each lev...