CVE-2026-96640

### 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 level repeats the same child `/Pages` reference in `/Kids`. Pillow treats the graph as a tree and expands each repeated reference in `PdfParser.linearize_page_tree()` without tracking visited or duplicate indirect references. In the bounded proof below, a `1410`-byte PDF expands to `8192` page references from one distinct page object and takes about `9.6` seconds through the append path. A comparable linear control with the same depth stays at one page reference and completes in about `0.02` seconds. This is in scope for Pillow's security policy because it is reproducible in `main`, affects the latest public release tested (`12.3.0`), and is not just an unexpected Python exception. The vulnerable operation accepts attacker-controlled PDF bytes through Pillow's PDF parser and causes deterministic resource consumption before normal completion. ### Details #### Affected versions - Package: `Pillow` - Ecosystem: `pip` / PyPI - Affected component: `src/PIL/PdfParser.py` page-tree traversal reached from `src/PIL/PdfImagePlugin.py` PDF append handling - Runtime-tested affected release: `12.3.0` - Runtime-tested affected development version: `13.0.0.dev0` - Tested commit: `b39b9649312fbc381060e8c7c7f9f0de4cc63daa` - Current source artifact hash: `sha256(git archive --format=tar HEAD) = 2ccefca23d432456765be8d3c34564c65ba016b0c9da1e44ca389762f7eda054` - Release `12.3.0` commit: `bb1d8e8ab8d29048624d96e3ee53cecf7c13d13d` - Release `12.3.0` source artifact hash: `sha256(git archive --format=tar 12.3.0) = 94bed33fd20123938dbc0153d295250bb4e3c09251b6d4a34f7595e25d409c81` - Source-level affected: `5.1.0`, `12.2.0`, `12.3.0`, and `HEAD` - Likely affected range: `>= 5.1.0` - Pa