GHSA-8g5g-8vmr-9782 - Pillow: Heap buffer overflow in ImagingJpegDecode

## Summary `JpegImageFile.draft()` — which `Image.thumbnail()` calls automatically — requests 1/2, 1/4 or 1/8 scaled JPEG output and shrinks `self._size` and the tile extents to match, so `_setimage()` allocates a single-row decode buffer of `ceil(W/s)` samples. `ImagingJpegDecode()` sets `cinfo.scale_num`/`scale_denom`, calls `jpeg_start_decompress()`, and then reads scanlines into that buffer without ever comparing `cinfo.output_width`, `cinfo.output_height` or `cinfo.out_color_components` against `state->xsize`, `state->ysize` and `state->bits`. libjpeg-turbo 3.x decodes lossless (SOF3) JPEG, but hardwires lossless streams to no scaling — the requested `scale_denom` is silently discarded. It therefore returns full-width `W`-sample rows into a buffer sized for `ceil(W/s)`. Each `jpeg_read_scanlines()` call writes `(W - ceil(W/s)) x components` bytes past the end of the heap allocation, and the overflowing bytes are the decoded pixel values, which are byte-exact for an 8-bit grayscale lossless image. The input is a standards-compliant JPEG, so `Image.open(..., formats=["JPEG"])` does not prevent it. Pillow's Python SOF parser already accepts marker `0xFFC3` ("SOF3", "Spatial lossless") at 8-bit precision, so the file opens normally as a `JPEG` / mode `L` image at its full declared size. ## Affected code At `e41083f383c9cd3db95de52564cc0b6452313d4a` (`main`, 2026-08-29): - `src/libImaging/JpegDecode.c:224-262` — `ImagingJpegDecode()`; `jpeg_start_decompress()` at :240, `jpeg_read_scanlines()` at :252, with no geometry check between them. - `src/decode.c:222-231` — `_setimage()`; `state->buffer = calloc(1, state->bytes)` with `state->bytes = (state->bits * state->xsize + 7) / 8`. - `src/PIL/JpegImagePlugin.py:428-464` — `JpegImageFile.draft()`; computes `s` and shrinks `_size`, the tile extents and `decoderconfig` with no notion of lossless coding. - `src/PIL/JpegImagePlugin.py:271` — `0xFFC3: ("SOF3", "Spatial lossless", SOF)`. ```c // src/libImaging/JpegDeco