GHSA-654x-cwwv-5j9c Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT > 1

• PyPI Pillow: >= 11.2.1, < 13.0.0 → fixed in 13.0.0

Summary When `PILLOW_ALIGNMENT` is set above 1, exporting an image to Arrow returns uninitialized heap memory mixed in with the pixel data. That memory can include pixels from images the process freed earlier. Details With an alignment above 1, the default allocator pads each row up to `aligned_linesize` and starts the first row at an aligned offset into the block. It then exports `im->blocks[0].ptr` as `xsize * ysize` tightly packed pixels, so exported data includes the padding bytes, and can include a few bytes from before the first row. Tested vulnerable on 11.2.1, 12.3.0, 4c1ed27a5. PoC Whether things leak may depend on allocator configuration. This does show a leak on my Mac. ```python import os os.environ["PILLOW_ALIGNMENT"] = "64" from PIL import Image Image A: 64 px wide, so its rows fill the whole aligned stride a = Image.new("L", (64, 40), 0x41).transpose(Image.Transpose.FLIP_LEFT_RIGHT) del a Image B: 61 px wide, so each row has 3 bytes of padding; its pixels are all 0 b = Image.new("L", (61, 40), 0).transpose(Image.Transpose.FLIP_LEFT_RIGHT) try: exported = Image.fromarrow(b, b.mode, b.size).tobytes() except ValueError as e: print("Not vulnerable:", e) else: if leaked := len(exported) - exported.count(0): raise RuntimeError(f"{leaked} of {len(exported)} exported bytes are not B's pixels") ``` Impact Information disclosure for applications that set `PILLOW_ALIGNMENT` and export images to Arrow. Likely any image whose row length isn't a multiple of the alignment will do. In a service handling several users' images, one user's exported data can contain pixels from another user's image, or other stale heap contents.