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BREW-aider-CVE-2026-112405 — Pillow: NULL pointer dereference in Image.merge via an unvalidated band 0

Affected: Homebrew: aider

### Summary `Image.merge()` checks the mode of every input band except band 0. If band 0 is a single-band `I` or `F` image, the native merge routine reads a pixel pointer that is NULL for those modes and the process crashes. The IPTC plugin reaches this from a plain `Image.open()` followed by a load, and IPTC has no magic-byte prefilter, so a crafted file with no recognizable header can drive it.

### Details The validation loop skips band 0:

- `src/PIL/Image.py:3795` iterates `for band in bands[1:]` and checks mode and size only for those.

For an `I` or `F` image the 8-bit plane `image8` is NULL (only `image32` is set). `ImagingMerge` dereferences `bands[0]->image8[y]` for the multi-band output modes:

- `src/libImaging/Bands.c:302`, `:311`, `:321`

The IPTC loader builds the band list from an attacker-typed sub-image and passes it to merge with the sub-image at index 0:

- `src/PIL/IptcImagePlugin.py:156-160` - `src/PIL/IptcImagePlugin.py:169` registers the opener with no accept function.

### PoC ```python from PIL import Image

# Band 0 is a single-band "I" image: its image8 plane is NULL. merge() validates only # bands[1:], so the native ImagingMerge dereferences the NULL plane for band 0. band0 = Image.new("I", (4, 4)) Image.merge("RGB", [band0, Image.new("L", (4, 4)), Image.new("L", (4, 4))]) ```

Run it: ``` $ python 02_poc.py Segmentation fault (core dumped) ```

Observed on a source build of commit `3e2ab96`.

Under AddressSanitizer: ``` ==ERROR: AddressSanitizer…

CVE-2026-112405 — Pillow: NULL pointer dereference in Image.merge via an unvalidated band 0

Affected: python-pillow Pillow

Pillow is a Python imaging library. From 12.0.0 until 13.0.0, Pillow's Image.merge() can dereference a null image8 plane. Validation skips the first input band, allowing a 32-bit integer or floating-point image backed by image32 storage to be merged as a narrower multiband image. A crafted IPTC file can reach this path when an application opens and loads it, or a caller can invoke Image.merge() directly with such a first band. This causes a native process crash. This issue is fixed in version 13.0.0.

PYSEC-2026-4333

Affected: PyPI: pillow