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fkie_cve-2026-96639

Pillow is a Python imaging library. From 11.0.0 until 13.0.0, multi-frame loaders can retain an ImagingCore allocation created for a narrow frame mode while a later frame of the same dimensions changes the logical mode and decoder tiles to wider RGB or CMYK data. ImageFile.load_prepare() reuses the stale native storage because _im is non-null without verifying that its mode and size still match. Crafted PSD, MPO, DCX, or MIC frame transitions can then write wider pixels into the narrow allocation, causing native out-of-bounds writes and crashes. The MPO path can also cause a native over-read and, in demonstrated runtime-specific profiles, command execution. This issue is fixed in version 13.0.0.

GHSA-v9pv-5gc8-w2m6 — Pillow multi-frame loaders can reuse stale native image storage after mode transitions

Affected: PyPI: Pillow

### Summary Pillow multi-frame image loaders can retain native `ImagingCore` storage allocated for one frame while changing the Python-side image `mode` and decoder tiles for a later frame.

If frame 0 allocates storage for a narrow mode such as `L`, and a later frame of the same dimensions switches to a wider mode such as `RGB` or `CMYK`, Pillow may decode wider pixels into the stale narrow native buffer. This causes native out-of-bounds writes and process crashes. In the MPO/JPEG path, I also demonstrated command execution in matching Python/Pillow runtime profiles through Pillow's documented `ImageSequence.Iterator` + `frame.load()` path.

### Details The root cause appears to be that `ImageFile.load_prepare()` only reallocates native storage when `_im is None`. It does not verify that the retained native `ImagingCore` storage still matches `self.mode` and `self.size`.

Several multi-frame `seek()` / `_open()` paths can update `self._mode` and decoder tiles for a later frame without invalidating `_im`. The decoder then writes or reads according to the new frame mode while the native storage still belongs to the old frame mode.

Verified affected surfaces:

1. **PSD**

`PsdImageFile.seek()` can switch from grayscale/native `L` storage to RGB layer decoding while retaining `_im`. This produces attacker-controlled native out-of-bounds writes and reliable crashes.

2. **MPO/JPEG**

A crafted two-frame MPO can make frame 0 allocate `L` storage, then make frame 1 decode `…

UBUNTU-CVE-2026-96639

Affected: Ubuntu:Pro:14.04:LTS: pillow, Ubuntu:Pro:16.04:LTS: pillow, Ubuntu:Pro:18.04:LTS: pillow, Ubuntu:Pro:20.04:LTS: pillow, Ubuntu:Pro:20.04:LTS: pillow-python2, Ubuntu:22.04:LTS: pillow, Ubuntu:24.04:LTS: pillow, Ubuntu:26.10: pillow, Ubuntu:26.04:LTS: pillow

(Pillow is a Python imaging library. From 11.0.0 until 13.0.0, multi-fr ...)