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fkie_cve-2026-96637

Pillow is a Python imaging library. Prior to 13.0.0, SpiderImageFile.seek() in src/PIL/SpiderImagePlugin.py reparses a later SPIDER stack frame and replaces self._size without rerunning Image._decompression_bomb_check(). A tiny file can use a safe first frame to pass Image.open() and then declare very large dimensions in a later mode F frame. Loading that frame allocates approximately four bytes per attacker-declared pixel and can consume multiple gigabytes of memory. This issue is fixed in version 13.0.0.

GHSA-g842-9rc4-69pw — Pillow: Decompression bomb check bypass in SpiderImagePlugin.seek() leads to unbounded allocation DoS

Affected: PyPI: pillow

## Summary SpiderImagePlugin's seek() bypasses the decompression bomb check, allowing a tiny (<1KB) crafted SPIDER stack file to trigger a multi-gigabyte allocation on im.load().

## Affected versions All versions up to and including current HEAD (v13.0.0.dev0, commit e7a7ea84)

## Root cause Image.open() runs Image._decompression_bomb_check(im.size) exactly once, in _open_core (src/PIL/Image.py:3668), right after the initial _open().

SPIDER stack files are multi-frame: SpiderImageFile.seek(frame) (SpiderImagePlugin.py:179-190) computes an attacker-influenced stkoffset and calls self._open() AGAIN directly (line 190) — NOT through Image.open(). _open() re-parses a fresh 27-float header at that offset and reassigns:

self._size = int(h[12]), int(h[2]) # SpiderImagePlugin.py:131

straight from unvalidated header floats (isSpiderHeader only checks internal labrec*lenbyt==labbyt consistency, never bounds nsam/nrow). No bomb check runs on this new size.

On the subsequent load(), ImageFile.load_prepare() (ImageFile.py:436-437) executes `self.im = Image.core.new(self.mode, self.size)` with the unchecked, attacker-chosen size. Mode is always "F" (4 bytes/pixel), so e.g. 40000x40000 = 6.4 GB.

## Why this is a real bug Sibling multi-frame plugins DO re-check on their alternate paths: - GifImagePlugin.py:286 - Image._decompression_bomb_check(self._size) (in seek) - TiffImagePlugin.py:1309 - Image._decompression_bomb_check(self._tile_size) - IcnsImagePlugin.py:121/138, IcoI…

UBUNTU-CVE-2026-96637

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. Prior to 13.0.0, SpiderImageFile.s ...)