Pillow: Decompression bomb check bypass in SpiderImagePlugin.seek() leads to unbounded allocation DoS

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, IcoImagePlugin.py:236/240 (sub-image selection)

SpiderImagePlugin has NO bomb check anywhere. This is the same class of bug just fixed for GdImageFile (GHSA-phj9-mv4w-65pm) and the FontFile classes (GHSA-5x94/8v84/45hq): an allocation reached on a non-Image.open() path sized by an attacker header field, independent of the one-time pixel bomb check.

Reproduction

Python 3.13, Pillow HEAD e7a7ea84

Attached: pillow_poc_spider_bomb.py

Sets Image.MAX_IMAGE_PIXELS=5000, then:

• CONTROL: Image.open(standalone 100x100 SPIDER) -> DecompressionBombWarning fires correctly.
• BYPASS: Image.open(stack, frame0=1x1) passes; im.seek(1) sets size (100,100) with NO DecompressionBombWarning/Error -> the check is bypassed.

Real-world impact: a large frame-1 nsam/nrow (e.g. 40000x40000) causes im.load() to allocate ~6.4 GB (mode F) with the bomb check fully bypassed, from a file under 1 KB.

Suggested fix

Call Image._decompression_bomb_check(self.size) at the end of SpiderImageFile._open() (or in seek() after _open()), mirroring GifImagePlugin / TiffImagePlugin. Optionally validate nsam/nrow are positive and sane in _open().

Credit: Yazan Balawneh - CyStack Security Team pillow_poc_spider_bomb.py