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fkie_cve-2026-96638

Pillow is a Python imaging library. From 9.1.0 until 13.0.0, BmpRleDecoder.decode() in src/PIL/BmpImagePlugin.py permits crafted RLE BMP delta escapes and absolute-mode runs to append beyond the remaining dest_length without a per-branch cap. A delta can add roughly 255 image rows in one iteration, so a kilobyte-sized file with a large but permitted width can expand the data bytearray to hundreds of megabytes or more, causing memory-exhaustion denial of service. This issue is fixed in version 13.0.0.

GHSA-24rc-8hj2-v98c — Pillow: BMP RLE decoder memory-amplification DoS via unbounded delta escape / absolute mode

Affected: PyPI: pillow

## Summary BmpRleDecoder can be forced to allocate gigabytes of memory from a kilobyte-sized crafted BMP file via unbounded growth in the delta escape and absolute mode branches.

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

## Root cause BmpRleDecoder.decode() (BmpImagePlugin.py:327-395) builds a Python bytearray `data` with dest_length = xsize*ysize, looping `while len(data) < dest_length`. The loop condition is only checked at the TOP; individual branches append attacker-controlled amounts UNBOUNDED by dest_length:

- delta escape (line 370): data += b"\x00" * (right + up * self.state.xsize) with right,up in 0..255 -> up to ~255*xsize bytes in ONE iteration. - absolute mode (line 385): data += bytes_read (up to 255).

Because a single delta can push len(data) to ~256 * (row size), a tiny RLE stream combined with a large-but-legal xsize (xsize*ysize just under MAX_IMAGE_PIXELS, ~89M) yields a multi-GB `data` bytearray (then copied again by bytes(data) and set_as_raw). No heap OOB (bytearray/set_as_raw are bounds-safe), but a severe memory-amplification DoS: the attacker file stays tiny.

## Reproduction Python 3.13, Pillow HEAD e7a7ea84

Attached: pillow_poc_bmp_rle_dos.py

A 1084-byte BMP (BI_RLE8, width=3,000,000, height=1 -> 3M px, passes bomb check) with one delta escape (00 02 00 150) produces ~429 MB `data`; measured peak RSS 911 MB. Scaling width toward the bomb limit and up=255 produces tens of GB from a ~1 KB…

UBUNTU-CVE-2026-96638

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 9.1.0 until 13.0.0, BmpRleDec ...)