---
id: "CVE-2026-96638"
kind: "vulnerability"
aliases: ["CVE-2026-96638", "GHSA-24rc-8hj2-v98c"]
title: "Pillow: BMP RLE decoder memory-amplification DoS via unbounded delta escape / absolute mode"
severity: "medium"
cvss: 6.5
epss: 0.00279999990016222
exploited: false
status: "open"
category: "package"
products: [{"name": "pillow", "fixed": ["13.0.0"], "affected": ">= 9.1.0, < 13.0.0", "ecosystem": "pypi"}]
ecosystems: ["pypi"]
tags: ["cwe-400", "cwe-770", "image-processing", "package", "pillow", "python", "self-hosted"]
not_affected_if: ["no-untrusted-bmp-input", "version-13.0.0-or-later", "version-below-9.1.0"]
published_at: "2026-10-21T21:20:32+00:00"
first_seen_at: "2026-10-23T19:11:32+00:00"
updated_at: "2026-10-23T21:57:29+00:00"
sources: [{"source": "ghsa", "url": "https://github.com/advisories/GHSA-24rc-8hj2-v98c", "first_seen_at": "2026-10-23T19:11:32+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-96638", "first_seen_at": "2026-10-23T21:57:29+00:00"}]
---

# Pillow: BMP RLE decoder memory-amplification DoS via unbounded delta escape / absolute mode

Pillow's BMP RLE decoder can be made to allocate gigabytes of memory from a kilobyte-sized crafted BMP file, because delta escape and absolute mode branches append attacker-controlled amounts without bounding growth by the image size. An attacker who can get a process to open a malicious BMP can cause a memory-exhaustion denial of service.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- pypi:pillow affected >= 9.1.0, < 13.0.0 fixed 13.0.0

## How to check

Run `python -c "import PIL; print(PIL.__version__)"` and check whether the version is >= 9.1.0 and < 13.0.0.
Search the codebase for Image.open or PIL usage on untrusted BMP uploads.

## Fix

Upgrade pillow to 13.0.0 or later.
Until upgraded, reject or limit untrusted BMP inputs before decoding.

## Timeline

- 2026-10-23T19:11:32+00:00 ghsa https://github.com/advisories/GHSA-24rc-8hj2-v98c
- 2026-10-23T21:57:29+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-96638

## From the sources

## 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 file.

## Suggested fix
Cap growth at dest_length in the delta and absolute branches, e.g. clamp the delta fill to (dest_length - len(data)) and stop appending once len(data) >= dest_length; reject deltas that would move the cursor beyond the image, matching the existing per-row clamp at line 341-343.

Credit: Yazan Balawneh - CyStack Security Team (GitHub: sonicnew)

[pillow_poc_bmp_rle_dos.py](https://github.com/user-attachments/files/30244974/pillow_poc_bmp_rle_dos.py)

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_