GHSA-654X-CWWV-5J9C

Vulnerability from github – Published: 2026-10-23 21:53 – Updated: 2026-10-23 21:53
VLAI
Summary
Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT > 1
Details

Summary

When PILLOW_ALIGNMENT is set above 1, exporting an image to Arrow returns uninitialized heap memory mixed in with the pixel data. That memory can include pixels from images the process freed earlier.

Details

With an alignment above 1, the default allocator pads each row up to aligned_linesize and starts the first row at an aligned offset into the block. It then exports im->blocks[0].ptr as xsize * ysize tightly packed pixels, so exported data includes the padding bytes, and can include a few bytes from before the first row.

Tested vulnerable on 11.2.1, 12.3.0, 4c1ed27a5.

PoC

Whether things leak may depend on allocator configuration. This does show a leak on my Mac.

import os

os.environ["PILLOW_ALIGNMENT"] = "64"

from PIL import Image

# Image A: 64 px wide, so its rows fill the whole aligned stride
a = Image.new("L", (64, 40), 0x41).transpose(Image.Transpose.FLIP_LEFT_RIGHT)
del a

# Image B: 61 px wide, so each row has 3 bytes of padding; its pixels are all 0
b = Image.new("L", (61, 40), 0).transpose(Image.Transpose.FLIP_LEFT_RIGHT)

try:
    exported = Image.fromarrow(b, b.mode, b.size).tobytes()
except ValueError as e:
    print("Not vulnerable:", e)
else:
    if leaked := len(exported) - exported.count(0):
        raise RuntimeError(f"{leaked} of {len(exported)} exported bytes are not B's pixels")

Impact

Information disclosure for applications that set PILLOW_ALIGNMENT and export images to Arrow. Likely any image whose row length isn't a multiple of the alignment will do. In a service handling several users' images, one user's exported data can contain pixels from another user's image, or other stale heap contents.

Severity
6.3 (Medium)
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N
Show details on source website
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "Pillow"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "11.2.1"
            },
            {
              "fixed": "13.0.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-114501"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-226",
      "CWE-908"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-10-23T21:53:21Z",
    "nvd_published_at": "2026-10-21T22:16:49Z",
    "severity": "MODERATE"
  },
  "details": "### Summary\n\nWhen [`PILLOW_ALIGNMENT`](https://pillow.readthedocs.io/en/stable/reference/block_allocator.html#memory-pools) is set above 1, exporting an image to Arrow returns uninitialized heap memory mixed in with the pixel data. That memory can include pixels from images the process freed earlier. \n\n### Details\n\nWith an alignment above 1, the default allocator pads each row up to `aligned_linesize` and starts the first row at an aligned offset into the block. It then exports `im-\u003eblocks[0].ptr` as `xsize * ysize` tightly packed pixels, so exported data includes the padding bytes, and can include a few bytes from before the first row.\n\nTested vulnerable on 11.2.1, 12.3.0, 4c1ed27a5.\n\n### PoC\n\nWhether things leak may depend on allocator configuration. This does show a leak on my Mac.\n\n```python\nimport os\n\nos.environ[\"PILLOW_ALIGNMENT\"] = \"64\"\n\nfrom PIL import Image\n\n# Image A: 64 px wide, so its rows fill the whole aligned stride\na = Image.new(\"L\", (64, 40), 0x41).transpose(Image.Transpose.FLIP_LEFT_RIGHT)\ndel a\n\n# Image B: 61 px wide, so each row has 3 bytes of padding; its pixels are all 0\nb = Image.new(\"L\", (61, 40), 0).transpose(Image.Transpose.FLIP_LEFT_RIGHT)\n\ntry:\n    exported = Image.fromarrow(b, b.mode, b.size).tobytes()\nexcept ValueError as e:\n    print(\"Not vulnerable:\", e)\nelse:\n    if leaked := len(exported) - exported.count(0):\n        raise RuntimeError(f\"{leaked} of {len(exported)} exported bytes are not B\u0027s pixels\")\n```\n\n### Impact\n\nInformation disclosure for applications that set `PILLOW_ALIGNMENT` and export images to Arrow. Likely any image whose row length isn\u0027t a multiple of the alignment will do. In a service handling several users\u0027 images, one user\u0027s exported data can contain pixels from another user\u0027s image, or other stale heap contents.",
  "id": "GHSA-654x-cwwv-5j9c",
  "modified": "2026-10-23T21:53:21Z",
  "published": "2026-10-23T21:53:21Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/python-pillow/Pillow/security/advisories/GHSA-654x-cwwv-5j9c"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-114501"
    },
    {
      "type": "WEB",
      "url": "https://github.com/python-pillow/Pillow/pull/10172"
    },
    {
      "type": "WEB",
      "url": "https://github.com/python-pillow/Pillow/commit/19f64946eb247b973dd4560be7dea3bf979d2e6a"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/python-pillow/Pillow"
    },
    {
      "type": "WEB",
      "url": "https://github.com/python-pillow/Pillow/releases/tag/13.0.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT \u003e 1"
}



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