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Response snapshot Oct 27, 2026, 01:01 UTC

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VulnerabilityCVE-2026-114501

Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT > 1

When [`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.

Published Updated

At a glance

Exploited
Not known
Severity
6.3 Medium (CVSS 4.0)
Chance of exploitation
0.5% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: Pillow: 13.0.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-654x-cwwv-5j9c (opens a new tab)

What happened

### Summary

When [`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.

### 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.

Continue reading

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.

```python 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.

Affected package ranges reported by GitHub: Pillow: >= 11.2.1, < 13.0.0

Scores and how they were measured

CVSS base severity

6.3

Version 4.0As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.5%

Scored Percentile 40.3%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-654x-cwwv-5j9c (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-654x-cwwv-5j9c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-114501, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 23). Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT > 1 (CVE-2026-114501). https://kevwire.com/record/CVE-2026-114501

MLA

"Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT > 1 (CVE-2026-114501)." KEVWire, 23 Oct. 2026, kevwire.com/record/CVE-2026-114501.

Chicago

KEVWire. "Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT > 1 (CVE-2026-114501)." Last modified October 23, 2026. https://kevwire.com/record/CVE-2026-114501.

Evidence summary (plain text)

Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT > 1 (Oct 23, 2026).
When [`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. 

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.
CVSS 6.3 (version 4.0; evidence as of Oct 23, 2026; https://github.com/advisories/GHSA-654x-cwwv-5j9c).
EPSS 0.5% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-114501&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-654x-cwwv-5j9c: https://github.com/advisories/GHSA-654x-cwwv-5j9c
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-114501&date=2026-10-26
Record ID: CVE-2026-114501.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 19:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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