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Response snapshot Oct 27, 2026, 01:01 UTC

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VulnerabilityCVE-2026-96639

Pillow multi-frame loaders can reuse stale native image storage after mode transitions

If frame 0 allocates storage for a narrow mode such as `L`, and a later frame of the same dimensions switches to a wider mode such as `RGB` or `CMYK`, Pillow may decode wider pixels into the stale narrow native buffer. This causes native out-of-bounds writes and process crashes.

Published Updated

At a glance

Exploited
Not known
Severity
9.8 Critical (CVSS 3.1)
Chance of exploitation
0.7% in the next 30 days
Sources
2 independent sources

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-v9pv-5gc8-w2m6 (opens a new tab)

What happened

### Summary Pillow multi-frame image loaders can retain native `ImagingCore` storage allocated for one frame while changing the Python-side image `mode` and decoder tiles for a later frame.

If frame 0 allocates storage for a narrow mode such as `L`, and a later frame of the same dimensions switches to a wider mode such as `RGB` or `CMYK`, Pillow may decode wider pixels into the stale narrow native buffer. This causes native out-of-bounds writes and process crashes. In the MPO/JPEG path, I also demonstrated command execution in matching Python/Pillow runtime profiles through Pillow's documented `ImageSequence.Iterator` + `frame.load()` path.

### Details The root cause appears to be that `ImageFile.load_prepare()` only reallocates native storage when `_im is None`. It does not verify that the retained native `ImagingCore` storage still matches `self.mode` and `self.size`.

Several multi-frame `seek()` / `_open()` paths can update `self._mode` and decoder tiles for a later frame without invalidating `_im`. The decoder then writes or reads according to the new frame mode while the native storage still belongs to the old frame mode.

Continue reading

Verified affected surfaces:

1. **PSD**

`PsdImageFile.seek()` can switch from grayscale/native `L` storage to RGB layer decoding while retaining `_im`. This produces attacker-controlled native out-of-bounds writes and reliable crashes.

2. **MPO/JPEG**

A crafted two-frame MPO can make frame 0 allocate `L` storage, then make frame 1 decode `CMYK` / `RGB` data into the retained `L` storage. The documented `ImageSequence.Iterator` path reaches this with `frame.load()`. Verified native memory corruption, crash, over-read behavior, and scoped harmless-marker command execution on Pillow 12.3.0.

3. **DCX**

`DcxImageFile.seek()` calls `PcxImageFile._open()`, which changes mode/tile state without a native-storage mode guard. A crafted DCX with `L` then `RGB` PCX frames produces retained-storage mismatch and crash.

4. **MIC**

MIC can call `TiffImageFile._open()` directly and bypass TIFF's normal guarded `seek()` behavior. With a `NextIFD`-based reachable frame transition, this produces retained-storage mismatch and crash.

TIFF appears to be a useful positive control because its `seek()` path checks both size and mode before reusing storage.

### Impact - process crash / denial of service, - native out-of-bounds write, - native over-read in the MPO path.

Stronger proven impact:

- command execution in matching Python/Pillow runtime profiles through the MPO/JPEG path.

Server-side deployment impact:

- Create a local vulnerable web upload service that accepts an unauthenticated MPO upload and automatically processes it with Pillow. - The attack is one HTTP request and requires no user interaction after upload. - The fixed harmless command-output marker succeeded 10/10 times. - The `_im=None` control was clean 10/10 times.

## Reproduction Evidence

Tested version:

```text Pillow 12.3.0 ```

MPO terminal proof environments:

```text Python 3.12.3 + Pillow 12.3.0, clean PyPI environment: marker 10/10, control 0/10

Python 3.13.14 + Pillow 12.3.0, prepared profile: marker 10/10, control 0/10 ```

Server-side web-upload proof:

```text control 1..10: MARKER_MISSING control_summary ok 0 missing 10 /10

vulnerable 1..10: MARKER_OK marker_output vulnerable_summary ok 10 missing 0 /10 ```

The terminal proof uses a fixed harmless marker command only. It does not use a reverse shell, persistence, data exfiltration, or arbitrary attacker-selected commands.

I can provide the following private artifacts to maintainers:

- minimal crash fixtures for PSD, MPO, DCX, and MIC, - non-weaponized MPO marker PoC, - local vulnerable web upload demo, - gdb/GEF geometry receipts, - control-run logs, - proposed regression-test shape.

## Minimal Reproduction Shape

For the MPO path, the vulnerable consumer shape is:

```python from io import BytesIO from PIL import Image, ImageSequence

source = BytesIO(open(path, "rb").read()) image = Image.open(source) iterator = ImageSequence.Iterator(image) next(iterator).load() next(iterator).load() ```

The crafted image shape is:

```text frame 0: mode L, dimensions W x H frame 1: same W x H, wider mode such as CMYK ```

Frame 0 allocates native `L` storage. Frame 1 changes the logical mode and decoder state but can reuse the old native storage. The frame 1 decoder writes using the wider mode into the narrow storage.

## Suggested Fix Direction

Before reusing native `ImagingCore` storage, Pillow should verify that the retained storage is compatible with the current logical image mode and size.

A central guard in `ImageFile.load_prepare()` could compare native storage mode/size against `self.mode` / `self.size` and allocate fresh storage when they differ. Alternatively, every mode-changing `seek()` / `_open()` path should explicitly invalidate `_im` before decoding.

Caution: TIFF has specialized tile-size behavior and already has a mode/size guard, so a central fix should avoid regressing TIFF tile-buffer handling.

## Credit

```text Rotem Kama ```

## Methodology Note

AI-assisted analysis was used for discovery and verification support. Claims in this report are limited to the local artifacts, controls, and command outputs described above.

Affected package ranges reported by GitHub: Pillow: >= 11.0.0, < 13.0.0

Scores and how they were measured

CVSS base severity

9.8

Version 3.1As of

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Measurement source (opens a new tab)

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

EPSS probability

0.7%

Scored Percentile 51.8%

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-v9pv-5gc8-w2m6 (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-v9pv-5gc8-w2m6, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-v9pv-5gc8-w2m6, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-v9pv-5gc8-w2m6, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-v9pv-5gc8-w2m6, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-v9pv-5gc8-w2m6, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  7. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-v9pv-5gc8-w2m6, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  8. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-v9pv-5gc8-w2m6, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  9. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-v9pv-5gc8-w2m6, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  10. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-v9pv-5gc8-w2m6, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  11. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-v9pv-5gc8-w2m6, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    NVD CVE-2026-96639 (opens a new tab)

    NVD record: description, CVSS (the CNA score is preferred), affected products, and publication date. Not evidence of exploitation.

    Published Checked

  13. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  14. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-96639, 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 22). Pillow multi-frame loaders can reuse stale native image storage after mode transitions (CVE-2026-96639). https://kevwire.com/record/CVE-2026-96639

MLA

"Pillow multi-frame loaders can reuse stale native image storage after mode transitions (CVE-2026-96639)." KEVWire, 22 Oct. 2026, kevwire.com/record/CVE-2026-96639.

Chicago

KEVWire. "Pillow multi-frame loaders can reuse stale native image storage after mode transitions (CVE-2026-96639)." Last modified October 22, 2026. https://kevwire.com/record/CVE-2026-96639.

Evidence summary (plain text)

Pillow multi-frame loaders can reuse stale native image storage after mode transitions (Oct 22, 2026).
If frame 0 allocates storage for a narrow mode such as `L`, and a later frame of the same dimensions switches to a wider mode such as `RGB` or `CMYK`, Pillow may decode wider pixels into the stale narrow native buffer. This causes native
out-of-bounds writes and process crashes. In the MPO/JPEG path, I also demonstrated command execution in matching Python/Pillow runtime profiles through Pillow's documented `ImageSequence.Iterator` + `frame.load()` path.

Several multi-frame `seek()` / `_open()` paths can update `self._mode` and decoder tiles for a later frame without invalidating `_im`. The decoder then writes or reads according to the new frame mode while the native storage still belongs to the old frame mode.
CVSS 9.8 (version 3.1; evidence as of Oct 22, 2026; https://nvd.nist.gov/vuln/detail/CVE-2026-96639).
EPSS 0.7% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-96639&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-v9pv-5gc8-w2m6: https://github.com/advisories/GHSA-v9pv-5gc8-w2m6
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo1MTozMFrOAAf0Mg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0OToxMlrOAAf0LQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NzoxN1rOAAf0Kg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NTozNlrOAAf0Jw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NTowNVrOAAf0JQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0Mzo0NVrOAAf0Iw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0MzozMVrOAAf0Ig%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODozNTowMVrOAAZRcw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODozMDoyN1rOAAY_5Q%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODoyOTowNVrOAAfz8g%3D%3D
NVD CVE-2026-96639: https://nvd.nist.gov/vuln/detail/CVE-2026-96639
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?pubStartDate=2026-10-12T13:35:55.142Z&pubEndDate=2026-10-26T13:35:55.142Z&resultsPerPage=500&startIndex=8000
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-96639&date=2026-10-26
Record ID: CVE-2026-96639.
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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