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Response snapshot Oct 26, 2026, 21:42 UTC

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VulnerabilityCVE-2026-96641

Pillow: Reachable undefined behavior in native integer operations, potentially leading to denial of service or logic integrity compromise

Package: Pillow 12.3.0 Source: PyPI sdist pillow-12.3.0.tar.gz SHA-256: 3b8182a766685eaa002637e28b4ec8d6b18819a0c71f579bf0dbaa5830297cce Sink: src/_imaging.c:2809:15, xsize += dx Run with an ordinary Pillow 12.3.

Published Updated

At a glance

Exploited
Not known
Severity
6.5 Medium (CVSS 3.1)
Chance of exploitation
0.4% 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-4rm8-3hmm-39fg (opens a new tab)

What happened

### Summary Affected Object and Source

Package: Pillow 12.3.0 Source: PyPI sdist pillow-12.3.0.tar.gz SHA-256: 3b8182a766685eaa002637e28b4ec8d6b18819a0c71f579bf0dbaa5830297cce Sink: src/_imaging.c:2809:15, xsize += dx

### Details The code in Pillow 12.3.0 is: ```c for (xsize = 0; *text; text++) { int dx = self->glyphs[*text].dx; if (dx > 0 && xsize > INT_MAX - dx) { PyErr_SetString(PyExc_OverflowError, "Width too large"); return -1; } xsize += dx; } ``` The PILfont glyph advance is parsed from a signed 16-bit field, so the valid range for `dx` is `[-32768, 32767]`.

The existing check only proves: `dx > 0 => xsize xsize >= INT_MIN - dx` Taking `dx = -32768` and repeating it 65,537 times, the mathematical sum is `-2,147,516,416`, which is less than `INT_MIN = -2,147,483,648`. On the 65,537th addition, the operation `-2147483648 + -32768` is performed, resulting in **C signed overflow undefined behavior**.

Continue reading

### PoC ```python """Minimal public-API reproducer for the Pillow 12.3.0 textwidth underflow.

Run with an ordinary Pillow 12.3.0 build to observe the wrapped width, or with the PyC-Contract UBSan build to obtain the exact native diagnostic. """

from __future__ import annotations

import struct from io import BytesIO

import PIL from PIL import Image, ImageFont

ADVANCE = -(2**15) CONTROL_REPEAT = 65_536 WITNESS_REPEAT = 65_537

def bitmap_font(advance: int) -> ImageFont.ImageFont: metrics = bytearray(256 * 20) glyph_offset = ord("A") * 20 metrics[glyph_offset : glyph_offset + 2] = struct.pack(">h", advance) font_data = b"PILfont\n;;;;;;1;\nDATA\n" + bytes(metrics) font = ImageFont.ImageFont() font._load_pilfont_data(BytesIO(font_data), Image.new("1", (1, 1))) return font

def main() -> None: font = bitmap_font(ADVANCE) print(f"Pillow={PIL.__version__}") print(f"control_math={ADVANCE * CONTROL_REPEAT}") print(f"control_return={font.getlength('A' * CONTROL_REPEAT)}") print(f"witness_math={ADVANCE * WITNESS_REPEAT}") print(f"witness_return={font.getlength('A' * WITNESS_REPEAT)}")

if __name__ == "__main__": main() ```

Observations from a regular 12.3.0 release build:

```text Pillow=12.3.0 control_math=-2147483648 control_return=0 witness_math=-2147516416 witness_return=2147450880 ```

The adjacent control uses 65,536 repetitions, producing a mathematical result of exactly `INT_MIN`, and the function subsequently returns 0 according to existing semantics. Adding just a single character flips the return value to `2,147,450,880`.

### Impact The directly confirmed impact is width semantic integrity violation: getlength() and boundary calculations based on getsize() may return a huge positive value, while the true mathematical width is negative.

The high-level getmask() typically passes through the decompression-bomb check, which mitigates the risk of direct massive allocation; however, internal font objects and applications that use the width for layout, quotas, serialization, or subsequent allocations may be affected by resource consumption or logic errors.

Therefore, the current recommended severity rating is "Reachable native integer undefined behavior, with potential DoS/logic integrity impact; specific security severity to be confirmed by the upstream threat model", rather than a memory corruption vulnerability.

### Recommended Patch and Regression Tests

It is recommended to extend the overflow check to cover both directions: ```c if ((dx > 0 && xsize > INT_MAX - dx) || (dx < 0 && xsize < INT_MIN - dx)) { PyErr_SetString(PyExc_OverflowError, "Width too large"); return -1; } ``` At a minimum, four test cases should be added: 1. `dx = 32767, repeat = 65538` — upper adjacent control (should succeed); 2. `dx = 32767, repeat = 65539` — should raise `OverflowError`; 3.

`dx = -32768, repeat = 65536` — lower adjacent control (should succeed); 4. `dx = -32768, repeat = 65537` — should raise `OverflowError`.

Additionally, it should be verified that exception propagation is consistent across `getlength()`, `getbbox()`, `getmask()`, and the underlying `font.getsize()`.

Affected package ranges reported by GitHub: pillow: < 13.0.0

Scores and how they were measured

CVSS base severity

6.5

Version 3.1As of

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

0.4%

Scored Percentile 28.4%

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-4rm8-3hmm-39fg (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-4rm8-3hmm-39fg, 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-4rm8-3hmm-39fg, 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-4rm8-3hmm-39fg, 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-4rm8-3hmm-39fg, 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-4rm8-3hmm-39fg, 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-4rm8-3hmm-39fg, 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-4rm8-3hmm-39fg, 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-4rm8-3hmm-39fg, 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-4rm8-3hmm-39fg, 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-4rm8-3hmm-39fg, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-96641, 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: Reachable undefined behavior in native integer operations, potentially leading to denial of service or logic integrity compromise (CVE-2026-96641). https://kevwire.com/record/CVE-2026-96641

MLA

"Pillow: Reachable undefined behavior in native integer operations, potentially leading to denial of service or logic integrity compromise (CVE-2026-96641)." KEVWire, 22 Oct. 2026, kevwire.com/record/CVE-2026-96641.

Chicago

KEVWire. "Pillow: Reachable undefined behavior in native integer operations, potentially leading to denial of service or logic integrity compromise (CVE-2026-96641)." Last modified October 22, 2026. https://kevwire.com/record/CVE-2026-96641.

Evidence summary (plain text)

Pillow: Reachable undefined behavior in native integer operations, potentially leading to denial of service or logic integrity compromise (Oct 22, 2026).
Package: Pillow 12.3.0
Source: PyPI sdist pillow-12.3.0.tar.gz
SHA-256: 3b8182a766685eaa002637e28b4ec8d6b18819a0c71f579bf0dbaa5830297cce
Sink: src/_imaging.c:2809:15, xsize += dx

Run with an ordinary Pillow 12.3.0 build to observe the wrapped width, or with
the PyC-Contract UBSan build to obtain the exact native diagnostic.
"""
CVSS 6.5 (version 3.1; evidence as of Oct 22, 2026; https://github.com/advisories/GHSA-4rm8-3hmm-39fg).
EPSS 0.4% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-96641&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-4rm8-3hmm-39fg: https://github.com/advisories/GHSA-4rm8-3hmm-39fg
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
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-96641&date=2026-10-26
Record ID: CVE-2026-96641.
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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