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fkie_cve-2026-96641

Pillow is a Python imaging library. Prior to 13.0.0, PILfont text-width accumulation in src/_imaging.c checks positive glyph advances against INT_MAX but does not check negative advances against INT_MIN. Repeating a signed 16-bit advance of -32768 can therefore execute a signed C integer overflow in xsize += dx, producing a large positive width from a negative mathematical result. Applications that use getlength(), getsize(), or derived layout, quota, serialization, or allocation calculations can experience logic-integrity errors or resource consumption, while the high-level mask path commonly retains a decompression-bomb check. This issue is fixed in version 13.0.0.

GHSA-4rm8-3hmm-39fg — Pillow: Reachable undefined behavior in native integer operations, potentially leading to denial of service or logic in…

Affected: PyPI: pillow

### 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 <= INT_MAX - dx` It does **not** prove the other half of the contract: `dx < 0 => 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**.

### 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)…

UBUNTU-CVE-2026-96641

Affected: Ubuntu:Pro:14.04:LTS: pillow, Ubuntu:Pro:16.04:LTS: pillow, Ubuntu:Pro:18.04:LTS: pillow, Ubuntu:Pro:20.04:LTS: pillow, Ubuntu:Pro:20.04:LTS: pillow-python2, Ubuntu:22.04:LTS: pillow, Ubuntu:24.04:LTS: pillow, Ubuntu:26.10: pillow, Ubuntu:26.04:LTS: pillow

(Pillow is a Python imaging library. Prior to 13.0.0, PILfont text-widt ...)