CVE-2026-96641

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

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;
}

"""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()

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

if ((dx > 0 && xsize > INT_MAX - dx) ||
    (dx < 0 && xsize < INT_MIN - dx)) {
    PyErr_SetString(PyExc_OverflowError, "Width too large");
    return -1;
}

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.

An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value. Typical impact: incorrect size calculations leading to heap overflows or logic errors.

CVE-2026-96641 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (13.0.0); upgrading removes the vulnerable code path.

Kodem intelligence

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1. What is CVE-2026-96641? CVE-2026-96641 is a medium-severity integer overflow or wraparound vulnerability in pillow (pip), affecting versions < 13.0.0. It is fixed in 13.0.0. An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value.
2. How severe is CVE-2026-96641? CVE-2026-96641 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of pillow are affected by CVE-2026-96641? pillow (pip) versions < 13.0.0 is affected.
4. Is there a fix for CVE-2026-96641? Yes. CVE-2026-96641 is fixed in 13.0.0. Upgrade to this version or later.
5. Is CVE-2026-96641 exploitable, and should I be worried? Whether CVE-2026-96641 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-96641 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-96641? Upgrade pillow to 13.0.0 or later.

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