---
id: CVE-2025-66418
title: urllib3 is a user-friendly HTTP client library for Python
summary: >-
  urllib3 is a user-friendly HTTP client library for Python. Starting in version
  1.24 and prior to 2.6.0, the number of links in the decompression chain was
  unbounded allowing a malicious server to insert a virtually unlimited number
  of co…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-770
vendor: python
product: urllib3
affected:
  - 'urllib3 >= 1.24, < 2.6.0'
patched:
  - urllib3 2.6.0
published: '2025-12-05'
updated: '2026-09-25'
sourceUpdated: '2026-09-25T23:10:00.463'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2025-66418'
references:
  - url: >-
      https://github.com/urllib3/urllib3/commit/24d7b67eac89f94e11003424bcf0d8f7b72222a8
    label: security-advisories@github.com
  - url: 'https://github.com/urllib3/urllib3/security/advisories/GHSA-gm62-xv2j-4w53'
    label: security-advisories@github.com
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2025-66418'
  - url: 'https://github.com/urllib3/urllib3'
tags:
  - nvd
  - osv
  - pip
epss: 0.00682
epssPercentile: 0.50505
aliases:
  - GHSA-gm62-xv2j-4w53
  - PYSEC-2026-1998
ecosystem: pip
ingestedAt: '2026-07-08T18:25:49.735Z'
---

## Overview

urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. This vulnerability is fixed in 2.6.0.

## Affected

- `urllib3 >= 1.24, < 2.6.0`

## Remediation

Upgrade past the affected range:

- `urllib3 2.6.0`

## Package advisory (CVE-2025-66418)

Affected packages:

- `urllib3 >= 1.24, < 2.6.0`

Patched in:

- `urllib3 2.6.0`

Source: https://osv.dev/vulnerability/GHSA-gm62-xv2j-4w53