CVE-2025-66418: PYSEC-2026-1998 - urllib3 allows an unbounded number of links in the decompression chain

CVE-2025-66418 · Severity: high · CVSS 7.5 · Published 2025-12-05

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.

Technologies: urllib3 (PyPI). Vendors: PyPI.

Quick answers

Is CVE-2025-66418 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2025-66418 being exploited in the wild.
How severe is CVE-2025-66418?
High severity, with a CVSS score of 7.5, published 5 December 2025.
Which products and versions are affected?
urllib3 (PyPI) ECOSYSTEM:introduced=1.24,fixed=2.6.0.
Is there a fix for CVE-2025-66418?
Yes. urllib3 (PyPI): upgrade to 2.6.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
Am I affected by CVE-2025-66418?
Check whether you run urllib3 (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • urllib3 (PyPI): upgrade to 2.6.0 or later
    pip install "urllib3>=2.6.0"

Affected products

  • urllib3 (PyPI) ECOSYSTEM:introduced=1.24,fixed=2.6.0

Identifiers

  • CVE-2025-66418
  • GHSA-gm62-xv2j-4w53
  • PYSEC-2026-1998

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