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

Source: https://junglewise.ai/threats/cve-2025-66418-urllib3-is-a-user-friendly-http-client-library-for-python  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2025-12-05  
License: CC BY 4.0 with a link back

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)](https://junglewise.ai/threats/technologies/urllib3). Vendors: [PyPI](https://junglewise.ai/threats/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

  ```sh
  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

## Related threats

- [urllib3 HTTP client unbounded memory allocation in chunked transfer](https://junglewise.ai/threats/cve-2026-97689-urllib3-is-an-http-client-library-for-python-from-1-10-3-until-2)
- [urllib3 infinite loop in Deflate decoder](https://junglewise.ai/threats/cve-2026-97688-urllib3-is-an-http-client-library-for-python-from-2-6-2-until-2-8)
- [urllib3 HTTPS proxy TLS settings isolation](https://junglewise.ai/threats/cve-2026-97687-urllib3-is-an-http-client-library-for-python-from-1-26-0-until-2)
- [PYSEC-2026-1997 - urllib3 does not control redirects in browsers and Node.js](https://junglewise.ai/threats/cve-2025-50182-urllib3-does-not-control-redirects-in-browsers-and-node-js)
- [PYSEC-2026-1999 - urllib3 redirects are not disabled when retries are disabled on PoolManager instantiation](https://junglewise.ai/threats/cve-2025-50181-urllib3-redirects-are-not-disabled-when-retries-are-disabled-on)