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CVE-2025-66418

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Date: December 5, 2025

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.

Severity Score

Related Resources (5)

https://nvd.nist.gov/vuln/detail/CVE-2025-66418
https://github.com/urllib3/urllib3
https://github.com/urllib3/urllib3/commit/24d7b67eac89f94e11003424bcf0d8f7b72222a8
https://github.com/urllib3/urllib3/security/advisories/GHSA-gm62-xv2j-4w53
https://www.mend.io/vulnerability-database/CVE-2025-66418

Severity Score

Weakness Type (CWE)

Allocation of Resources Without Limits or Throttling

CWE-770

Top Fix

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Upgrade Version

Upgrade to version urllib3 - 2.6.0;urllib3 - 2.6.0;https://github.com/urllib3/urllib3.git - 2.6.0

Learn More

CVSS v3.1

Base Score:
Attack Vector (AV): NETWORK
Attack Complexity (AC): LOW
Privileges Required (PR): NONE
User Interaction (UI): NONE
Scope (S): UNCHANGED
Confidentiality (C): NONE
Integrity (I): NONE
Availability (A): HIGH

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