CVE-2025-66418: Python URLLIB3

High severity, CVSS 7.5. EPSS: 0.7% chance of exploitation in the next 30 days.

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 products

• Python URLLIB3: from 1.24, before 2.6.0 (fixed in 2.6.0)

Published 2025-12-05. Last modified 2026-09-25.