urllib3 2.6.0: Unbounded Decompression Chain Causing DoS (CPU/Memory)
CVE-2025-66418 Published on December 5, 2025
urllib3 allows an unbounded number of links in the decompression chain
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.
Weakness Type
Allocation of Resources Without Limits or Throttling
The software allocates a reusable resource or group of resources on behalf of an actor without imposing any restrictions on the size or number of resources that can be allocated, in violation of the intended security policy for that actor.
Products Associated with CVE-2025-66418
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Affected Versions
urllib3 Version >= 1.24, < 2.6.0 is affected by CVE-2025-66418Vulnerable Packages
The following package name and versions may be associated with CVE-2025-66418
| Package Manager | Vulnerable Package | Versions | Fixed In |
|---|---|---|---|
| pip | urllib3 | >= 1.24, < 2.6.0 | 2.6.0 |
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.