CVE-2025-66471
urllib3 Streaming API improperly handles highly compressed data
Description
urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.0 and prior to 2.6.0, the Streaming API improperly handles highly compressed data. urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once. When streaming a compressed response, urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). The library must read compressed data from the network and decompress it until the requested chunk size is met. Any resulting decompressed data that exceeds the requested amount is held in an internal buffer for the next read operation. The decompression logic could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This can result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data.
INFO
Published Date :
Dec. 5, 2025, 5:16 p.m.
Last Modified :
Dec. 10, 2025, 4:10 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | HIGH | [email protected] | ||||
| CVSS 4.0 | HIGH | [email protected] |
Solution
- Update urllib3 to version 2.6.0 or later.
- Monitor system resource usage after updating.
- Review decompression logic for large files.
Public PoC/Exploit Available at Github
CVE-2025-66471 has a 3 public
PoC/Exploit available at Github.
Go to the Public Exploits tab to see the list.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2025-66471.
| URL | Resource |
|---|---|
| https://github.com/urllib3/urllib3/commit/c19571de34c47de3a766541b041637ba5f716ed7 | Patch |
| https://github.com/urllib3/urllib3/security/advisories/GHSA-2xpw-w6gg-jr37 | Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2025-66471 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2025-66471
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
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Python Just
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2025-66471 vulnerability anywhere in the article.
-
Daily CyberSecurity
urllib3 Flaws Risk Client DoS via Unbounded Decompression and Streaming Resource Exhaustion
The maintainers of urllib3, the ubiquitous HTTP client for Python, have issued a security advisory detailing two high-severity vulnerabilities that could allow malicious servers to crash client applic ... Read more
The following table lists the changes that have been made to the
CVE-2025-66471 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Dec. 10, 2025
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Added CPE Configuration OR *cpe:2.3:a:python:urllib3:*:*:*:*:*:*:*:* versions from (including) 1.0 up to (excluding) 2.6.0 Added Reference Type GitHub, Inc.: https://github.com/urllib3/urllib3/commit/c19571de34c47de3a766541b041637ba5f716ed7 Types: Patch Added Reference Type GitHub, Inc.: https://github.com/urllib3/urllib3/security/advisories/GHSA-2xpw-w6gg-jr37 Types: Vendor Advisory -
New CVE Received by [email protected]
Dec. 05, 2025
Action Type Old Value New Value Added Description urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.0 and prior to 2.6.0, the Streaming API improperly handles highly compressed data. urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once. When streaming a compressed response, urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). The library must read compressed data from the network and decompress it until the requested chunk size is met. Any resulting decompressed data that exceeds the requested amount is held in an internal buffer for the next read operation. The decompression logic could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This can result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data. Added CVSS V4.0 AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Added CWE CWE-409 Added Reference https://github.com/urllib3/urllib3/commit/c19571de34c47de3a766541b041637ba5f716ed7 Added Reference https://github.com/urllib3/urllib3/security/advisories/GHSA-2xpw-w6gg-jr37