Is there a public exploit for CVE-2025-66471?

No public exploit for CVE-2025-66471 was found in Exploit-DB, Metasploit, Nuclei templates or public proof-of-concept repositories. That is an absence of evidence rather than proof that none exists.

How severe is CVE-2025-66471?

CVE-2025-66471 scores CVSS 8.9 (v4.0), rated high. EPSS puts the chance of exploitation in the next 30 days at 0.73%.

What does CVE-2025-66471 affect?

CVE-2025-66471 affects urllib3 urllib3.

8.9 High

urllib3 is a user-friendly HTTP client library for Python.

CVE-2025-66471

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.

Assessment

CVSS: 8.9 (v4.0)
Severity: High
EPSS: 0.73% chance of exploitation in the next 30 days
Known exploited: Not listed in CISA KEV
Exploit availability: Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
Published: 5 December 2025
Last updated: 25 September 2026
Affected: urllib3 · urllib3
Weakness: CWE-409
Assigner: security-advisories@github.com

CVSS vectorCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H

At the source

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