CVE-2025-66471Network attack vector

Denial of service via unbounded streaming decompression

Published Dec 5, 2025 · Updated Dec 5, 2025

Denial of service in urllib3 before 2.6.0 allows remote attackers to exhaust client CPU and memory via compressed HTTP responses. HTTPResponse streaming reads fully decode a small, highly compressed input in one operation and buffer output beyond the requested chunk size. Exploitation requires an application to stream a compressed response from an untrusted source with decoding enabled, and the resulting resource exhaustion can interrupt the client process.

CVSS severity8.9
High
EPSS probability0.68%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in urllib3 before 2.6.0 allows remote attackers to exhaust client CPU and memory via compressed HTTP responses. HTTPResponse streaming reads fully decode a small, highly compressed input in one operation and buffer output beyond the requested chunk size. Exploitation requires an application to stream a compressed response from an untrusted source with decoding enabled, and the resulting resource exhaustion can interrupt the client process.

The record

CVE
CVE-2025-66471
Published
Dec 5, 2025
Updated
Dec 5, 2025
Vendor
urllib3
Product
urllib3
Classifications
CWE-409, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 5, 2025CVE publishedPublication date reported by the CVE source.
  2. Dec 5, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=>= 1.0, < 2.6.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

urllib3 · urllib3version=>= 1.0, < 2.6.0

Published CVSS scores

8.9GitHubCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-409Improper Handling of Highly Compressed Data (Data Amplification)
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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