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[USN-7927-3] urllib3 regression

Posted on 13 January 2026
Ubuntu Security

==========================================================================Ubuntu Security Notice USN-7927-3
January 13, 2026

python-urllib3 regression
==========================================================================
A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 25.10
- Ubuntu 25.04
- Ubuntu 24.04 LTS

Summary:

USN-7927-1 introduced a regression in urllib3

Software Description:
- python-urllib3: HTTP library with thread-safe connection pooling

Details:

USN-7927-1 fixed vulnerabilities in urllib3. The update for CVE-2025-66471
introduced a regression in urllib3 when decompressing zstd data. This
update fixes the problem.

We apologize for the inconvenience.

Original advisory details:

Illia Volochii discovered that urllib3 did not limit the steps in a
decompression chain. An attacker could possibly use this issue to cause
urllib3 to use excessive resources, causing a denial of service.
(CVE-2025-66418)

Rui Xi discovered that urllib3 incorrectly handled highly compressed data.
An attacker could possibly use this issue to cause urllib3 to use
excessive resources, causing a denial of service. This issue only affected
Ubuntu 24.04 LTS, Ubuntu 25.04, and Ubuntu 25.10. (CVE-2025-66471)

For the brotli encoding, the fix for CVE-2025-66471 requires an additional
security update in the brotli package.

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 25.10
python3-urllib3 2.3.0-3ubuntu0.4

Ubuntu 25.04
python3-urllib3 2.3.0-2ubuntu0.5

Ubuntu 24.04 LTS
python3-urllib3 2.0.7-1ubuntu0.6

In general, a standard system update will make all the necessary changes.

References:
https://ubuntu.com/security/notices/USN-7927-3
https://ubuntu.com/security/notices/USN-7927-2
https://ubuntu.com/security/notices/USN-7927-1
CVE-2025-66471, https://bugs.launchpad.net/bugs/2136906

Package Information:
https://launchpad.net/ubuntu/+source/python-urllib3/2.3.0-3ubuntu0.4
https://launchpad.net/ubuntu/+source/python-urllib3/2.3.0-2ubuntu0.5
https://launchpad.net/ubuntu/+source/python-urllib3/2.0.7-1ubuntu0.6

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