urllib3 streaming API improperly handles highly compressed data

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.9 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H · score source

EPSS

• CVE-2025-66471: 0.68% estimated exploitation activity in the next 30 days; percentile 52.50 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact 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; CWE-409) on the client side, even if the application only requested a small chunk of data. Affected usages Applications and libraries using urllib3 version…

Published by the source

5 Dec 2025, 18:15 UTC

Last modified by the source

25 Sep 2026, 17:15 UTC

Source last checked

26 Oct 2026, 13:54 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-2xpw-w6gg-jr37

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-2xpw-w6gg-jr37

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# Patch Next advisory brief: CVE-2025-66471

Advisory: urllib3 streaming API improperly handles highly compressed data
IDs: CVE-2025-66471, GHSA-2xpw-w6gg-jr37
Record: https://patchnext.com/advisories/cve-2025-66471 (revision 3, source checked 26 Oct 2026, 13:54 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: urllib3 2.6.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI urllib3: affected >= 1.0, < 2.6.0; fixed in 2.6.0

## Suggested inspection (read-only)
- `python -m pip show urllib3`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on urllib3.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-2xpw-w6gg-jr37 (OSV record GHSA-2xpw-w6gg-jr37 lists affected ranges: PyPI urllib3 >=1.0 <2.6.0 (fixed))
- Verified: https://github.com/advisories/GHSA-2xpw-w6gg-jr37 (GitHub-reviewed advisory GHSA-2xpw-w6gg-jr37 lists affected ranges: PyPI urllib3 >=1.0 <2.6.0)

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