urllib3: Chunked Deflate streaming can enter an infinite loop

Exploitation

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

CVSS 4.0

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

EPSS

• CVE-2026-97688: 0.29% estimated exploitation activity in the next 30 days; percentile 21.82 (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 content in chunks instead of loading the entire response body into memory at once. urllib3 can decompress response bodies according to the HTTP Content-Encoding header. When streaming a compressed, chunked response, urllib3 first consumes data already buffered by the decoder before reading the next HTTP chunk. However, urllib3 versions from 2.6.2 through 2.7.0 could enter an infinite loop when a chunked response contained bytes after the end of the Deflate stream. If the decompressed body exceeded the requested streaming chunk size, Python's zlib implementation could retain the trailing bytes as unconsumed input after reaching the end of the compressed stream. urllib3 would repeatedly attempt to decode those same bytes without making progress or reading more data from the network. A malicious server could exploit this behavior to cause excessive CPU usage and prevent the affected…

Published by the source

30 Sep 2026, 15:00 UTC

Last modified by the source

1 Oct 2026, 17:55 UTC

Source last checked

26 Oct 2026, 13:54 UTC

1. Revision 4 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-gh4c-6fx4-qh6g

2. Revision 3 · amended · 25 Oct 2026

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

3. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-gh4c-6fx4-qh6g

4. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-gh4c-6fx4-qh6g

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# Patch Next advisory brief: CVE-2026-97688

Advisory: urllib3: Chunked Deflate streaming can enter an infinite loop
IDs: CVE-2026-97688, GHSA-gh4c-6fx4-qh6g
Record: https://patchnext.com/advisories/cve-2026-97688 (revision 4, 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: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: urllib3 2.8.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI urllib3: affected >= 2.6.2, < 2.8.0; fixed in 2.8.0

## 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://github.com/advisories/GHSA-gh4c-6fx4-qh6g (GitHub-reviewed advisory GHSA-gh4c-6fx4-qh6g lists affected ranges: PyPI urllib3 >=2.6.2 <2.8.0)
- Verified: https://osv.dev/vulnerability/GHSA-gh4c-6fx4-qh6g (OSV record GHSA-gh4c-6fx4-qh6g lists affected ranges: PyPI urllib3 >=2.6.2 <2.8.0 (fixed))

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