urllib3: HTTPResponse.stream()/read_chunked() buffers an unbounded chunk-size line into memory

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-2026-97689: 0.34% estimated exploitation activity in the next 30 days; percentile 26.89 (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 efficiently handling large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once. When decoding a chunked-transfer-encoded response, this API reads each chunk's size field by buffering until it sees \n or EOF. A malicious HTTP server can return Transfer-Encoding: chunked and then send a very long run of bytes without any newline, causing the streaming client to buffer that entire run before urllib3 can reject the chunk size as invalid and allocate more memory than intended. To fix the issue, we'll reject chunk-size fields larger than 65536 bytes, as already done in the non-streaming case, which is currently handled by the Python standard library. Note that this only applies to reading the chunk-size field, not the chunk data, which is already handled correctly. Thus, there shouldn't be any impact for non-malicious servers. Affected usages Applications and libraries using urllib3…

Published by the source

30 Sep 2026, 14:46 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-vxq7-64xx-v4gw

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-vxq7-64xx-v4gw

4. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-vxq7-64xx-v4gw

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# Patch Next advisory brief: CVE-2026-97689

Advisory: urllib3: HTTPResponse.stream()/read_chunked() buffers an unbounded chunk-size line into memory
IDs: CVE-2026-97689, GHSA-vxq7-64xx-v4gw
Record: https://patchnext.com/advisories/cve-2026-97689 (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: 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 2.8.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI urllib3: affected >= 1.10.3, < 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-vxq7-64xx-v4gw (GitHub-reviewed advisory GHSA-vxq7-64xx-v4gw lists affected ranges: PyPI urllib3 >=1.10.3 <2.8.0)
- Verified: https://osv.dev/vulnerability/GHSA-vxq7-64xx-v4gw (OSV record GHSA-vxq7-64xx-v4gw lists affected ranges: PyPI urllib3 >=1.10.3 <2.8.0 (fixed))

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