h11 accepts some malformed Chunked-Encoding bodies

Exploitation

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

CVSS

9.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS

• CVE-2025-43859: 0.59% estimated exploitation activity in the next 30 days; percentile 48.45 (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 A leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions.

Details HTTP/1.1 Chunked-Encoding bodies are formatted as a sequence of "chunks", each of which consists of: - chunk length - `\r\n` - `length` bytes of content - `\r\n` In versions of h11 up to 0.14.0, h11 instead parsed them as: - chunk length - `\r\n` - `length` bytes of content - any two bytes i.e. it did not validate that the trailing `\r\n` bytes were correct, and if you put 2 bytes of garbage there it would be accepted, instead of correctly rejecting the body as malformed. By itself this is harmless. However, suppose you have a proxy or reverse-proxy that tries to analyze HTTP requests, and your proxy has a _different_ bug in parsing Chunked-Encoding, acting as if the format is: - chunk length - `\r\n` - `length` bytes of content - more bytes of content, as many as it takes until you find a `\r\n` For example,…

Published by the source

24 Apr 2025, 16:07 UTC

Last modified by the source

27 Oct 2026, 16:45 UTC

Source last checked

28 Oct 2026, 06:10 UTC

1. Revision 4 · amended · 28 Oct 2026

   Amended from osv record GHSA-vqfr-h8mv-ghfj

2. Revision 3 · amended · 27 Oct 2026

   Amended from ghsa record GHSA-vqfr-h8mv-ghfj

3. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-vqfr-h8mv-ghfj

4. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-vqfr-h8mv-ghfj

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# Patch Next advisory brief: CVE-2025-43859

Advisory: h11 accepts some malformed Chunked-Encoding bodies
IDs: CVE-2025-43859, GHSA-vqfr-h8mv-ghfj
Record: https://patchnext.com/advisories/cve-2025-43859 (revision 4, source checked 28 Oct 2026, 06:10 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: h11 0.16.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI h11: affected < 0.16.0; fixed in 0.16.0

## Suggested inspection (read-only)
- `python -m pip show h11`: 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 h11.
- 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-vqfr-h8mv-ghfj (OSV record GHSA-vqfr-h8mv-ghfj lists affected ranges: PyPI h11 >=0 <0.16.0 (fixed))
- Verified: https://github.com/advisories/GHSA-vqfr-h8mv-ghfj (GitHub-reviewed advisory GHSA-vqfr-h8mv-ghfj lists affected ranges: PyPI h11 >=0 <0.16.0)

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