h2: Duplicate Host header could facilitate request smuggling

Exploitation

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

CVSS

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

EPSS

• CVE-2026-71554: 0.42% estimated exploitation activity in the next 30 days; percentile 36.17 (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 h2 <=4.4.0 accepts request header blocks containing more than one Host header, and forwards every Host header to the consuming application. Where the consumer downgrades HTTP/2 to HTTP/1.1, the resulting request carries two Host header lines, which is a request smuggling primitive (CWE-444). Patches Patched and fixed in v4.4.1 Workarounds Users of the h2 library are advised to check and follow HTTP semantics best practices in their application code. h2 provides best effort sanity checks, but ultimately the calling code is responsible to ensure proper and safe usage of HTTP/2 as provided by h2, hyperframe, and hpack. References Similar to the previously disclosed and fixed duplicate content-length issue.

Published by the source

6 Aug 2026, 21:53 UTC

Last modified by the source

6 Aug 2026, 21:53 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-6hr6-w5qg-qmwg

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# Patch Next advisory brief: CVE-2026-71554

Advisory: h2: Duplicate Host header could facilitate request smuggling
IDs: CVE-2026-71554, GHSA-6hr6-w5qg-qmwg
Record: https://patchnext.com/advisories/cve-2026-71554 (revision 1, source checked 26 Oct 2026, 13:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: h2 4.4.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI h2: affected <= 4.4.0; fixed in 4.4.1

## Suggested inspection (read-only)
- `python -m pip show h2`: 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 h2.
- 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-6hr6-w5qg-qmwg (GitHub-reviewed advisory GHSA-6hr6-w5qg-qmwg lists affected ranges: PyPI h2 >=0 <=4.4.0)

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