h2 allows HTTP Request Smuggling due to illegal characters in headers

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:N/SC:N/SI:L/SA:N · score source

EPSS

• CVE-2025-57804: 1.88% estimated exploitation activity in the next 30 days; percentile 79.51 (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.

Summary HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls.

Published by the source

25 Aug 2025, 20:44 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:35 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-847f-9342-265h

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-847f-9342-265h

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# Patch Next advisory brief: CVE-2025-57804

Advisory: h2 allows HTTP Request Smuggling due to illegal characters in headers
IDs: CVE-2025-57804, GHSA-847f-9342-265h
Record: https://patchnext.com/advisories/cve-2025-57804 (revision 3, source checked 26 Oct 2026, 13:35 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: h2 4.3.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI h2: affected < 4.3.0; fixed in 4.3.0

## 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://osv.dev/vulnerability/GHSA-847f-9342-265h (OSV record GHSA-847f-9342-265h lists affected ranges: PyPI h2 >=0 <4.3.0 (fixed))
- Verified: https://github.com/advisories/GHSA-847f-9342-265h (GitHub-reviewed advisory GHSA-847f-9342-265h lists affected ranges: PyPI h2 >=0 <4.3.0)

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