HTTPX2: Conflicting Content-Length and Transfer-Encoding headers can be auto-generated

Exploitation

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

CVSS

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

EPSS

• CVE-2026-84380: 0.22% estimated exploitation activity in the next 30 days; percentile 12.76 (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 HTTPX2 can automatically add a Content-Length header to a request that already contains a caller-supplied Transfer-Encoding header. The resulting HTTP/1.1 request contains both framing headers, which can create an ambiguous message boundary and enable request smuggling or connection desynchronization when processed by intermediaries that disagree about which header takes precedence. Details When a request body has a known size, HTTPX2's content encoder returns a default Content-Length. Request.prepare() applies each default header with setdefault(), which only checks whether that same header is already present. It does not check whether the mutually exclusive Transfer-Encoding header is present. For example: The request contains both: On an HTTP/1.1 connection, the body is serialized using chunked transfer coding while both headers are sent on the wire. This violates HTTP message-framing requirements. Fixed-size byte, JSON, form, and known-length multipart bodies can reach…

Published by the source

8 Sep 2026, 20:46 UTC

Last modified by the source

10 Sep 2026, 12:25 UTC

Source last checked

26 Oct 2026, 13:51 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-pf96-p4fj-6566

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-pf96-p4fj-6566

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-pf96-p4fj-6566

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# Patch Next advisory brief: CVE-2026-84380

Advisory: HTTPX2: Conflicting Content-Length and Transfer-Encoding headers can be auto-generated
IDs: CVE-2026-84380, GHSA-pf96-p4fj-6566
Record: https://patchnext.com/advisories/cve-2026-84380 (revision 3, source checked 26 Oct 2026, 13:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.6)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: httpx2 2.11.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI httpx2: affected < 2.11.0; fixed in 2.11.0

## Suggested inspection (read-only)
- `python -m pip show httpx2`: 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 httpx2.
- 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-pf96-p4fj-6566 (GitHub-reviewed advisory GHSA-pf96-p4fj-6566 lists affected ranges: PyPI httpx2 >=0 <2.11.0)
- Verified: https://osv.dev/vulnerability/GHSA-pf96-p4fj-6566 (OSV record GHSA-pf96-p4fj-6566 lists affected ranges: PyPI httpx2 >=0 <2.11.0 (fixed))

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