h2: Duplicate Host header could facilitate request smuggling
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.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | h2 | < 4.4.1 | 4.4.1 |
References
- https://github.com/python-hyper/h2/security/advisories/GHSA-6hr6-w5qg-qmwg
- https://github.com/python-hyper/h2/commit/292a40829feefda98c8509dcdbbb4a57af9bd6a6
- https://github.com/python-hyper/h2
- https://pypi.org/project/h2
- https://github.com/advisories/GHSA-6hr6-w5qg-qmwg
- https://nvd.nist.gov/vuln/detail/CVE-2026-71554
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L- Weakness (CWE)
CWE-444- Platform
- Python (PyPI)
- Package
h2- Disclosed
- 06.08.2026
- Updated
- 11.10.2026 15:18
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-3628 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-6hr6-w5qg-qmwg - CC BY 4.0
Vulnerability history: h2
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 2
- First → latest
- 08.2025 08.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Medium 2
Disclosures per year
- 2025: 1
- 2026: 1
-
h2: Duplicate Host header could facilitate request smuggling this entry
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