h2: Duplicate Host header could facilitate request smuggling
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Impact type
- —
- Confidentiality · integrity · availability
- None · None · Low
- Fix status
- Partial Red Hat: 6 without a fix
Attack profile from CVSS 3.1 · CNA. Fix status from CNA, GitHub and Red Hat.
Summary
h2 is a pure-Python implementation of a HTTP/2 protocol stack. Versions up to and including 4.4.0 accept request header blocks containing more than one Host header, and forward 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, providing a request smuggling primitive. This issue is fixed in version 4.4.1.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- Yes
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| python-hyper | h2 | before 4.4.1 | 4.4.1 | Patch available | CNA |
| — | h2 (PyPI) | before 4.4.1 | 4.4.1 | Patch available | GHSA-6hr6-w5qg-qmwg |
- h2 Vendorpython-hyper Affected before 4.4.1 Fixed in 4.4.1 Patch available CNA
- h2 (PyPI) Vendor— Affected before 4.4.1 Fixed in 4.4.1 Patch available GHSA-6hr6-w5qg-qmwg
Remediation
- Fix · CNA h2 (python-hyper) Fixed in 4.4.1
- Fix · GHSA-6hr6-w5qg-qmwg h2 (PyPI) Fixed in 4.4.1 Advisory →
Vendor assessments
Red Hat
Red Hat rating: Moderate. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Migration Toolkit for Containers | Affected | No fix yet: Fix deferred |
| Red Hat Ansible Automation Platform 2 | Affected | No fix yet: Fix deferred |
| Red Hat Hardened Images | Affected | No fix yet: Fix deferred |
| Red Hat OpenShift AI (RHOAI) | Affected | No fix yet: Fix deferred |
| Red Hat OpenStack Platform 17.1 | Affected | No fix yet: Fix deferred |
| Red Hat OpenStack Platform 18.0 | Affected | No fix yet: Fix deferred |
Timeline
- 2026-10-02 Threatist record created from CVE-2026-71554
- 2026-10-03 Alias added: GHSA-6hr6-w5qg-qmwg
- 2026-10-16 Affected versions changed · Red Hat
References
More python-hyper vulnerabilities
- TI-2026-79288 Out-of-bounds write in Google Brotli decoder KEVCVSS 4.0CRITICAL 9.2
- TI-2025-28443 h11 accepts some malformed Chunked-Encoding bodies CVSS 3.1CRITICAL 9.1
- TI-2025-35687 h2 allows HTTP Request Smuggling due to illegal characters in headers CVSS 4.0MEDIUM 6.9
- TI-2026-42767 hpack: Unbounded variable integer decoding can cause run-away computation on malformed input CVSS 4.0MEDIUM 6.3
Sources
- CVE-2026-71554 · CVE Program
- CVE-2026-71554 · NVD
- FIRST EPSS
- GHSA-6hr6-w5qg-qmwg · GitHub Advisory Database · CC BY 4.0
- CVE-2026-71554 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences