h2 allows HTTP Request Smuggling due to illegal characters in headers
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Attack requirements
- None
- Impact type
- —
- Confidentiality · integrity · availability
- None · None · None
- Fix status
- Partial Red Hat: 2 without a fix
Attack profile from CVSS 4.0 · CNA. Fix status from CNA, GitHub and Red Hat.
Summary
h2 is a pure-Python implementation of a HTTP/2 protocol stack. Prior to version 4.3.0, an 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. This issue has been patched in version 4.3.0.
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 4.3.0 | 4.3.0 | Patch available | CNA |
| — | h2 (PyPI) | before 4.3.0 | 4.3.0 | Patch available | GHSA-847f-9342-265h |
- h2 Vendorpython-hyper Affected before 4.3.0 Fixed in 4.3.0 Patch available CNA
- h2 (PyPI) Vendor— Affected before 4.3.0 Fixed in 4.3.0 Patch available GHSA-847f-9342-265h
Remediation
- Fix · CNA h2 (python-hyper) Fixed in 4.3.0
- Fix · GHSA-847f-9342-265h h2 (PyPI) Fixed in 4.3.0 Advisory →
Vendor assessments
Red Hat
Red Hat rating: Moderate. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| OpenShift Service Mesh 3 | Affected | No fix yet: Fix deferred |
| Red Hat Ansible Automation Platform 2 | Affected | No fix yet: Fix deferred |
Timeline
- 2026-10-02 Threatist record created from CVE-2025-57804
- 2026-10-03 Alias added: GHSA-847f-9342-265h
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-2026-50126 h2: Duplicate Host header could facilitate request smuggling CVSS 3.1MEDIUM 5.3
- TI-2026-42767 hpack: Unbounded variable integer decoding can cause run-away computation on malformed input CVSS 4.0MEDIUM 6.3
Sources
- CVE-2025-57804 · CVE Program
- CVE-2025-57804 · NVD
- FIRST EPSS
- GHSA-847f-9342-265h · GitHub Advisory Database · CC BY 4.0
- CVE-2025-57804 · 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