h11 accepts some malformed Chunked-Encoding bodies
Priority MEDIUM 48CWE-444Published · Updated
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Impact type
- —
- Confidentiality · integrity · availability
- High · High · None
- Fix status
- Partial Red Hat: 5 fixed, 3 without a fix
Attack profile from CVSS 3.1 · CNA. Fix status from CNA, GitHub and Red Hat.
Summary
h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.
Why it matters now
Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00594 48th percentileScored 2026-11-07
Fix Partial h11 (python-hyper): 0.16.0h11 (PyPI): 0.16.0Red Hat: 4 advisoriesRed Hat: 5 fixed, 3 without a fix
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- Yes
- Technical impact
- Total
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 | h11 | before 0.16.0 | 0.16.0 | Patch available | CNA |
| — | h11 (PyPI) | before 0.16.0 | 0.16.0 | Patch available | GHSA-vqfr-h8mv-ghfj |
- h11 Vendorpython-hyper Affected before 0.16.0 Fixed in 0.16.0 Patch available CNA
- h11 (PyPI) Vendor— Affected before 0.16.0 Fixed in 0.16.0 Patch available GHSA-vqfr-h8mv-ghfj
Remediation
- Fix · CNA h11 (python-hyper) Fixed in 0.16.0
- Fix · GHSA-vqfr-h8mv-ghfj h11 (PyPI) Fixed in 0.16.0 Advisory →
Red Hat 4 advisories
- RHSA-2025:8615 Released 2025-06-05 · Red Hat rating ImportantRed Hat Ansible Automation Platform 2.4 Container Release Update
- Red Hat Ansible Automation Platform 2.4 for RHEL 8 ansible-automation-platform-24/lightspeed-rhel8
- RHSA-2025:8221 Released 2025-05-27 · Red Hat rating ImportantRed Hat Ansible Automation Platform 2.5 Container Release Update
- Red Hat Ansible Automation Platform 2.5 for RHEL 8 ansible-automation-platform-25/ee-supported-rhel8, ansible-automation-platform-25/lightspeed-chatbot-rhel8, ansible-automation-platform-25/lightspeed-rhel8
- Red Hat Ansible Automation Platform 2.5 for RHEL 9 ansible-automation-platform-25/ee-cloud-services-rhel9, ansible-automation-platform-25/ee-supported-rhel9
- RHSA-2025:7536 Released 2025-05-14 · Red Hat rating ImportantRed Hat OpenStack Platform 17.1 (python-h11) security update
- Red Hat OpenStack Platform 17.1 python-h11-0.12.0-2.1.el9ost, python3-h11-0.12.0-2.1.el9ost
Show 1 more Red Hat advisories
- RHSA-2025:7535 Released 2025-05-14 · Red Hat rating ImportantRed Hat OpenStack Platform 18.0 (python-h11) security update
- Red Hat OpenStack Services on OpenShift 18.0 python-h11-0.12.0-4.el9ost, python3-h11-0.12.0-4.el9ost
Vendor assessments
Red Hat
Red Hat rating: Important. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| OpenShift Lightspeed | Affected | No fix yet |
| Red Hat Ansible Automation Platform 2 | Affected | No fix yet |
| Red Hat Enterprise Linux AI (RHEL AI) | Affected | No fix yet |
| Red Hat Ansible Automation Platform 2.4 for RHEL 8 | Fixed | RHSA-2025:8615 |
| Red Hat Ansible Automation Platform 2.5 for RHEL 8 | Fixed | RHSA-2025:8221 |
| Red Hat Ansible Automation Platform 2.5 for RHEL 9 | Fixed | RHSA-2025:8221 |
| Red Hat OpenStack Platform 17.1 | Fixed | RHSA-2025:7536 |
| Red Hat OpenStack Services on OpenShift 18.0 | Fixed | RHSA-2025:7535 |
Not affected: 8 products
- Red Hat Ansible Automation Platform 2
- Red Hat Ansible Automation Platform 2.4 for RHEL 8
- Red Hat Ansible Automation Platform 2.4 for RHEL 9
- Red Hat Ansible Automation Platform 2.5 for RHEL 8
- Red Hat Ansible Automation Platform 2.5 for RHEL 9
- Red Hat Enterprise Linux AI (RHEL AI)
- Red Hat OpenShift AI (RHOAI)
- Red Hat Satellite 6
Timeline
- 2026-10-02 Threatist record created from CVE-2025-43859
- 2026-10-03 Alias added: GHSA-vqfr-h8mv-ghfj
References
Other 5
- https://github.com/python-hyper/h11/security/advisories/GHSA-vqfr-h8mv-ghfj
- https://github.com/python-hyper/h11/commit/114803a29ce50116dc47951c690ad4892b1a36ed
- https://github.com/pypa/advisory-database/tree/main/vulns/h11/PYSEC-2026-348.yaml
- https://github.com/python-hyper/h11
- https://pypi.org/project/h11
More python-hyper vulnerabilities
- TI-2026-79288 Out-of-bounds write in Google Brotli decoder KEVCVSS 4.0CRITICAL 9.2
- TI-2025-35687 h2 allows HTTP Request Smuggling due to illegal characters in headers CVSS 4.0MEDIUM 6.9
- 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-2025-43859 · CVE Program
- CVE-2025-43859 · NVD
- FIRST EPSS
- GHSA-vqfr-h8mv-ghfj · GitHub Advisory Database · CC BY 4.0
- CVE-2025-43859 · 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