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Fencer Vulns
medium 5.3

CVE-2026-71554 json

h2: Duplicate Host header could facilitate request smuggling

published 2026-08-06 3 mo ago modified 2026-10-24 16 d ago also GHSA-6hr6-w5qg-qmwg , PYSEC-2026-3628 , EUVD-2026-54177

cvss
5.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 36%
public exploits
0
none known

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.

Patch / fix

  • pypi h2 4.4.1
  • python-h2 SLE Module for Server Applications 15 SP7 3.2.0-150200.3.8.1
  • python-h2 SLE Module for Python 3 15 SP7 4.1.0-150400.8.9.1
  • python-h2 SLES for SAP applications 16.0, SLES 16.0 4.2.0-160000.4.1

suse advisories SUSE-SU-2026:3972-1SUSE-SU-2026:3970-1SUSE-SU-2026:23563-1

  • python-h2 openSUSE Leap 16.0 4.2.0-160000.4.1
  • python-h2 openSUSE Tumbleweed 4.4.1-1.1

opensuse advisories openSUSE-SU-2026:21693-1openSUSE-SU-2026:11523-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-71554 updated cvss 5.3

h2: h2: Request smuggling due to duplicate Host headers

  • Red Hat Migration Toolkit for Containers rhmtc/openshift-migration-hook-runner-rhel8 Red Hat Ansible Automation Platform 2 ansible-automation-platform-24/ee-supported-rhel8, ansible-automation-platform-24/ee-supported-rhel9, ansible-automation-platform-25/ee-supported-rhel8, ansible-automation-platform-25/ee-supported-rhel9, ansible-automation-platform-26/ee-supported-rhel9, ansible-automation-platform-27/ee-supported-rhel9, +2 more Red Hat Hardened Images python-urllib3 Red Hat OpenShift AI (RHOAI) rhai/guidellm-rhel9, rhoai/odh-ogx-core-rhel9, rhoai/odh-feature-server-rhel9, rhoai/odh-llama-stack-core-rhel9 Red Hat OpenStack Platform 17.1 python3-h2 Red Hat OpenStack Platform 18.0 python-h2, rhoso/openstack-tempest-all-rhel9
    affected: workaround, no fix available “Fix deferred”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
3 mo ago

Exploit prediction

0.42% chance of exploitation in the next 30 days, higher than 36% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
GitHub 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
OSV 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Affected packages

ecosystem package affected fixed in
pypi h2
<4.4.1
4.4.1

Affected products

  • python-hyper h2

Classification

Weakness
  • CWE-444 Inconsistent Interpretation of HTTP Requests

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.