CVE-2026-71554: H2 Duplicate Host Header Could Facilitate Vulnerability

MODERATE
Advisory Identifier: GHSA-6hr6-w5qg-qmwg
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CVSS 5.3
CWE-444
Published
Advisory Summary

CVE-2026-71554 is a MODERATE (CVSS 5.3) duplicate host header could facilitate vulnerability (CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')) affecting H2 libraries in the PyPI ecosystem. The issue involves duplicate host header could facilitate request smuggling. Remediation is available by upgrading affected packages to version 4.4.1.

CVE-2026-71554 at a Glance

CVE Identifier CVE-2026-71554
GHSA Identifier GHSA-6hr6-w5qg-qmwg
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
Affected Product H2
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 5.3

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability Low (A:L)

How to Fix CVE-2026-71554

To remediate this vulnerability, upgrade all affected H2 dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

h2 fixed: 4.4.1 
pip install h2>=4.4.1
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show h2

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-71554, inspect your installed packages using pip:

pip show h2

Analyze the terminal output to locate any version of H2 in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 4.4.1), your application is affected.

Note: The pip show h2 command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling'), where the product acts as an intermediary http agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed http requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination. In H2, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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.

CVE-2026-71554 Affected H2 Versions

1 component(s)
Fixed Versions:
4.4.1
Product / Package Ecosystem Affected Versions Fixed Version
h2
pypi/h2
PyPI
< 4.4.1
4.4.1

References

GitHub Advisory / Wiki ADVISORY
https://github.com/python-hyper/h2/security/advisories/GHSA-6hr6-w5qg-qmwg
Security Fix Commit FIX COMMIT
https://github.com/python-hyper/h2/commit/292a40829feefda98c8509dcdbbb4a57af9bd6a6
H2 Source Repository REPOSITORY
https://github.com/python-hyper/h2

Frequently Asked Questions

What is CVE-2026-71554?
CVE-2026-71554 is a security vulnerability in H2 involving Duplicate Host header could facilitate request smuggling under CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling').
Which H2 versions are affected?
Affected versions include releases prior to 4.4.1 across affected packages: h2.
What version fixes CVE-2026-71554?
Upgrading to 4.4.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is h2 affected?
Yes, h2 is affected in releases prior to 4.4.1. Upgrade your dependency to 4.4.1 or later.
What is the CVSS score of CVE-2026-71554?
CVE-2026-71554 has a CVSS v3.1 base score of 5.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.
What CWE is associated with CVE-2026-71554?
CVE-2026-71554 is classified under CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') (The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.).
How can I check PyPI dependencies for CVE-2026-71554?
Run the dependency tree command: 'pip show h2'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-6hr6-w5qg-qmwg",
  "modified" : "2026-08-06T21:53:41Z",
  "published" : "2026-08-06T21:53:41Z",
  "aliases" : [ "CVE-2026-71554" ],
  "summary" : "h2: Duplicate Host header could facilitate request smuggling",
  "details" : "### Impact\nh2 <=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).\n\n### Patches\nPatched and fixed in v4.4.1\n\n### Workarounds\nUsers 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.\n\n### References\nSimilar to the previously disclosed and fixed duplicate content-length issue.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "h2"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.4.1"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.4.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/python-hyper/h2/security/advisories/GHSA-6hr6-w5qg-qmwg"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/python-hyper/h2/commit/292a40829feefda98c8509dcdbbb4a57af9bd6a6"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/python-hyper/h2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-444" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-06T21:53:41Z",
    "nvd_published_at" : null
  }
}