CVE-2026-71554 is a medium-severity (CVSS 5.3) CWE-444 vulnerability in h2. A fix is available for h2 — see the affected versions and patch details below.
h2: Duplicate Host header could facilitate request smuggling
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-71554.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-71554 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 379,842 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
h2Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | h2 | all versions | 4.4.1pip install --upgrade 'h2==4.4.1' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for h2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update h2 to 4.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-71554 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
h2 is a pure-Python HTTP/2 protocol stack. Versions <= 4.4.0 accept request header blocks containing more than one Host header and forward every Host header to the consuming application. When that application downgrades HTTP/2 to HTTP/1.1, the resulting request carries duplicate Host header lines, providing an HTTP…
Update the bundled h2 Python library to 4.4.1 or later. Where an immediate upgrade is not possible, avoid HTTP/2 to HTTP/1.1 downgrade in front-end proxies for affected services, or reject requests carrying more than one Host header at the edge.Source: Red Hat security advisory for CVE-2026-71554 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-71554 in your dependencies?
Find it across PyPI, including transitive dependencies.