CVE-2026-71554: h2 is a pure-Python implementation of a HTTP/2 protocol stack

Routine

MEDIUM 5.3REMOTE

CVE-2026-71554 is a medium-severity flaw (CVSS 5.3) affecting affected software. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 65%)
CISA exploited list	No
Weakness	CWE-444
Published	2026-08-06
Last modified	2026-10-24

Publisher (CNA): GitHub, Inc.

Last updated 2026-10-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Timeline

2026-08-06 22:18 UTCPublished CVE record published
2026-10-24 15:10 UTCUpdated Record last modified

Exact fixes by distribution

Python (pip) · h2 · FIXED · 4.4.1 · vulnerable: <= 4.4.0 · source

pip install --upgrade "h2>=4.4.1" pip show h2

Fedora 46 · sugar-clock · FIXED · 22.1-17.fc46 · FEDORA-2026-57948849df · source

sudo dnf upgrade --advisory=FEDORA-2026-57948849df

Fedora ELN · selinux-policy · FIXED · 45.23-1.eln159 · FEDORA-2026-3668c969a2 · source

sudo dnf upgrade --advisory=FEDORA-2026-3668c969a2

Debian 12 · python-h2 · NO FIX YET

Debian 13 · python-h2 · NO FIX YET

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

GitHub PoC: SunandM/poc-h2-CVE-2026-71554 (1 stars)

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-71554?

No fixed release is listed yet. Watch the vendor advisory and apply any mitigation shown on this page.

Is CVE-2026-71554 being exploited?

CVE-2026-71554 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-71554 affect?

CVE-2026-71554 affects affected software. Severity is medium (CVSS 5.3). 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 consu

How urgent is CVE-2026-71554?

Priority 32/100. Remotely exploitable, no login, no user action

Software pages

Python vulnerabilities

References

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