🛡️ CVE-2026-71554 — h2

🟡 CVSS 5.3 — Medium ✅ No Known Exploit CWE-444 NVD
5.3
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

h2: Duplicate Host header could facilitate request smuggling

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.

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs no privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality none, integrity none, availability low.

Weakness class

CVE-2026-71554 is classified as CWE-444: HTTP Request Smuggling. A proxy and a server disagree on where one request ends, letting an attacker slip a second request past the front end.

Affected software

CVE-2026-71554 is recorded against 2 packages.

  • h2 (fixed in 4.4.1)
  • unknown

Timeline and source

Published on 6 August 2026 and last revised on 10 August 2026. No public exploit is currently recorded for this entry. Record sourced from NVD.

References

github.com (Web)
github.com (Web)
github.com (Package)

Details

Severity Medium
CVSS Score 5.3
CVSS Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
CWE CWE-444
Public Exploit ✅ No
Source NVD
Published 2026-08-06
Updated 2026-08-12
Modified 2026-08-10
Fix URL N/A

Affected Packages

Software From version Fixed in
h2 — 4.4.1
unknown — —

Similar Threats

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