CVE-2026-71554
Medium
Description
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.
SeverityMedium
CVSS5.3
Exploit probability—
ProductH2
PublishedAug 6, 2026
First seenAug 6, 2026
Weakness typeCWE-444CWE-444
How to fix
Update
- h24.4.1 or later
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| h2PyPI | v < 4.4.1 | 4.4.1 |
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