CVE Catalog

CVE-2026-71554

MediumCVSS 5.3
Published: Updated: Translated: NVD NIST

Exploitation Probability (EPSS)

Low risk
0.42%

35th percentile - higher than 35% of all known CVEs

Summary

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.

Risk Assessment

The risk for the organization is the possibility of a request smuggling attack, which can lead to bypassing security controls, intercepting requests, or manipulating HTTP traffic.

Recommendation

It is recommended to update the h2 library to version 4.4.1 or later, which fixes this vulnerability, and to review server configurations for handling multiple Host headers.

Related vulnerabilities

Original NVD description (English source)

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.

Vulnerability data from NVD (NIST) · CISA KEV · EPSS