What’s Vulnerable › Vulnerabilities

CVE-2026-71554: h2: Duplicate Host header could facilitate request smuggling

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-06 · updated 2026-10-24
• Weakness: CWE-444

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.

Affected products

• python-hyper h2: before 4.4.1

References

• https://github.com/python-hyper/h2/commit/292a40829feefda98c8509dcdbbb4a57af9bd6a6
• https://github.com/python-hyper/h2/security/advisories/GHSA-6hr6-w5qg-qmwg

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