{"id":"CVE-2026-71554","title":"h2 is a pure-Python implementation of a HTTP/2 protocol stack","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 co…","severity":"medium","cvss":5.3,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","cwe":["CWE-444"],"vendor":"h2","product":"h2","affected":["h2 < 4.4.1"],"patched":["h2 4.4.1"],"published":"2026-08-06","updated":"2026-09-18","sourceUpdated":"2026-09-18T20:09:01.757","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-71554","references":[{"url":"https://github.com/python-hyper/h2/commit/292a40829feefda98c8509dcdbbb4a57af9bd6a6","label":"security-advisories@github.com"},{"url":"https://github.com/python-hyper/h2/security/advisories/GHSA-6hr6-w5qg-qmwg","label":"security-advisories@github.com"},{"url":"https://github.com/python-hyper/h2"},{"url":"https://github.com/advisories/GHSA-6hr6-w5qg-qmwg"}],"tags":["nvd","exploit-available","osv","pip","ghsa"],"epss":0.00418,"epssPercentile":0.33364,"exploits":{"github":1,"githubRepos":["https://github.com/SunandM/poc-h2-CVE-2026-71554"],"checkedAt":"2026-09-25T08:21:10.671Z"},"exploitAvailable":true,"aliases":["GHSA-6hr6-w5qg-qmwg","PYSEC-2026-3628"],"ecosystem":"pip","ingestedAt":"2026-08-06T22:05:23.027Z","slug":"CVE-2026-71554","body":"## Overview\n\nh2 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.\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.\n\n## Package advisory (CVE-2026-71554)\n\nAffected packages:\n\n- `h2 < 4.4.1`\n\nPatched in:\n\n- `h2 4.4.1`\n\nSource: https://osv.dev/vulnerability/GHSA-6hr6-w5qg-qmwg","depth":"twilight","depthScore":41,"depthScoreParts":{"impact":29.2,"likelihood":0.1,"exploitation":12,"ransomware":0},"changes":[{"seq":5462,"id":"CVE-2026-71554","ts":1788887284676,"field":"exploit_available","old":"false","new":"true"},{"seq":4345,"id":"CVE-2026-71554","ts":1788886398562,"field":"exploit_available","old":"true","new":"false"},{"seq":3071,"id":"CVE-2026-71554","ts":1788883061758,"field":"exploit_available","old":"false","new":"true"},{"seq":2100,"id":"CVE-2026-71554","ts":1788882466638,"field":"exploit_available","old":"true","new":"false"},{"seq":1166,"id":"CVE-2026-71554","ts":1788881903420,"field":"exploit_available","old":"false","new":"true"}]}