{"cve":"CVE-2026-71554","nvd":{"published":"2026-08-06T22:18:32.403","modified":"2026-10-24T15:10:00.190","status":"Deferred","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.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","score":5.3},"cwes":["CWE-444"],"references":["https://github.com/python-hyper/h2/commit/292a40829feefda98c8509dcdbbb4a57af9bd6a6","https://github.com/python-hyper/h2/security/advisories/GHSA-6hr6-w5qg-qmwg"],"products":[],"source":"nvd","synced_at":"2026-10-24T16:53:34.184483516+00:00"},"epss":{"cve":"CVE-2026-71554","score":0.00418,"percentile":0.35824,"date":"2026-10-25"},"kev":null,"osv":[{"id":"GHSA-6hr6-w5qg-qmwg","url":"https://osv.dev/vulnerability/GHSA-6hr6-w5qg-qmwg","summary":"h2: Duplicate Host header could facilitate request smuggling","modified":"2026-09-10T03:51:13.136510577Z","packages":[{"ecosystem":"PyPI","name":"h2","purl":"pkg:pypi/h2","ranges":["<4.4.1"],"fixed":["4.4.1"]}]},{"id":"PYSEC-2026-3628","url":"https://osv.dev/vulnerability/PYSEC-2026-3628","summary":"h2: Duplicate Host header could facilitate request smuggling","modified":"2026-08-10T11:30:11.413688625Z","packages":[{"ecosystem":"PyPI","name":"h2","purl":"pkg:pypi/h2","ranges":["<4.4.1"],"fixed":["4.4.1"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}