{"cve":"CVE-2025-57804","nvd":{"published":"2025-08-25T21:15:37.983","modified":"2026-06-17T09:43:28.050","status":"Deferred","description":"h2 is a pure-Python implementation of a HTTP/2 protocol stack. Prior to version 4.3.0, an HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls. This issue has been patched in version 4.3.0.","cvss_v4":{"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N","score":6.9},"cvss_v3":null,"cwes":["CWE-93"],"references":["https://github.com/python-hyper/h2/commit/035e9899f95e3709af098f578bfc3cd302298e3a","https://github.com/python-hyper/h2/security/advisories/GHSA-847f-9342-265h","https://lists.debian.org/debian-lts-announce/2025/09/msg00004.html"],"products":[],"source":"nvd","synced_at":"2026-10-19T22:07:46.309729541+00:00"},"epss":{"cve":"CVE-2025-57804","score":0.01883,"percentile":0.79485,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-847f-9342-265h","url":"https://osv.dev/vulnerability/GHSA-847f-9342-265h","summary":"h2 allows HTTP Request Smuggling due to illegal characters in headers","modified":"2026-09-10T03:50:26.965133425Z","packages":[{"ecosystem":"PyPI","name":"h2","purl":"pkg:pypi/h2","ranges":["<4.3.0"],"fixed":["4.3.0"]}]},{"id":"PYSEC-2026-1435","url":"https://osv.dev/vulnerability/PYSEC-2026-1435","summary":"h2 allows HTTP Request Smuggling due to illegal characters in headers","modified":"2026-07-07T17:47:39.182570446Z","packages":[{"ecosystem":"PyPI","name":"h2","purl":"pkg:pypi/h2","ranges":["<4.3.0"],"fixed":["4.3.0"]}]}],"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."]}