CVE-2026-86205

h3 versions before 2.0.1-rc.18 contain an open redirect vulnerability in the redirectBack() utility that fails to sanitize protocol-relative paths in the Referer header pathname. Attackers can craft a same-origin URL with a double-slash path segment that passes origin validation but produces a Location header interpreted by browsers as a protocol-relative redirect to an external domain.

• https://github.com/h3js/h3/security/advisories/GHSA-fp4x-ggrf-wmc6
• https://www.vulncheck.com/advisories/h3-before-2.0.1-rc.18-open-redirect-via-redirectback