Is CVE-2026-86205 being exploited?

No public exploit is currently tracked.

How severe is CVE-2026-86205?

CVE-2026-86205 has a CVSS v3 severity of MEDIUM (5.4).

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CVE-2026-86205

Description

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.