CVE-2026-86205: h3js h3

Medium severity, CVSS 5.4. EPSS: 0.3% chance of exploitation in the next 30 days.

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.

Affected products

• h3js h3: from 2.0.1-rc.17, before 2.0.1-rc.18 (fixed in 2.0.1-rc.18)

Published 2026-09-06. Last modified 2026-09-08.