Vulnerability · Published Sep 6, 2026 · Assigned by VulnCheck

CVE-2026-86205h3 before 2.0.1-rc.18 Open Redirect via redirectBack()

Medium 5.4

In plain English

A medium-severity open redirect in h3 from h3js. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 19% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.4Medium

Exploit odds (30 days)0.3%top 80.6% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
npm	h3	2.0.1-rc.18	GHSA-fp4x-ggrf-wmc6

Affected products (as reported)

Vendor	Product	Versions
h3js	h3	2.0.1-rc.17 – before 2.0.1-rc.18

Weakness: CWE-601 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N

References

GitHub Security Advisory (GHSA-fp4x-ggrf-wmc6) vendor-advisory
VulnCheck Advisory: h3 before 2.0.1-rc.18 Open Redirect via redirectBack() third-party-advisory