CVE-2026-86205 h3 before 2.0.1-rc.18 Open Redirect via redirectBack()
Attackers can exploit unsanitized Referer headers in h3's redirectBack() to redirect users to external domains, bypassing origin validation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2026-86205?
CVE-2026-86205 is a medium-severity vulnerability in h3js h3, published on 6 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
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.
Weakness: CWE-601
Which versions are affected by CVE-2026-86205?
2 open source packages ship an affected version, including h3.
| Package | Affected | Fixed in |
|---|---|---|
h3 npm | From 2.0.1-rc.17 | 2.0.1-rc.18 |
h3 npm | From 2.0.1-rc.17 | 2.0.1-rc.18 |
How do I fix CVE-2026-86205?
Upgrade h3 to 2.0.1-rc.18 or later.
Is CVE-2026-86205 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-86205 exists.
Sources
Also known as GHSA-fp4x-ggrf-wmc6, GHSA-vhx4-2r84-67h4.