# 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.

- **Severity:** Medium 5.3 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N`)
- **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 available:** No
- **Published:** 6 Sep 2026
- **Updated:** 8 Sep 2026
- **Also known as:** GHSA-fp4x-ggrf-wmc6, GHSA-vhx4-2r84-67h4

## 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 (URL Redirection to Untrusted Site)

## Which versions are affected by CVE-2026-86205?

2 open source packages ship an affected version, including h3.


| Package | Ecosystem | 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-86205 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-86205 exists.

Affected packages and fixed versions:
- h3 (npm): affected from 2.0.1-rc.17; fixed in 2.0.1-rc.18
- h3 (npm): affected from 2.0.1-rc.17; fixed in 2.0.1-rc.18

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-86205/ (Markdown: https://insights.sbomify.com/CVE-2026-86205/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-86205
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-86205
- OSV: https://osv.dev/vulnerability/CVE-2026-86205

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-86205/
Summaries are generated and may contain mistakes; the official description above is the record.