# CVE-2026-33131: h3 has a middleware bypass with one gadget

> Attackers can bypass authentication middleware in H3 applications by spoofing the Host header, allowing unauthorized access to protected routes.

- **Severity:** High 7.4 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 20 Mar 2026
- **Updated:** 20 Mar 2026
- **Also known as:** GHSA-3vj8-jmxq-cgj5

## What is CVE-2026-33131?

CVE-2026-33131 is a high-severity authentication bypass vulnerability in h3js h3, published on 20 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

H3 is a minimal H(TTP) framework. Versions 2.0.0-0 through 2.0.1-rc.14 contain a Host header spoofing vulnerability in the NodeRequestUrl (which extends FastURL) which allows middleware bypass. When event.url, event.url.hostname, or event.url._url is accessed, such as in a logging middleware, the _url getter constructs a URL from untrusted data, including the user-controlled Host header. Because H3's router resolves the route handler before middleware runs, an attacker can supply a crafted Host header (e.g., Host: localhost:3000/abchehe?) to make the middleware path check fail while the route handler still matches, effectively bypassing authentication or authorization middleware. This affects any application built on H3 (including Nitro/Nuxt) that accesses event.url properties in middleware guarding sensitive routes. The issue requires an immediate fix to prevent FastURL.href from being constructed with unsanitized, attacker-controlled input. Version 2.0.1-rc.15 contains a patch for this issue.

Weakness: CWE-290 (Authentication Bypass by Spoofing)

## Which versions are affected by CVE-2026-33131?

2 open source packages ship an affected version, including h3. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `h3` | npm | From 2.0.0-0 | 2.0.1-rc.15 |
| `h3` | npm | From 2.0.0-0 | 2.0.1-rc.15 |

Products: H3 H3.

## How do I fix CVE-2026-33131?

Upgrade h3 to 2.0.1-rc.15 or later.

## Is CVE-2026-33131 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33131 exists.

## Other vulnerabilities in h3js h3

- [CVE-2026-23527](https://insights.sbomify.com/CVE-2026-23527/): h3 v1 has Request Smuggling (TE.TE) issue (high 8.9)
- [CVE-2026-33128](https://insights.sbomify.com/CVE-2026-33128/): h3 has a Server-Sent Events Injection via Unsanitized Newlines in Event Stream Fields (high 7.5)
- [CVE-2026-33129](https://insights.sbomify.com/CVE-2026-33129/): h3 has an observable timing discrepancy in basic auth utils (medium 5.9)
- [CVE-2026-33490](https://insights.sbomify.com/CVE-2026-33490/): h3: Missing Path Segment Boundary Check in `mount()` Causes Middleware Execution on Unrelated Prefix-Matching Routes (low 3.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33131 and fix it if it is affected.

What it is: CVE-2026-33131 is a high-severity authentication bypass vulnerability in h3js h3, published on 20 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33131 exists.

Affected packages and fixed versions:
- h3 (npm): affected from 2.0.0-0; fixed in 2.0.1-rc.15
- h3 (npm): affected from 2.0.0-0; fixed in 2.0.1-rc.15

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-2026-33131/ (Markdown: https://insights.sbomify.com/CVE-2026-33131/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33131
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33131
- OSV: https://osv.dev/vulnerability/CVE-2026-33131

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33131/
Summaries are generated and may contain mistakes; the official description above is the record.