# CVE-2026-33490: h3: Missing Path Segment Boundary Check in `mount()` Causes Middleware Execution on Unrelated Prefix-Matching Routes

> Attackers can trigger unintended middleware on prefix-matching routes, potentially escalating privileges or bypassing security checks in h3 applications.

- **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/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:** 26 Mar 2026
- **Updated:** 26 Mar 2026
- **Also known as:** CLSA-2026-1791201400, GHSA-2j6q-whv2-gh6w

## What is CVE-2026-33490?

CVE-2026-33490 is a low-severity missing authorization vulnerability in h3js h3, published on 26 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. In versions 2.0.0-0 through 2.0.1-rc.16, the `mount()` method in h3 uses a simple `startsWith()` check to determine whether incoming requests fall under a mounted sub-application's path prefix. Because this check does not verify a path segment boundary (i.e., that the next character after the base is `/` or end-of-string), middleware registered on a mount like `/admin` will also execute for unrelated routes such as `/admin-public`, `/administrator`, or `/adminstuff`. This allows an attacker to trigger context-setting middleware on paths it was never intended to cover, potentially polluting request context with unintended privilege flags. Version 2.0.2-rc.17 contains a patch.

Weakness: CWE-706 (Use of Incorrectly-Resolved Name or Reference)

## Which versions are affected by CVE-2026-33490?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `h3` | npm | From 2.0.1-alpha.0 | 2.0.1-rc.17 |
| `h3` | npm | From 2.0.1-alpha.0 | 2.0.1-rc.17 |

Products: H3 H3.

## How do I fix CVE-2026-33490?

Upgrade h3 to 2.0.1-rc.17 or later.

## Is CVE-2026-33490 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33490 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-2026-33131](https://insights.sbomify.com/CVE-2026-33131/): h3 has a middleware bypass with one gadget (high 7.4)
- [CVE-2026-33129](https://insights.sbomify.com/CVE-2026-33129/): h3 has an observable timing discrepancy in basic auth utils (medium 5.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33490 and fix it if it is affected.

What it is: CVE-2026-33490 is a low-severity missing authorization vulnerability in h3js h3, published on 26 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-2026-33490 exists.

Affected packages and fixed versions:
- h3 (npm): affected from 2.0.1-alpha.0; fixed in 2.0.1-rc.17
- h3 (npm): affected from 2.0.1-alpha.0; fixed in 2.0.1-rc.17

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-2026-33490/ (Markdown: https://insights.sbomify.com/CVE-2026-33490/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33490
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33490
- OSV: https://osv.dev/vulnerability/CVE-2026-33490

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33490/
Summaries are generated and may contain mistakes; the official description above is the record.