# CVE-2026-33490: h3 missing path segment boundary check in mount()

Source: https://junglewise.ai/threats/cve-2026-33490-h3js-h3-incorrect-path-validation-in-mount-method  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-03-20. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-33490 · Severity: low · CVSS 3.1 · Published 2026-03-20

h3 is a minimal HTTP framework used to build web applications. The framework's mount() method, which mounts sub-applications at specific path prefixes, fails to properly validate path boundaries. An attacker can exploit this by requesting URLs that share only a string prefix with a mounted path (e.g., /admin-public when /admin is mounted), causing security middleware intended only for the /admin path to execute on unrelated routes. This could allow unauthorized access to protected functionality if the application relies on middleware to set permission flags.

Technologies: [h3 (npm)](https://junglewise.ai/threats/technologies/h3), [Uber H3-Js](https://junglewise.ai/threats/technologies/h3-js). Vendors: [npm](https://junglewise.ai/threats/vendors/npm), [Uber](https://junglewise.ai/threats/vendors/uber).

## Quick answers

**Is CVE-2026-33490 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-33490 being exploited in the wild.

**How severe is CVE-2026-33490?** Low severity, with a CVSS score of 3.1, published 20 March 2026.

**Which products and versions are affected?** h3 (npm) SEMVER:introduced=2.0.1-alpha.0,fixed=2.0.1-rc.17; Uber H3-Js >= 2.0.1-alpha.0, <= 2.0.1-rc.16.

**Is there a fix for CVE-2026-33490?** Yes. h3 (npm): upgrade to 2.0.1-rc.17 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-33490 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-33490?** Check whether you run h3 (npm) or Uber H3-Js at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- h3 (npm): upgrade to 2.0.1-rc.17 or later

  ```sh
  npm install h3@2.0.1-rc.17
  ```


## Technical details

The vulnerability is a path traversal/bypass issue rooted in improper string prefix matching. The mount() method at src/h3.ts:127 and withoutBase() utility at src/utils/internal/path.ts:40 both use startsWith() without checking that the character immediately following the base prefix is a forward slash (/), query string (?), fragment (#), or end-of-string. This causes /admin middleware to incorrectly match /admin-public, /administrator, /adminstuff, etc. A mounted sub-app's middleware chain then executes for these unrelated paths, polluting the request context with privileges it should never have. The attack requires no authentication or user interaction—a simple HTTP GET request to a prefix-matching path triggers the context pollution. A patch adding segment boundary validation is available in version 2.0.1-rc.17 and later.

## Affected products

- h3js h3 >=2.0.1-alpha.0 through 2.0.1-rc.16

## Identifiers

- CVE-2026-33490
- GHSA-2j6q-whv2-gh6w

## Timeline

- 2026-03-20: disclosed: Security advisory GHSA-2j6q-whv2-gh6w published
- 2026-03-20: patched: Patch available in h3 >= 2.0.1-rc.17

## References

- [https://github.com/h3js/h3/security/advisories/GHSA-2j6q-whv2-gh6w](https://github.com/h3js/h3/security/advisories/GHSA-2j6q-whv2-gh6w)
- [https://github.com/h3js/h3](https://github.com/h3js/h3)

## Related threats

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- [h3 serveStatic path traversal via percent-encoded dot segments](https://junglewise.ai/threats/cve-2026-86253-h3-servestatic-path-traversal-via-percent-encoded-dot-segments)
- [h3 SSE event injection via unsanitized carriage return](https://junglewise.ai/threats/cve-2026-86252-h3-sse-event-injection-via-unsanitized-carriage-return)
- [h3 serveStatic path traversal via double-encoded dot segments](https://junglewise.ai/threats/cve-2026-86251-h3-servestatic-path-traversal-via-double-encoded-dot-segments)
- [H3: Unbounded Chunked Cookie Count in Session Cleanup Loop may Lead to Denial of Service](https://junglewise.ai/threats/cve-2026-86250-h3-unbounded-chunked-cookie-denial-of-service)