# CVE-2026-86251: h3 serveStatic path traversal via double-encoded dot segments

Source: https://junglewise.ai/threats/cve-2026-86251-h3-servestatic-path-traversal-via-double-encoded-dot-segments  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-06. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-86251 · Severity: medium · CVSS 5.9 · Published 2026-09-06

h3's serveStatic utility, used to serve static assets from web servers and CDNs, contains a path traversal vulnerability that allows attackers to bypass security checks and read arbitrary files. An attacker can craft requests with double-encoded paths (e.g., `%252e%252e`) that survive validation but are later decoded by backend systems (CDNs, S3, object storage), enabling unauthorized access to sensitive files outside the intended asset directory.

Technologies: [h3 (npm)](https://junglewise.ai/threats/technologies/h3). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-86251 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-86251 being exploited in the wild.

**How severe is CVE-2026-86251?** Medium severity, with a CVSS score of 5.9, published 6 September 2026.

**Which products and versions are affected?** h3 (npm) SEMVER:introduced=0,fixed=1.15.9.

**Is there a fix for CVE-2026-86251?** Yes. h3 (npm): upgrade to 1.15.9 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-86251 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-86251?** Check whether you run h3 (npm) 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 1.15.9 or later

  ```sh
  npm install h3@1.15.9
  ```


## Technical details

The vulnerability stems from conflicting decode stages in h3's static file serving. When `serveStatic` processes a URL like `/%252e%252e/etc/passwd`, the H3Event constructor first decodes it to `/%2e%2e/etc/passwd` (preserving %25 sequences). A second redundant `decodeURI()` call in serveStatic does not further decode `%2e` (since encodeURI never encodes the dot character), leaving `%2e%2e` intact. The `resolveDotSegments()` function only checks for literal `.` characters and fast-returns without traversal protection. URL-based backends (CDNs, S3) then interpret `%2e%2e` as `..` per RFC 3986/URL Standard, enabling path traversal. Affected versions: h3 ≤ 1.15.8. Patched in 1.15.9. Only URL-based backends are vulnerable; filesystem-based backends are not affected.

## Affected products

- h3js h3 ≤ 1.15.8

## Identifiers

- CVE-2026-86251
- GHSA-72gr-qfp7-vwhw

## Timeline

- 2026-03-20: disclosed
- 2026-03-20: patched: Version 1.15.9 released

## References

- [https://github.com/h3js/h3/security/advisories/GHSA-72gr-qfp7-vwhw](https://github.com/h3js/h3/security/advisories/GHSA-72gr-qfp7-vwhw)
- [https://github.com/h3js/h3](https://github.com/h3js/h3)

## Related threats

- [H3 denial of service via unbounded chunked cookie count](https://junglewise.ai/threats/duplicate-advisory-h3-unbounded-chunked-cookie-count-in-session-cleanup-11041bf9)
- [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: 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)
- [H3 open redirect via protocol-relative path in redirectBack()](https://junglewise.ai/threats/cve-2026-86205-h3-open-redirect-via-protocol-relative-path-in-redirectback)