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CVE-2026-86253 – Path Traversal / Arbitrary File Read – h3 (h3js) npm package before 1.15.6 / 2.0.1-

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Summary

CVE-2026-86253 is a path traversal vulnerability (CWE-22) in the serveStatic() function of h3, a minimal HTTP framework for Node.js published as the h3 npm package (org: h3js). The flaw allows an unauthenticated remote attacker to read arbitrary files from the underlying server filesystem by sending percent-encoded directory-traversal sequences in the request path. It has been assigned a CVSS score of 8.2 (High).

Technical details

  • Root cause: On Node.js, h3 relies on srvx’s FastURL class for URL parsing. Unlike standard WHATWG URL parsing, FastURL performs raw string slicing for performance and does not normalize dot segments (./..) or resolve percent-encoded equivalents (%2e).
  • Trigger: event.url.pathname is not normalized before being handled by serveStatic(). The unnormalized path is subsequently passed to decodeURI(), which decodes %2e%2e sequences into literal ../ segments without any further sanitization.
  • Attack vector: Network — a remote, unauthenticated attacker can send a crafted HTTP GET request containing chained percent-encoded dot-segments (e.g. GET /%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/etc/passwd) to any endpoint served via serveStatic().
  • Impact: Successful exploitation allows disclosure of arbitrary files outside the intended static-file directory, including system files (e.g. /etc/passwd), environment variable files, application configuration, credentials/API keys, and source code. No user interaction or privileges are required. Only confidentiality is impacted — there is no reported integrity or availability impact.

Affected software

  • h3 (npm package, h3js): all versions before 1.15.6
  • h3 (npm package, h3js): all versions from 2.0.0-beta.0 before 2.0.1-rc.15

Severity

  • CVSS v3.1 Base Score: 5.9 (Moderate) — CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
  • CVSS v4.0 Score: 8.2 (High) — 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

Mitigation and recommended actions

  • Immediate: Upgrade the h3 npm package to 1.15.6 or later (for the 1.x line) or 2.0.1-rc.15 or later (for the 2.x pre-release line). These versions correctly normalize percent-encoded dot segments before path resolution in serveStatic().
  • If immediate patching is not possible:
    • Avoid exposing serveStatic() (or any static-file-serving route built on it) to untrusted network traffic until patched.
    • Deploy a reverse proxy or WAF rule that blocks/normalizes requests containing percent-encoded traversal sequences (%2e%2e, %252e, etc.) before they reach the application.
    • Run the application under a restricted filesystem user/container with least-privilege access to reduce the blast radius of any file read.
    • Audit access logs for suspicious request paths containing repeated %2e%2e or ../ sequences targeting sensitive files.

References

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