better-helperjs Vulnerable to Directory Traversal via String Prefix Bypass in Static Server

Summary

A directory traversal vulnerability exists in the production static file server of better-helperjs (<= 3.0.5). Attackers can read arbitrary files located in adjacent directory structures that share the same string prefix as the intended static root directory.

Details

The framework utilizes a custom static file server engine used when running in NODE_ENV=production (src/ssr/site-server.ts). Inside the safeStaticPath() method, the requested path is checked against the static root directory to prevent directory traversal out of the designated public folder.

However, the validation uses String.prototype.startsWith():

  const root = path.resolve(rootDir);
  const resolved = path.resolve(root, `.${decodedPath}`);
  
  if (!resolved.startsWith(root)) {
    return null;
  }

This is logically flawed because startsWith evaluates plain strings rather than structural directory paths. If the application's assigned rootDir is /app/dist/client, and an attacker tries to access /app/dist/client-secrets/database.sqlite, the string "/app/dist/client-secrets/database.sqlite" successfully starts with "/app/dist/client".

Because of this bypass, an attacker can read sensitive files stored inside any adjacent directory traversing through the parent, as long as the adjacent directory name begins with the exact same prefix as the target public directory.

Impact

• Severity: High (7.5)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
• Affected Versions: <= 3.0.5
• Patched Version: >= 3.0.6

Proof of Concept

To reproduce this locally on a vulnerable installation:

1. Assume the framework static configuration serves from a .dist/client directory.
2. Build the project and create an adjacent prefix secret simulating sensitive deployment structure:
   bash

   npm run build
   mkdir -p dist/client-secrets
   echo "EXPOSED_SECRET" > dist/client-secrets/secret.txt

3. Start the application in Production mode (NODE_ENV=production tsx server.ts).
4. Using an HTTP client that doesn't pre-normalize /../ paths (e.g., netcat or raw sockets), send a GET request spanning into the prefix-sharing adjacent folder:
   http

   GET /%2e%2e%2fclient-secrets/secret.txt HTTP/1.1
   Host: localhost:4174
   Connection: close

5. The server incorrectly validates the path and responds with HTTP/1.1 200 OK exposing EXPOSED_SECRET.

(Note: Developer environment npm run dev servers are immune as static requests are handled defensively by Vite's Dev Middlewares. This vulnerability only triggers upon production starts).

Remediation / Patches

The path validation block has been updated to mandate exact path separation bounds by enforcing path.sep onto the evaluated traversal string.

  // Enforces separator OR exact root matches preventing prefix extension
  if (!resolved.startsWith(root + path.sep) && resolved !== root) {
    return null;
  }

Workarounds

If upgrading is temporarily impossible, users can safeguard their environment by ensuring no sensitive directories are deployed adjacent to their static build client directories sharing the identical word-prefix string.