Fix Improper Assets Management in Polka

Improper asset management in Polka-based applications typically manifests as directory traversal or sensitive file exposure. When developers serve the root directory or fail to restrict file types, attackers can exfiltrate .env files, source code, or internal configuration. In a minimalist framework like Polka, the responsibility of path sanitization and asset scoping falls entirely on the middleware configuration.

The Vulnerable Pattern

const polka = require('polka');
const sirv = require('sirv');
// VULNERABLE: Serving the current directory (root) exposes sensitive files
// like .env, package.json, and .git folders.
polka()
.use(sirv(’.’, { dev: true }))
.listen(3000, err => {
if (err) throw err;
console.log(‘Insecure server on localhost:3000’);
});

The Secure Implementation

The fix implements three layers of defense: 1. Perimeter Isolation: By using path.join(__dirname, 'public'), we ensure the middleware cannot traverse outside the intended directory. 2. Dotfile Restriction: Setting dotfiles: false explicitly blocks requests for hidden system/config files (e.g., .env, .htaccess). 3. Subpath Mounting: Mounting the middleware on '/static' instead of the root '/' prevents the asset handler from intercepting and potentially exposing logic intended for other API routes.

const polka = require('polka');
const sirv = require('sirv');
const path = require('path');
// SECURE: Scope assets to a specific ‘public’ directory and disable dotfiles.
const assets = sirv(path.join(__dirname, ‘public’), {
dotfiles: false,
immutable: true,
single: false
});
polka()
.use(‘/static’, assets)
.listen(3000, err => {
if (err) throw err;
console.log(‘Secure server on localhost:3000’);
});