Fix Business Logic Errors in Polka

Polka's minimalist footprint means zero guardrails for your application's state machine. Business logic errors in Polka typically arise when developers trust client-side input for sensitive operations like identity, pricing, or permissions. In a 'hacker-style' context, we exploit these by manipulating request parameters to bypass intended workflows. To secure Polka, you must enforce server-side truth and validate the state transition at every middleware hop.

The Vulnerable Pattern

const polka = require('polka');
const app = polka();
// VULNERABLE: Trusting user-provided ID and price
app.post(‘/checkout’, (req, res) => {
const { userId, productId, price } = req.body;
// Logic Error: Attacker can send price: 0.01
db.orders.create({ userId, productId, amount: price });
res.end(‘Order processed’);
});
app.listen(3000);

The Secure Implementation

The vulnerable snippet suffers from 'Parameter Tampering'. It trusts the 'price' and 'userId' sent in the POST body, allowing an attacker to purchase items for pennies or charge them to other accounts. The secure version fixes this by: 1. Using session-based authentication to determine 'userId', 2. Discarding the client-provided price in favor of a database lookup (Server-Side Truth), and 3. Implementing status checks to ensure the business workflow is followed correctly. Never let the client define the cost or the owner of a transaction.

const polka = require('polka');
const { authenticate } = require('./auth-middleware');
const app = polka();
// SECURE: Server-side validation and session-based identity
app.post(‘/checkout’, authenticate, async (req, res) => {
const { productId } = req.body;
const userId = req.user.id; // Identity from secure session, not body
const product = await db.products.find(productId);
if (!product) {
res.statusCode = 404;
return res.end(‘Product not found’);
}
// Logic Fix: Use server-side price, ignore client-provided price
const actualPrice = product.price;
await db.orders.create({
userId,
productId,
amount: actualPrice,
status: ‘PENDING’
});
res.end(‘Order secured’);
});
app.listen(3000);