Fix Business Logic Errors in Koa
Business Logic Errors (BLE) in Koa are logic flaws that allow attackers to manipulate application flow. Unlike syntax errors, the code is 'correct' but the logic is broken. In Koa's middleware-heavy architecture, these usually manifest as state manipulation, parameter tampering, or broken access control due to trusting client-side input in stateful transactions. If you trust the client to define the state of a transaction, you've already lost.
The Vulnerable Pattern
const router = require('@koa/router')();// VULNERABLE: Trusting the client to provide the price router.post(‘/api/checkout’, async (ctx) => { const { cartItems, discountCode, totalToPay } = ctx.request.body;
// Logic Flaw: The server trusts ‘totalToPay’ from the request body. // An attacker can set totalToPay to 0.01 regardless of the items. const order = await db.orders.create({ userId: ctx.state.user.id, items: cartItems, amount: totalToPay, status: ‘PENDING’ });
ctx.status = 201; ctx.body = { success: true, orderId: order.id }; });
The Secure Implementation
The vulnerability is a 'Trust Boundary Violation'. The vulnerable code allows the client to dictate the financial outcome of a transaction. To fix this in Koa: 1. Implement 'Server-Side Truth' by re-fetching all price/state data from your database rather than the request body. 2. Use schema validators (like Zod or Joi) to ensure only expected fields are processed, preventing 'Mass Assignment' where an attacker might try to inject a 'role: admin' or 'isPaid: true' field into a request. 3. Implement strict state machine checks to ensure a user cannot jump from 'Cart' to 'Shipped' without the 'Payment' middleware firing.
const router = require('@koa/router')(); const { calculateTotal } = require('./logic/pricing');// SECURE: Server-side source of truth router.post(‘/api/checkout’, async (ctx) => { const { cartItems, discountCode } = ctx.request.body;
// 1. Fetch current prices from DB, do not trust client-provided amounts const productIds = cartItems.map(item => item.id); const products = await db.products.findMany({ id: { in: productIds } });
// 2. Re-calculate total on the backend const actualTotal = calculateTotal(products, cartItems, discountCode);
// 3. Enforce invariants (e.g., total cannot be negative) if (actualTotal < 0) { ctx.throw(400, ‘Invalid transaction state’); }
const order = await db.orders.create({ userId: ctx.state.user.id, items: cartItems, amount: actualTotal, status: ‘PENDING’ });
ctx.status = 201; ctx.body = { success: true, orderId: order.id }; });
Your Koa API
might be exposed to Business Logic Errors
74% of Koa apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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