Client-Side Price Manipulation: Pay Whatever You Want at Checkout

Exploiting a server-side validation failure in OopsSec Store's checkout process to purchase products at arbitrary prices.

OopsSec Store's checkout sends the order total straight from the browser. The server saves whatever it receives without recalculating from actual product prices. Change it to a penny, the order goes through at a penny.

Lab setup

From an empty directory:

npx create-oss-store oss-store
cd oss-store
npm start

Or with Docker (no Node.js required):

docker run -p 3000:3000 leogra/oss-oopssec-store

The app runs at http://localhost:3000.

Vulnerability overview

When you buy something on OopsSec Store, the browser sends a POST to /api/orders with the cart items and a total field. That total is calculated by frontend JavaScript. The server takes it at face value and creates the order.

The product prices are in the database. The server could look them up and do the math itself. It doesn't.

Locating the attack surface

Add some products to your cart and go through checkout. The payment page shows your order summary with the total.

Click "Complete Payment" and the browser fires off a POST with the order details, including the total the frontend calculated.

Exploitation

Configuring the proxy

Set up Burp Suite as an intercepting proxy (browser traffic through 127.0.0.1:8080). Leave interception off for now.

Preparing the order

Add products to your cart. Higher-priced items make the result more obvious. Go through checkout until you hit the payment page.

Intercepting the request

Turn on interception in Burp, then click "Complete Payment". Burp catches the POST to /api/orders before it hits the server.

Looking at the request

The request body is JSON with the order details:

The total field is the price the frontend calculated. The server uses this number directly.

Modifying the price

Change total to whatever you want. 0.1 works:

Completing the attack

Forward the modified request and turn off interception. The server processes the order at your price.

Capturing the flag

The order confirmation shows the purchase at the modified total. The server notices the mismatch and returns the flag:

OSS{cl13nt_s1d3_pr1c3_m4n1pul4t10n}

Vulnerable code analysis

The checkout handler pulls total straight out of the request body and saves it:

const { total } = await request.json();

const order = await prisma.order.create({
  data: {
    userId: user.id,
    total: total, // Client-provided value used directly
  },
});

The frontend does calculate the right number. But the server never checks it. Anyone with a proxy, devtools, or curl can send whatever total they want.

The product prices and cart quantities are right there in the database. The server just doesn't use them.

Remediation

Recalculate the total server-side

Pull the cart from the database and compute the total from actual prices:

const cart = await prisma.cart.findFirst({
  where: { userId: user.id },
  include: {
    cartItems: {
      include: { product: true },
    },
  },
});

const calculatedTotal = cart.cartItems.reduce(
  (sum, item) => sum + item.product.price * item.quantity,
  0
);

const order = await prisma.order.create({
  data: {
    userId: user.id,
    total: calculatedTotal, // Server-calculated value
  },
});

Detect tampering

If you still want the client total for logging or display, compare it against the server calculation:

const clientTotal = requestBody.total;
const serverTotal = calculateTotalFromCart(cart);

if (Math.abs(clientTotal - serverTotal) > 0.01) {
  return NextResponse.json(
    { error: "Price validation failed" },
    { status: 400 }
  );
}

The frontend total is fine for UX. The backend should never trust it for the actual charge.

Lab

Disclaimers

Do not deploy OopsSec Store on a production server. This application is intentionally vulnerable and should only be used in isolated, local environments for educational purposes.

Do not exploit vulnerabilities on systems you don’t have explicit authorization to test. Unauthorized access to computer systems is illegal. Always obtain proper permission before performing security testing.

Feedback & Support

Having trouble following this writeup? Found a typo or have suggestions for improvement?

Feel free to open an issue or start a discussion on GitHub.

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