Critical Business Logic Vulnerability in E-commerce Checkout

I'm Mehran Kia, also known as cyb3rsem. During a security assessment of an e-commerce platform, I discovered a business logic vulnerability caused by insufficient server-side validation of product quantities.

After adding a product to the cart, I modified the quantity parameter in the request to an unusually large value, such as:

quantity=1000000

quantity=1000000

The backend accepted the value, recalculated the order total, and allowed the transaction to continue to the payment gateway without rejecting the request.

This behavior could lead to:

• Creation of invalid or oversized orders;
• Incorrect order and payment calculations;
• Inventory inconsistencies;
• Excessive resource consumption;
• Potential financial or operational impact.

The root cause was the absence of strict backend controls for maximum quantity, stock availability, per-user limits, and final payment validation.

The recommended fix is to validate every quantity server-side, enforce product and order limits, compare the requested amount with inventory, and recalculate the final price immediately before payment.

Category: Business Logic / Improper Input Validation Severity: Critical

Client-side restrictions are not sufficient. All order-related values must be validated and calculated securely on the server.

— Mehran Kia / cyb3rsem

telegram Chnnel : https://t.me/hakfateam

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