Add-cart.php Num

The Hidden Dangers of `add-cart.php?num=1`: Why Simple Shopping Carts Fail

In the world of e-commerce development, few scripts are as ubiquitous—and as notoriously vulnerable—as add-cart.php. At first glance, it seems harmless: a simple backend handler that adds a product to a user’s shopping cart. But when you see a URL like https://example.com/add-cart.php?num=1, alarms should go off for any experienced developer.

This article dissects the add-cart.php script, focusing specifically on the num parameter. We will explore what it does, why it’s a red flag for security, how attackers exploit it, and how to rebuild it correctly.

The Exploit: Negative Quantities

The most common exploitation method for the num parameter involves Integer Underflow or Logic Errors. add-cart.php num

While most developers remember to prevent a user from ordering 0 items, they often forget to handle negative numbers.

1. Mass Assignment & Cart Flooding (DoS)

By sending a single request with an absurdly high num value, or by sending thousands of sequential requests via a simple script, an attacker can flood the cart session. The Hidden Dangers of add-cart

Attack: add-cart.php?num=999999999
Result: If the server tries to allocate an array with a key of 999 million, it could exhaust memory. Even worse, if num controls quantity, the attacker sets num=10000 on a $0.01 item, forcing your tax or shipping calculation functions into integer overflows.

Step 1: Initialization and Input Sanitization

The script usually receives data via a GET or POST request. Let's assume the request looks like add-cart.php?id=123.

<?php
session_start();
// 1. Include Database Connection
require_once 'db_connect.php'; // Assume $pdo is the connection object Attack:   add-cart
// 2. Check if the request is valid
if (isset($_GET['id'])) 
// 3. Sanitize the Product ID
// We use filter_var to ensure 'id' is an integer.
$product_id = filter_var($_GET['id'], FILTER_SANITIZE_NUMBER_INT);
// Validate that the ID is not empty after sanitization
if (empty($product_id)) 
    header("Location: products.php?error=invalid_id");
    exit();
// (Optional) Check if user is logged in. 
// If not, you might use $_SESSION['cart'] for guest users.
// For this article, we assume a logged-in user.
$user_id = $_SESSION['user_id'];
// ... Logic continues below


?>

1. Strict Type Validation

Never trust input. The num parameter must be validated to ensure it is a positive integer.

// Vulnerable Code
$quantity = $_GET['num']; 
// If user sends ?num=-5, this is accepted.
// Secure Code
$quantity = intval($_GET['num']);
if ($quantity <= 0) 
    // Reject the request
    die("Error: Quantity must be at least 1.");