Add-cart.php: Num

Tonight, he'd let the ghost walker win. The next morning, a new commit appeared on the main branch: fix: add unique constraint and row-level locking to add-cart.php (thanks gh0st_walk3r for the pentest)

He checked gh0st_walker 's IP address. Traced it back to a residential block in Akron, Ohio. Not a botnet. Not a competitor. Someone sitting in a basement, probably using a simple bash script:

Leo leaned back in his creaking office chair, the glow of three monitors painting his tired face in pale blue light. He was the senior backend engineer for Velvet & Sole , a boutique online shoe retailer that had, against all odds, become a cult hit. Their signature "Dragonhide 7X" boot sold out in eleven minutes every restock.

Leo smiled. He opened a new terminal and manually reduced the three rows to one. Then he added a note to the user's account: "Loyal customer. Approved for second pair on next restock. Also, nice race condition." add-cart.php num

– 11:34:02.447 POST /add-cart.php HTTP/1.1 – 11:34:02.451 POST /add-cart.php HTTP/1.1 – 11:34:02.453

<?php // Legacy code. No locking. No transactions. $product_id = $_POST['product_id']; $user_id = $_SESSION['user_id']; $quantity = 1; // default // Check if item exists in cart $result = $db->query("SELECT * FROM cart WHERE user_id=$user_id AND product_id=$product_id"); if($result->num_rows == 0) { $db->query("INSERT INTO cart (user_id, product_id, quantity) VALUES ($user_id, $product_id, $quantity)"); } else { $db->query("UPDATE cart SET quantity = quantity + $quantity WHERE user_id=$user_id AND product_id=$product_id"); } ?>

Three separate line items for the same boot. Quantity: 1. Three times. Tonight, he'd let the ghost walker win

Leo clicked through to the checkout table. The order hadn't been placed yet. But the cart's total? $1,197.00. The user had effectively bypassed the "max 1 per customer" rule without triggering a single alarm. Not a hack. Not an SQL injection. Just the ugly poetry of concurrency.

He opened the source file: add-cart.php .

But he didn't type a single line.

Instead, he clicked over to the user's profile. gh0st_walker had been a member for four years. Bought three pairs of boots, left glowing reviews each time. Their last order was a size 11—the same size in the ghost cart.

He closed the file. He'd fix add-cart.php tomorrow.

Leo swore under his breath. No BEGIN TRANSACTION . No FOR UPDATE . Just two naïve queries and a prayer. The three simultaneous POSTs had each run the SELECT , seen an empty cart, and each fired an INSERT . Three rows. Same product. Not a botnet