John visits an online shop that stores the IDs and prices of the items to...

John visits an online shop that stores the IDs and prices of the items to buy in a cookie. After selecting the items that he wants to buy, the attacker changes the price of the item to 1.

Original cookie values:

ItemID1=2

ItemPrice1=900

ItemID2=1

ItemPrice2=200

Modified cookie values:

ItemID1=2

ItemPrice1=1

ItemID2=1

ItemPrice2=1

Now, he clicks the Buy button, and the prices are sent to the server that calculates the total price.

Which of the following hacking techniques is John performing?