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?