ECSA v10: EC-Council Certified Security Analyst v10

Thomas is an attacker who reviewed the HTML source code of an online shopping website to look for vulnerabilities he could exploit.

He already knows that when a user selects any items on the online shopping webpage, those selections are typically stored as form-field values and sent to the application in an HTTP request (GET or POST) after the user clicks the Submit button. He also knows that some fields related to the selected items can be modified by the user (such as quantity and color), while others cannot (such as price). While reviewing the HTML code, he found that item price field values were present in the HTML code. He changed the price field values for certain items from $200 to $2 in the HTML code and successfully submitted the request to the application.

Identify the type of attack Thomas performed on the online shopping website?