While using a Web-based order form, an attacker enters an unusually large value in the Quantity field. The value she entered is so large that it exceeds the maximum value supported by the variable type used to store the quantity in the Web application. This causes the value of the quantity variable to wrap around to the minimum possible value, which is a negative number. As a result, the Web application processes the order as a return instead of a purchase, and the attacker's account is refunded a large sum of money. What type of attack has occurred in this scenario

The type of attack has occurred in this scenario is a integer overflow. An integer overflow occurs when there's an attempt by an arithmetic operation to create a numeric value which is outside the range which the given numbers are able to represent.

In this, the integer overflow occurs when the result of the integer operation cannot be able to fit in the allocated memory space which then brings about an unexpected result.