Which type of attack involves overwhelming a system with traffic to render it inoperable?

A Denial of Service (DoS) attack is specifically designed to overwhelm a system with excessive traffic, rendering it unable to process legitimate requests and ultimately causing it to become inoperable. The purpose of this attack is to disrupt the services of a targeted server, service, or network, often resulting in downtime that can lead to significant financial and reputational damage for the affected organization.

In contrast, a SQL Injection attack focuses on exploiting vulnerabilities in a web application's database by inserting malicious SQL code. This type of attack can compromise data integrity and confidentiality but does not primarily target system availability.

A Phishing attack attempts to trick users into providing sensitive information such as usernames and passwords, primarily targeting individuals rather than overwhelming a system.

A Man-in-the-Middle attack involves intercepting and altering communications between two parties without their knowledge, aiming to eavesdrop or manipulate the data being exchanged. While this can lead to data breaches, it does not align with the goal of overwhelming a system to prevent functionality.

Therefore, the distinction lies in the targeted outcome of the attack; DoS specifically seeks to disrupt service availability, which defines its nature and purpose.