Which type of attack involves an attacker who intercepts and potentially alters the communications between two parties?

The type of attack where an attacker intercepts and potentially alters communications between two parties is known as a Man-in-the-Middle (MitM) attack. In this scenario, the attacker secretly relays and possibly alters the communication between the two parties without either of them knowing that a third party is involved. This can occur in various contexts, such as when connecting to unsecured networks, allowing the attacker to capture sensitive information like passwords or credit card numbers.

A Denial of Service (DoS) attack, on the other hand, aims to make a system or service unavailable to its intended users by overwhelming it with a flood of traffic or requests, rather than intercepting communications.

Port Scanning is a technique used to discover available ports and services on a networked device, not to intercept or alter communications. It primarily serves as a method for an attacker to gather information about potential vulnerabilities.

SQL Injection exploits vulnerabilities in an application's software by inserting malicious SQL code into a query, allowing the attacker to manipulate the database, but this does not involve intercepting communications between two parties.

Thus, the Man-in-the-Middle attack distinctly fits the description of intercepting and altering communications, differentiating it from the other types of attacks listed.