Which of the following is an attack in which the attacker secretly relays and possibly alters communication between two parties?

The attack described in the question involves an attacker secretly intercepting and possibly altering the communication between two parties, which is characteristic of a Man-in-the-Middle (MITM) attack. In a MITM scenario, the attacker positions themselves between the two communicating parties without their knowledge, allowing them to listen in on the conversation or modify the messages being exchanged. This can enable various malicious activities, such as stealing sensitive information, injecting malicious content, or manipulating transactions.

MITM attacks are particularly dangerous because victims often remain unaware that their communications have been compromised. This type of attack can happen over unsecured networks or even on some secured connections if the attacker can exploit vulnerabilities in the encryption or authentication processes.

Other types of attacks listed do not fit this definition. For example, Cross-Site Scripting (XSS) involves injecting malicious scripts into webpages viewed by users, SQL Injection (SQLi) targets databases via malicious SQL code to manipulate or extract data, and brute-force attacks involve attempting to guess passwords or encryption keys through exhaustive trial and error. None of these methods involve secretly relaying or altering direct communication between two parties as MITM does.