Which of the following is NOT a major attack type?

The correct choice is identified as a Trojan horse, which is not categorized as a major attack type. A Trojan horse refers to a malicious software program that disguises itself as a legitimate application to deceive users into installing it. Unlike the other options, which represent more direct attack methodologies employed against systems or data, a Trojan horse operates through deception, often leading to unauthorized access or control once installed.

In contrast, man-in-the-middle attacks involve an attacker secretly intercepting and relaying messages between two parties, making it a direct attack on the communication process. Replay attacks involve the interception and retransmission of valid data to trick systems into accepting it as a legitimate request. Spam typically refers to unsolicited messages, often used to either spread malware or gather personal information, which can also be seen as a significant threat in the cybersecurity realm. Thus, the Trojan horse is best understood as a delivery mechanism for malware rather than a major attack type itself.