How can Trojans be installed on a target system?

• A. Through self-replicating emails
• B. When a user runs an infected program
• C. By accessing infected removable media
• D. By displaying malicious advertisements

Answer: (B)

Trojans are a type of malware that disguise themselves as legitimate software, making it easier to trick users into executing them. The primary method of installation for Trojans involves a user running an infected program. When the user is persuaded to download and execute such a program, the Trojan gains access to the system and can perform various malicious actions without the user's knowledge.

This method relies heavily on social engineering—trusted-looking applications or games can be made to appear harmless or even beneficial. Once activated, the Trojan can steal confidential information, create backdoors for future access, or use system resources for other malicious activities, such as sending spam or participating in Distributed Denial of Service (DDoS) attacks.

While options like self-replicating emails or accessing infected removable media may facilitate the spread of other types of malware, they are not characteristic of how Trojans typically operate. Displaying malicious advertisements often leads to click fraud or adware but does not directly install a Trojan without user intervention. Therefore, the process of a user running an infected program defines the core method by which Trojans are installed on a target system.