A computer program that functions normally while quietly installing malicious software on a machine is known as a:

A DDoS (Distributed Denial of Service) attack floods a target system with traffic from multiple sources to make it unavailable, which does not involve disguising software as legitimate.

A worm is self-replicating malware that spreads across networks autonomously without needing to disguise itself as a normal program.

A phishing attack is a social engineering technique that deceives users into revealing credentials or clicking malicious links, not a program that installs malware while appearing to function normally.

A Trojan horse derives its name from the Greek myth and operates by appearing to be a useful or benign program - such as a game, utility, or installer - while secretly performing unauthorized actions such as installing backdoors, keyloggers, or other malware. Unlike worms, Trojans do not self-replicate; they depend on user interaction to be installed and rely on their deceptive appearance to bypass user suspicion. The key defining characteristic is the combination of apparent legitimate functionality with hidden malicious behavior.