What is the name of the type of malicious software that fools a user into downloading and running it by pretending to be a useful or legitimate application?

Introduction / Context:
Cyber security often involves recognising different types of malicious software, or malware. Some malware spreads by exploiting network weaknesses, while others depend on tricking the user. This question focuses on a type of malware that disguises itself as something helpful, such as a free utility or game, to persuade the user to download and execute it. Knowing this term helps users identify social engineering techniques and avoid dangerous downloads.

Given Data / Assumptions:

• The malware pretends to be a useful or legitimate application.
• The user is deceived into downloading and running it voluntarily.
• The focus is on the method of entry, not on self replication behaviour.
• Options include different classes of malware such as keyloggers, worms and viruses.

Concept / Approach:
A Trojan horse is a type of malware that hides its malicious intent behind a harmless or attractive facade. The name comes from the ancient Greek story of the Trojan Horse, where soldiers hid inside a wooden horse given as a gift. Similarly, a digital Trojan horse appears to be a useful program but, once executed, can install backdoors, steal data or damage the system. Unlike worms and many viruses, Trojans do not typically self replicate across systems by themselves; they rely on tricking users into running them.

Step-by-Step Solution:

Verification / Alternative check:
Security guides and antivirus vendors commonly define a Trojan as malware that misleads users of its true intent. Examples include fake antivirus software that pretends to scan for threats but actually installs more malware or phishing programs disguised as banking apps. Technical classifications separate Trojans from viruses, which modify other legitimate files, and worms, which are self contained and self replicating. The constant theme with Trojans is social engineering and disguise, which exactly matches the description in the question.

Why Other Options Are Wrong:

• Keylogger: A program that records keystrokes to capture passwords and other information, but it may or may not be disguised as a useful app; the question emphasises the disguise aspect, not the specific function.
• Worm: A self replicating program that spreads through networks without needing the user to install it as a helpful application.
• File virus: Attaches itself to executable files and spreads when those files run; it is not defined primarily by pretending to be a useful standalone program.
• Macro virus: Infects macro code in documents such as word processor or spreadsheet files; again, the key feature is not pretending to be a helpful full program.

Common Pitfalls:
Beginners sometimes use virus as a general word for all malware. In exams and security discussions, however, different terms highlight different behaviours. Another confusion is thinking that any program that records keystrokes or steals data must be a Trojan. In fact, Trojan horse describes the delivery method, while keylogger describes a specific payload. Remember that if the emphasis is on disguise as a useful application to trick the user, the correct term is Trojan horse.

Final Answer:
The malware that fools users by pretending to be a useful application is called a Trojan horse.