Civil Aviation Authority of Bangladesh (CAAB) Assistant Programmer (AP) Exam: 2022প্রশ্ন ও সমাধান

A Trojan horse is malicious software disguised as a legitimate, harmless program (a game, utility, or email attachment) that tricks the user into installing or running it. Once active, it can open a backdoor, steal login credentials, log keystrokes, or hand an attacker remote control of the machine.

Unlike a virus or worm, a Trojan does not self-replicate — it relies entirely on social engineering (the user willingly runs it) to spread, and it does not attach itself to other files.

Malware type	Self-replicates?	Needs a host file?	Typical goal
Virus	Yes	Yes (attaches to a file)	Corrupt/spread via infected files
Worm	Yes	No (spreads over the network)	Consume bandwidth, spread rapidly
Trojan	No	No (masquerades as a legitimate app)	Steal data, create backdoor access

Example: a fake "browser update" installer that, once run, silently records keystrokes typed into a bank's login page and sends them to the attacker.

Prevention: install antivirus/anti-malware software, avoid downloading programs from untrusted sources, never open unexpected attachments, and keep the OS and applications patched.

Exam takeaway: a Trojan's defining traits are disguise plus no self-replication — this is exactly what distinguishes it from viruses and worms, which spread on their own.