Trojan

A trojan is malware disguised as legitimate software or a harmless file, tricking users into installing it so that it can carry out hidden malicious actions.

Named after the wooden horse of Greek legend, a trojan relies on deception rather than spreading on its own. It might look like a PDF invoice, a free utility, a cracked copy of paid software or a fake browser update. Once opened, it can install other malware, steal data, record keystrokes or open a backdoor that gives attackers remote access.

Trojans are a common first step in larger attacks against small businesses, including banking fraud and ransomware. Because users install them willingly, they can slip past defences that focus only on blocking intrusions from outside.

Practical tip: only install software from official sources, and restrict installation rights so ordinary users cannot add programs without IT approval. Be particularly wary of attachments that ask you to enable macros or editing, and of pop-ups urging you to install an update. If something unexpected installs, disconnect the computer and contact IT right away.