Trojan Horses take advantage of this, hiding a malicious program inside something apparently harmless. If software has been installed in good faith, it can get around almost any firewall, authentication system, or virus scanner.
Trojans vary in the nefarious acts they perform once inside a machine.
They can be harmless pranks that display an obscene or political message, or logic bombs that erase data and try to damage hardware. Some are coupled with viruses, spreading between systems by e-mail. The most insidious are stealthier, and often have a purpose beyond wreaking havoc. As well as hacking, Trojans have been used to spy on people, and have acted as the culprits in some spectacular frauds. No one is safe.