The trojan horse is a legendary Greek tale of how the Greeks used stealth to enter the city of Troy during the Trojan War. The Greeks built a huge hollow wooden horse that secretly housed a select band of Greek warriors. The Greek army pretended to sail away from Troy and left the trojan horse, the trojans pulled the trojan horse into their city as a trophy of victory over the Greeks. The trojan horse ultimately sealed the destruction of the city of Troy, as the Greeks destroyed the city from the inside.
Within computing, a specific type of computer program has been referred to as trojan horse program. As with the Greek's wooden trojan horse, a trojan horse program appears to be innocent, will pretend to serve a purpose, but will ultimately attack a computer system from the inside by stealth. A trojan horse can pose a significant security risk to computer systems. Trojan horses tend to be downloaded from the Internet; a user will download a file they think has a genuine and useful purpose, but, infact, is designed for another purpose: to gain access, corrupt or attack a computer system. One of the most famous trojan horse programs is Back Orifice.
The purpose of a trojan horse program "runs the gamut" of security threats. Trojan horses can be used to: destroy and corrupt data; steal personal details; deliver adverts; monitor computer usage; remotely control a computer system to engage in illegal computer activity; create a zombie machine to engage in denial of service attacks; and to harm an individual out of spite or amusement.
Trojan horse programs are a type of malware; trojan horse programs differ from viruses and worms because they do not attempt to replicate and spread themselves to other computer systems. Trojan horse programs can be removed by anti-virus and anti-malware programs. It is important for Internet users to regularly update an anti-virus program and scan their system weekly. Trojan horses are continually evolving, and still (2013) pose a significant risk to home computers.