A trojan (or trojan horse) is a program in which malicious or harmful code is disguised as seemingly legitimate software that looks apparently harmless, allowing it to access the user's data in such a way that it can get control and do its chosen form of damage, such as ruining the file allocation table on their hard disk. In one celebrated case, a trojan was a program that was supposed to find and destroy computer viruses. A trojan may be widely redistributed as part of a computer virus. Trojans often are not intended to be harmful but due to hijacking, severe or multiple security flaws, the victim downloading the software could also get a trojan.
A backdoor trojan gives malicious users remote control over the infected computer. They enable the author to do anything they wish on the infected computer – including sending, receiving, launching and deleting files, displaying data and rebooting the computer. Backdoor Trojans are often used to unite a group of victim computers to form a botnet or zombie network that can be used for criminal purposes.
Rootkits are designed to conceal certain objects or activities in users' systems. Often their main purpose is to prevent malicious programs being detected – in order to extend the period in which programs can run on an infected computer.
Trojan-Banker programs are designed to steal users' account data for online banking systems, e-payment systems and credit or debit cards. ===Trojan-DDoS=== These programs conduct DoS (Denial of Service) attacks against a targeted web address. By sending multiple requests – from the user's computer and several other infected computers – the attack can overwhelm the target address, leading to a denial of service.
Trojan-Downloaders can download and install new versions of malicious programs onto users' computers – including other Trojans and adware.
These programs are used by hackers in order to install Trojans and/or viruses – or to prevent the detection of malicious programs. Not all antivirus programs are capable of scanning all of the components inside this type of Trojan.
Trojan-FakeAV are also commonly known as Rogue Antivirus programs and simulate the activity of antivirus software. They are designed to extort money from users – in return for the detection and removal of threats, even though the threats that they report are actually non-existent.
This type of program steals user account information from online gamers.
Trojan-IM programs steal logins and passwords for instant messaging programs – such as ICQ, MSN Messenger, AOL Instant Messenger, Yahoo Pager, Skype and many more.
This type of Trojan can modify data on users' computers – so that their computers don’t run correctly or they can no longer use specific data. The criminal will only restore the user's computer’s performance or unblock their data, after they have paid them the ransom money that they demand.
These programs can cost users money – by sending text messages from theirmobile device to premium rate phone numbers.
Trojan-Spy programs can spy on how users are using their computers – for example, by tracking the data they enter via their keyboard (also known as keylogging), taking screen shots or getting a list of running applications.
These programs can harvest email addresses from users' computers.
- Other types of Trojans include:
The term "Trojan" comes from Greek mythology about the Trojan War, as told in the Aeneid by Virgil and mentioned in the Odyssey by Homer. According to legend, the Greeks presented the citizens of Troy with a large wooden horse as a gift, the citizens took the horse into the city which was a big mistake. Since expert warriors were inside it, they had the perfect position to attack. During the night, the warriors emerged from the wooden horse and overran the city.