What is a Trojan Virus?

A Trojan horse or Trojan is a type of malware that is often disguised as legitimate software. Trojans can be employed by cyber-thieves and hackers trying to gain access to users' systems. Users are typically tricked by some form of social engineering into loading and executing Trojans on their systems. Once activated, Trojans can enable cyber-criminals to spy on you, steal your sensitive data, and gain backdoor access to your system. These actions can include:

• Deleting data
• Blocking data
• Modifying data
• Copying data
• Disrupting the performance of computers or computer networks

Unlike computer viruses and worms, Trojans are not able to self-replicate.

How Trojans can impact you

Trojans are classified according to the type of actions that they can perform on your computer:

• Backdoor  
  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.
• Exploit  
  Exploits are programs that contain data or code that takes advantage of a vulnerability within application software that’s running on your computer.
• Rootkit  
  Rootkits are designed to conceal certain objects or activities in your system. 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  
  Trojan-Banker programs are designed to steal your 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 your computer and several other infected computers – the attack can overwhelm the target address… leading to a denial of service.
• Trojan-Downloader  
  Trojan-Downloaders can download and install new versions of malicious programs onto your computer – including Trojans and adware.
• Trojan-Dropper  
  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  
  Trojan-FakeAV programs simulate the activity of antivirus software. They are designed to extort money from you – in return for the detection and removal of threats… even though the threats that they report are actually non-existent.
• Trojan-GameThief  
  This type of program steals user account information from online gamers.
• Trojan-IM  
  Trojan-IM programs steal your logins and passwords for instant messaging programs – such as ICQ, MSN Messenger, AOL Instant Messenger, Yahoo Pager, Skype and many more.
• Trojan-Ransom  
  This type of Trojan can modify data on your computer – so that your computer doesn’t run correctly or you can no longer use specific data. The criminal will only restore your computer’s performance or unblock your data, after you have paid them the ransom money that they demand.
• Trojan-SMS  
  These programs can cost you money – by sending text messages from your mobile device to premium rate phone numbers.
• Trojan-Spy  
  Trojan-Spy programs can spy on how you’re using your computer – for example, by tracking the data you enter via your keyboard, taking screen shots or getting a list of running applications.
• Trojan-Mailfinder  
  These programs can harvest email addresses from your computer.
• Other types of Trojans include:
  • Trojan-ArcBomb
  • Trojan-Clicker
  • Trojan-Notifier
  • Trojan-Proxy
  • Trojan-PSW