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 

How to protect yourself against Trojans

By installing effective anti-malware software, you can defend your devices – including PCs, laptops, Macs, tablets and smartphones – against Trojans. A rigorous anti-malware solution – such as Kaspersky Anti-Virus – will detect and prevent Trojan attacks on your PC, while Kaspersky Mobile Security can deliver world-class virus protection for Android smartphones. Kaspersky Lab has anti-malware products that defend the following devices against Trojans:

• Windows PCs
• Linux computers
• Apple Macs
• Smartphones
• Tablets

Other articles and links related to Trojans

• What is a Computer Virus or a Computer Worm?
• Adware, Pornware and Riskware
• Who Creates Malware?
• Kaspersky Total Security
• Kaspersky Internet Security
• Kaspersky Anti-Virus
• Kaspersky Internet Security for Mac
• Kaspersky Internet Security for Android