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. A Trojan, or Trojan horse, is a type of malware that conceals its true content to fool a user into thinking it’s a harmless file. Like the wooden horse used to sack Troy, the “payload” carried by a Trojan is unknown to the user, but it can act as a delivery vehicle for a variety of threats. 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

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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 recognize and detect a Trojan virus

Because Trojans are used as a delivery device for a number of different types of malware, if you suspect your device may have been breached by a Trojan, you should look for many of the same telltale signs of malicious software. These may include:

  • Poor device performance- Is your computer or mobile device running slowly or crashing more frequently than normal?
  • Strange device behavior- Are programs running you didn’t initiate or are other unexplained processes being executed on your device?
  • Pop-up and spam interruptions- Are you noticing an uptick in the number of interruptions from browser pop-ups or email spam?

If your device is exhibiting these symptoms, it’s possible a Trojan virus has managed to sneak its payload onto your computer. Try searching your computer for any programs or applications you don’t remember installing yourself. Enter any unrecognized file names or programs into a search engine to determine if they are recognized Trojans. 

Finally, if you haven’t already, scan your computer with antivirus software to see if it has uncovered a malicious file.

Examples of Trojans 

  • Zeus – Also known as Zbot, Zeus is a successful Trojan malware package with many variants used to carry out a number of different types of attack. It’s perhaps most well-known for its successful hack of the U.S. Department of Transportation
  • Wirenet – Wirenet is a password-stealing Trojan notable for being among the first to target Linux and OSX users, many of whom were migrating from Windows operating systems based on perceived security flaws.
  • Mobile banking Trojans – Webroot has documented a number of Trojans written to target mobile banking apps for the purpose of stealing login credentials or replacing legitimate apps with malicious ones. 

How to protect yourself against Trojans

As with protecting against most common cybersecurity threats, effective cybersecurity software should be your front line of protection. An effective internet security solution should run fast, frequent scans and alert you as soon as a Trojan virus is detected. 

If you’re reading this because it’s already too late, see our page on removing malware infecting your computer. If you’re reading this to stay safe from these types of attacks in the future, there are a few best practices in addition to installing cybersecurity software to help keep yourself safe:

  • Never download or install software from a source you don’t trust completely
  • Never open an attachment or run a program sent to you in an email from someone you don’t know. 
  • Keep all software on your computer up to date with the latest patches
  • Make sure a Trojan antivirus is installed and running on your computer

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
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