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What is a Trojan horse?
Trojan horse attacks pose one of the most serious threats to computer security. This page will teach you how to avoid falling prey to them, and how to repair the damage if you already did. According to legend, the Greeks won the Trojan war by hiding in a huge, hollow wooden horse to sneak into the fortified city of Troy. In today's computer world, a Trojan horse is defined as a "malicious, security-breaking program that is disguised as something benign". For example, you download what appears to be a movie or music file, but when you click on it, you unleash a dangerous program that erases your disk, sends your credit card numbers and passwords to a stranger, or lets that stranger hijack your computer to commit illegal denial of service attacks like those that have virtually crippled the DALnet IRC network for months on end.
The following general information applies to all operating systems, but by far most of the damage is done to / with Windows users due to its vast popularity and many weaknesses.
1. How did I get infected?
Trojans are executable programs, which means that when you open the file, it will perform some action(s). In Windows, executable programs have file extensions like "exe", "vbs", "com", "bat", etc. Some actual trojan filenames include: "dmsetup.exe" and "LOVE-LETTER-FOR-YOU.TXT.vbs"
Trojans can be spread in the guise of literally ANYTHING people find desirable, such as a free game, movie, song, etc. Victims typically downloaded the trojan from a WWW or FTP archive, got it via peer-to-peer file exchange using IRC/instant messaging/Kazaa etc., or just carelessly opened some email attachment. Trojans usually do their damage silently. The first sign of trouble is often when others tell you that you are attacking them or trying to infect them!
2. How do I avoid getting infected in the future?
You must be certain of both the source and content of each file you download! In other words, you need to be sure that you trust not only the person or file server that you downloaded the file from, but also the contents of the file itself.
Here are some practical tips to avoid getting infected.
4. How do I get rid of trojans?
Here are your many options, none of them are perfect. I strongly suggest you read through all of them before rushing out and trying to run some program blindly.
Clean Re-installation:
Although arduous, this will always be the only sure way to eradicate a trojan or virus. Back up your entire hard disk, reformat the disk, re-install the operating system and all your applications from original CDs, and finally, if you're certain they are not infected, restore your user files from the backup. If you are not up to the task, you can pay for a professional repair service to do it.
Anti-Virus Software:
Some of these can handle most of the well known trojans, but none are perfect, no matter what their advertising claims. You absolutely must make sure you have the very latest update files for your programs, or else they will miss the latest trojans. Compared to traditional viruses, today's trojans evolve much quicker and come in many seemingly innocuous forms, so anti-virus software is always going to be playing catch up. Also, if they fail to find every trojan, anti-virus software can give you a false sense of security, such that you go about your business not realizing that you are still dangerously compromised. There are many products to choose from, but the following are generally effective: PCSecurity Shield, Kaspersky Anti-Virus, and Trend Micro ( PC-Cillin ).
All are available for immediate downloading typically with a 30 day free trial. For a more complete review of all major anti-virus programs, visit our anti-virus software review section.
Anti-Trojan Programs:
These programs are the most effective against trojan horse attacks, because they specialize in trojans instead of general viruses.
Trojan horse attacks pose one of the most serious threats to computer security. This page will teach you how to avoid falling prey to them, and how to repair the damage if you already did. According to legend, the Greeks won the Trojan war by hiding in a huge, hollow wooden horse to sneak into the fortified city of Troy. In today's computer world, a Trojan horse is defined as a "malicious, security-breaking program that is disguised as something benign". For example, you download what appears to be a movie or music file, but when you click on it, you unleash a dangerous program that erases your disk, sends your credit card numbers and passwords to a stranger, or lets that stranger hijack your computer to commit illegal denial of service attacks like those that have virtually crippled the DALnet IRC network for months on end.
The following general information applies to all operating systems, but by far most of the damage is done to / with Windows users due to its vast popularity and many weaknesses.
1. How did I get infected?
Trojans are executable programs, which means that when you open the file, it will perform some action(s). In Windows, executable programs have file extensions like "exe", "vbs", "com", "bat", etc. Some actual trojan filenames include: "dmsetup.exe" and "LOVE-LETTER-FOR-YOU.TXT.vbs"
Trojans can be spread in the guise of literally ANYTHING people find desirable, such as a free game, movie, song, etc. Victims typically downloaded the trojan from a WWW or FTP archive, got it via peer-to-peer file exchange using IRC/instant messaging/Kazaa etc., or just carelessly opened some email attachment. Trojans usually do their damage silently. The first sign of trouble is often when others tell you that you are attacking them or trying to infect them!
2. How do I avoid getting infected in the future?
You must be certain of both the source and content of each file you download! In other words, you need to be sure that you trust not only the person or file server that you downloaded the file from, but also the contents of the file itself.
Here are some practical tips to avoid getting infected.
4. How do I get rid of trojans?
Here are your many options, none of them are perfect. I strongly suggest you read through all of them before rushing out and trying to run some program blindly.
Clean Re-installation:
Although arduous, this will always be the only sure way to eradicate a trojan or virus. Back up your entire hard disk, reformat the disk, re-install the operating system and all your applications from original CDs, and finally, if you're certain they are not infected, restore your user files from the backup. If you are not up to the task, you can pay for a professional repair service to do it.
Anti-Virus Software:
Some of these can handle most of the well known trojans, but none are perfect, no matter what their advertising claims. You absolutely must make sure you have the very latest update files for your programs, or else they will miss the latest trojans. Compared to traditional viruses, today's trojans evolve much quicker and come in many seemingly innocuous forms, so anti-virus software is always going to be playing catch up. Also, if they fail to find every trojan, anti-virus software can give you a false sense of security, such that you go about your business not realizing that you are still dangerously compromised. There are many products to choose from, but the following are generally effective: PCSecurity Shield, Kaspersky Anti-Virus, and Trend Micro ( PC-Cillin ).
All are available for immediate downloading typically with a 30 day free trial. For a more complete review of all major anti-virus programs, visit our anti-virus software review section.
Anti-Trojan Programs:
These programs are the most effective against trojan horse attacks, because they specialize in trojans instead of general viruses.
a. Never download blindly from people or sites which you aren't 100% sure about.
In other words, as the old saying goes, don't accept candy from strangers. If you do a lot of file downloading, it's often just a matter of time before you fall victim to a trojan.
b. Even if the file comes from a friend, you still must be sure what the file is before opening it, because many trojans will automatically try to spread themselves to friends in an email address book or on an IRC channel. There is seldom reason for a friend to send you a file that you didn't ask for. When in doubt, ask them first, and scan the attachment with a fully updated anti-virus program.
c. Beware of hidden file extensions! Windows by default hides the last extension of a file, so that innocuous-looking "susie.jpg" might really be "susie.jpg.exe" - an executable trojan! To reduce the chances of being tricked, unhide the extensions. This can be different for your version of OS, but most can be changed through the folders option in your control panel. Their will be a box you will need to check that says something like "show all files". You can also do a search in Google if yours is different.
d. Never use features in your programs that automatically get or preview files. Those features may seem convenient, but they let anybody send you anything which is extremely reckless. For example, never turn on "auto DCC get" in mIRC, instead always screen every single file you get manually. Likewise, disable the preview mode in Outlook and other email programs.
e. Never blindly type commands that others tell you to type, or go to web addresses mentioned by strangers, or run pre-fabricated programs or scripts (not even popular ones). If you do so, you are potentially trusting a stranger with control over your computer, which can lead to trojan infection or other serious harm.
f. Don't be lulled into a false sense of security just because you run anti-virus programs. Those do not protect perfectly against many viruses and trojans, even when fully up to date. Anti-virus programs should not be your front line of security, but instead they serve as a backup in case something sneaks onto your computer.
g. Finally, don't download an executable program just to "check it out" - if it's a trojan, the first time you run it, you're already infected!
In other words, as the old saying goes, don't accept candy from strangers. If you do a lot of file downloading, it's often just a matter of time before you fall victim to a trojan.
b. Even if the file comes from a friend, you still must be sure what the file is before opening it, because many trojans will automatically try to spread themselves to friends in an email address book or on an IRC channel. There is seldom reason for a friend to send you a file that you didn't ask for. When in doubt, ask them first, and scan the attachment with a fully updated anti-virus program.
c. Beware of hidden file extensions! Windows by default hides the last extension of a file, so that innocuous-looking "susie.jpg" might really be "susie.jpg.exe" - an executable trojan! To reduce the chances of being tricked, unhide the extensions. This can be different for your version of OS, but most can be changed through the folders option in your control panel. Their will be a box you will need to check that says something like "show all files". You can also do a search in Google if yours is different.
d. Never use features in your programs that automatically get or preview files. Those features may seem convenient, but they let anybody send you anything which is extremely reckless. For example, never turn on "auto DCC get" in mIRC, instead always screen every single file you get manually. Likewise, disable the preview mode in Outlook and other email programs.
e. Never blindly type commands that others tell you to type, or go to web addresses mentioned by strangers, or run pre-fabricated programs or scripts (not even popular ones). If you do so, you are potentially trusting a stranger with control over your computer, which can lead to trojan infection or other serious harm.
f. Don't be lulled into a false sense of security just because you run anti-virus programs. Those do not protect perfectly against many viruses and trojans, even when fully up to date. Anti-virus programs should not be your front line of security, but instead they serve as a backup in case something sneaks onto your computer.
g. Finally, don't download an executable program just to "check it out" - if it's a trojan, the first time you run it, you're already infected!
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