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Whatís a Trojan Horse virus?

By May 31, 2006

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I came across this gem on the Visa website:
Trojan Horse virus
Whatís a Trojan Horse virus?
A Trojan Horse is an email virus usually released by an email attachment. If opened, it will scour your hard drive for any personal and financial information such as your social security, account, and PIN numbers. Once it has collected your info, it is sent to a thiefís database.

Now, there are Trojan Horses and there are viruses, but there's no such thing as a Trojan Horse virus. In fact, the very definition of each precludes any chance of there being such a thing. A Trojan does not replicate. Viruses do. That fact alone means there can never be a "Trojan Horse virus".

The Visa description continues with, "A Trojan Horse is an email virus usually released by an email attachment." Not so. A Trojan may be sent as an attachment in email, but it's certainly not an email virus. (In fact there are few true email viruses, but that's a whole other topic). So it may or may not arrive in email, and it's equally likely to have been downloaded from a website or resulted from a P2P file transfer. In other words, vector has nothing to do with whether something is or isn't a Trojan.

Just what is a Trojan then? A Trojan is a program that appears to be legitimate, but in fact does something malicious. Quite often, that something malicious involves gaining remote, surreptitious access to a user's system. Unlike viruses, a Trojan does not replicate (i.e. infect other files), nor does it make copies of itself as worms do.

There are several different types of Trojans. Some of these include: remote access Trojans (RATs), backdoor Trojans (backdoors), IRC Trojans (IRCbots), and keylogging Trojans. Many Trojan encompass multiple types. For example, a Trojan may install both a keylogger and a backdoor. IRC Trojans are often combined with backdoors and RATs to create collections of infected computers known as botnets.

But one thing you probably won't find a Trojan doing is scouring your hard drive for personal details, as the Visa description alleges. Contextually, that would be a bit of a trick for a Trojan. Instead, this is where the keylogging functionality most often comes into play - capturing the user's keystrokes as they type and sending the logs to the attackers. Some of these keyloggers can be pretty sophisticated, targeting only certain websites (for example) and capturing any keystrokes involved with that particular session.

But why is it important to know the difference between a virus, a worm, and a Trojan? Because a virus infects legitimate files, thus if antivirus software detects a virus, that file should be cleaned. Conversely, if antivirus software detects a worm or a Trojan, there is no legitimate file involved and action should be to delete the file.

May 31, 2006 at 7:29 pm
(1) EAguer says:

After a recent incident ,an unknown entity trying to download a bank command into my computer to get my bank accounts information, I suspended all Internet banking operations.
It does not make sense the financial institutions to secure their systems if the weak link in the security is me ,and I do not have or will have the means to secure my computer.Also I concluded that is logically and physically imposible to secure any computer system.

June 5, 2006 at 9:45 pm
(2) Maryellen Jones says:

I read your explanation of Trojan several times. But I still don’t know what to do with a trojan. Your instructions say “delete the file”. How will I know which file to delete? How will I recognize the trojan? If my antivirus doesn’t quantine the trojan, how do I protect my computer from it.

Your article is full of information, but I haven’t a clue what to do with the information. Could you write something in plain English, please?
submitted respectfully,

June 6, 2006 at 12:52 pm
(3) Mary says:

“…if antivirus software detects a virus, that file should be cleaned… if antivirus software detects a worm or a Trojan (the) action should be to delete the file.”

When the antivirus software detects the threat, it may prompt your for an action. Choose ‘clean’ for viruses, and ‘delete’ for worms and Trojans.

June 8, 2006 at 2:25 pm
(4) Sharon says:

I contracted the BackDoor-AWV virus, even though I have McAfee Suite installed and current. I cannot Delete, Quarantine or Remove. McAfee warns me that the virus is present, but cannot help with the removal, nor does it recognize the name when I enter the name on their customer support site. Any suggestions?

June 8, 2006 at 8:18 pm
(5) Ms. Truschel says:

Not the most lucid explanation I’ve ever read.
If folks didn’t have a pretty good idea what viruses, Trojan horses, and worms were before they read this explanation, they sure didn’t pick up much from this piece.
I think you know what you’re talking about, but you could have expressed yourself a lot more clearly.

June 9, 2006 at 1:24 am
(6) Mary says:

I am sorry you are having difficulty understanding the material. Perhaps if you can explain just what part of it you did not understand, I can better respond to your comments. You may find “What is a Virus of help.

June 9, 2006 at 1:29 am
(7) Mary says:

Hi Sharon,

If you can, please post your question to the antivirus forum. That way, other members will benefit as well. Plus, it may take a bit of back and forth to resolve your issue, and that’s outside of the scope of this comments section. :-)

June 9, 2006 at 6:18 am
(8) Terry says:

So a infector is a worm or virus?

June 9, 2006 at 12:24 pm
(9) Andrew says:

I found the info fairly informative although I need to know a lot more in this area. As the lady said, if your computer says I have a Trogen, how do I know which file(s) it is in so that I can only delete those affected files. I am also interested in knowing more about GAIN.exe and other support cookies regarding which one’s do I have to use and need and which one’s will be a problem for me. I have accessed some websites on them, but found myself bogged down in unfamiliar information. I plan on signing up for your website for a while. This might help.

June 12, 2006 at 10:19 am
(10) Bill says:

The article was well written and totally understandable. Thank you!

June 16, 2006 at 12:16 pm
(11) Holly says:

Thanks for the article. I got a trojan horse and was able to delete the file using Avast! antivirus software, but now I cannot get onto the internet.. I’ve tried unplugging the modem and plugging it back in as a friend advised, but it didn’t help.. any suggestions?

September 21, 2006 at 9:54 am
(12) Keetta says:

I found this article to be very informative. I THINK I got rid of the Trojan last night using AVG, but I’m afraid that it will return. Is that possible? Is it ok to have more than one anti-virus agent for extra protection?

March 11, 2008 at 9:43 pm
(13) John says:

of course it can always be a good thing to have more than 1 program for virus protection, i reccommend the following ones:

spybot – search & destroy (free download)
ad-aware SE personal (free download)
webroot’s spysweeper (costs $40 for 2 years, it really is the best thing out there in my opinion. worth the money)

the main place you’re gonna get those viruses (of course you’ve heard it a million times) porn, and downloads from ‘suspicious’ sites. limewire of course, but that’s really only if you’re not careful about how you use it. bottom line, stop downloading so many odd/illegal programs/videos/pics/porn/songs

(i’m a hypcrite myself on that part, lol. but i know what i’m doing.)

March 11, 2008 at 9:44 pm
(14) John says:


September 28, 2008 at 10:37 pm
(15) Laina says:

I have my antivirus software find two trojan viruses today and two a couple weeks ago and it says it quarantined them. They were in my temporary internet files. I deleted these files after finding out that’s where my antivirus software found them but, should I do anything else?

November 10, 2008 at 7:06 am
(16) jhenyfer says:

i have this trojan horse downloader agent that infects my system, i have it scanned and removed everytime the AVG detects threats from it but now that i have idea that this virus came from a website for downloading games, does this mean i cant download anything on that site anymore? can i just healed the virus and install the game to my computer anyway? thanks..

December 3, 2008 at 8:59 pm
(17) Aejey says:

I’m currently writing a case study on this topic and trying to understand it, therefore starting from scratch. can these trojans be found on social networking sites? If so, do you happen to know any specific ocurences in the past where this has been proven an issue?

January 2, 2009 at 4:13 pm
(18) lisa says:

My computer just detected a trojan and said that disinfect was recommended; when I click okay it said that it could only rename it .. but I have just scanned my computer and its not picking anything up. does this mean its still there? and can the trojan tamper with my anti virus software so that it doesn’t show up?

January 2, 2009 at 11:54 pm
(19) this says:

there is such thing as trojan horse i almost got infected with it on myspace good thing i have avast and it stoped it from anything from downloading : )
but if you want to check it out go to FIND FRIENDS then look for this name Proxy – “spotproxy.com”. TROJAN HORSE IS REAL BE ON THE LOOK OUT OK

January 3, 2009 at 12:08 am
(20) this says:

this has nothing to do with this but if anyone can help me with the name of this virus

discription: thing comes out of egg; starts growling; makes a hole in the backround;and destroys my computer.

please help

January 21, 2009 at 9:51 pm
(21) lexa says:

what is an example of the trojan virus??

March 18, 2009 at 1:44 am
(22) Nik says:

well well well its not a virus aye well thats what it’s called so ummm i presume it it 1

April 1, 2009 at 10:25 pm
(23) Krystin says:

It says I have a Trojan virus on my iPod but idk how I could recieve an email virus on it when I have not used it for email.

April 3, 2009 at 9:30 am
(24) Mary Landesman says:

As the topic of the article discusses, a trojan isn’t a virus. So one cannot have a trojan virus. I’m not sure what the email question is regarding. Malware spreads via many vectors. An iPod is pretty much just a portable hard drive with specialized software. If you can transfer files onto something, you can transfer malware as well.

April 8, 2009 at 11:23 pm
(25) brad says:

A Trojan virus attached to my flash drive after I used it on an infected computer. If a Trojan does not infect other areas, how is this possible? The exact name was “Trojan horse Dropper.Generic.ZMN” I have scanned my computer several times and itís not on my computer. So why would it transfer to a flash drive and not a computer?

April 9, 2009 at 11:50 am
(26) Andrea says:

my mom says that Trojan infestation, but can you honestly know what to look for when trying to find the source of the infestation?

April 18, 2009 at 11:25 pm
(27) Jennie says:

this is all very confusing.
today someone got into my hotmail account and e mailed all of my contacts using my name as the sender… the only reason i knew this is because I have another screen name and it is on my contact list as well. so I was shocked to see mail from myself. how did they do this??? and what is this called and how do I prevent this from happening again.
I have AVG and when i scanned my pc it did not show anything odd except cookies. So what do I do now????

April 21, 2009 at 9:04 am
(28) BJ says:

Okay, I did a scan and I have 32 viruses. I have disconnected the iternet already, but not sure what to do know. Is there anyay to get rid of it wihout getting an antivirus? I need some help and fast.

April 27, 2009 at 2:32 pm
(29) Aaron Baker says:


April 27, 2009 at 2:33 pm
(30) w/e says:


May 6, 2009 at 3:17 pm
(31) Geoff Powell says:

Last week the computer flashed up on the Screen TROJAN in very large letters.This was accompanyed by a verl lound Klaxon sound from the computer.Does this mean that the Trojan has breached the computers defense system.?
All seems to be operating as Normal

May 25, 2009 at 5:42 pm
(32) Leah says:

theres a serious problem with my computer. ok so i start it up and the dell screen comes up. then it says that windows did not start successfully and that i should select last known good configuration. so i pick that and a blue screen comes up and it says that there is a problem on my computer and it needs to be shut down to protect my computer. i have no idea whats wrong with it but i cant get past the blue screen. if anyone has even the slighest idea how to fix this problem i would really appreciate it. thanks:)

June 11, 2009 at 10:37 pm
(33) dar says:

i have a trojan virus on my computer,it will not let me delete it & when i tried to use your free virus removal tool,it blocks me,i just want this to be deleted from my computer

June 15, 2009 at 8:28 pm
(34) MacLeod says:

Ok so last night i was busy watching prison break (shit i didnt finish it) online and then all of a sudden a pop-up appered on my screen and it says my internet has a trojan virus, i was panickin and i dont know what i did. All i remember was i click is thing and suddenly asked me to install it and that was the PAV(personal anti-virus) so yeah i was so upset and i cant get rid of it. I browsed the and i found out it was a fake software… i tried to do the CTRL+ALT+DELETE thing and by the help of my computer antivirus and its gone. I deleted it, and im still worried they going to steal my identiy and steal my money in my account(waaaaahhhhh is that possible?)because i use my computer to check my accout and stuff… but i always log out after, i need help and im still worried like hell =(

June 20, 2009 at 3:10 pm
(35) Rawr!? says:

Trojan needs to Die. D:

June 29, 2009 at 3:46 am
(36) Lindsey says:

Tonight my computer was infected with “Trojan.Win32.Agent.azsy” what is that? It says it copies its body to the current user’s window startup directory and attempts to steal passwords. We have 385 threats. What should I do? Could I have gotten this from a website, because it all came up right after I had clicked on a Site about Farrah Faucets death. Our Pc has been running a little slow, so we had already thought that something was going on. So Please tell me what you think! I am scared and in need of desperate help. Thanks

September 7, 2009 at 8:30 am
(37) BlackCharizard says:

it also diguised as a game or something like that

September 7, 2009 at 8:32 am
(38) BlackCharmander says:

my antivirus is kaspersky that is a nice antivirus

February 22, 2010 at 6:32 am
(39) anthony says:

well i got 466 trojans on my computer says norton and i was wondering norton cannot remove them so what should i do i dont got nothing to hide but id hate for my computer to crash

April 15, 2010 at 5:57 am
(40) Travis says:

anthony: If you want to get rid of a trojan and etc. Get Malwarebytes, Link: malwarebytes.org

June 2, 2010 at 11:46 pm
(41) Chris says:

can a trojan be attached to your ip adress

June 18, 2010 at 4:53 pm
(42) Sunflower says:

I clicked on the Visa link to see what you were talking about, and Internet stopped working, I received a DEP notice, (several actually), and I read what that was since I had never received one before. The link was blocked – so I shut that tab.
I do not know if that was MSE, Trend Micro I.S., or one of the two antiviruses that I have, or even the firewall blocking the incoming.

I was reading this looking for info on Webroot – did I miss it?

July 20, 2010 at 2:29 am
(43) robert says:

Hey i recently got a trojan from a protected website. i did a complete scan of the computer and it said that the trojan was trying to block norton and windows defender and the computer started downloading an antimalware system to stop it, so i turned off the internet since i didnt aprove of it. for the trojan i had when i did the scan and it tried 2 shut down norton i clicked remove and the computer went back to normal for 5min and it tried to stop me from restoring it. but once it got restored (this all happened at 2am) it worked fine for another 24 hours ad then at the same time while i was on another secured connection website i got the trojan alert. if this helps neone by me saying this *make sure your norton has all checks or it will not work as well* run complete scans with the internet off and find a window. *the trojan i had must of had a hard time getting threw the piece of crap computer i have* but u will get a small window use that window to f11 your computer and restore it to the earliest restore point such as 2009 if its 2010 if u can. or whenever your computer was not invaded.

August 3, 2010 at 5:17 pm
(44) Mike says:

Mary, thank you for taking the time to set up a blog that educates people on internet security. However, it appears there is an instance of inconsistency and redundancy with your initial posting: In your second paragraph, you say “A Trojan does not replicate. Viruses do.” In the fourth paragraph, you say “Unlike viruses, a Trojan does not replicate (i.e. infect other files), nor does it make copies of itself as worms do.”

The terms “replicate” and “copy” are synonymous. Viruses do not replicate (copy) themselves onto other files; they require human interaction to infect a computer, most often by inadvertently running an infected executable file. There are what’s called Blended Threats which possess the most dangerous characteristics of viruses, worms, and trojan horses; I believe this is the term you were looking for in your initial posting – a virus that can copy itself like a worm can.

I believe you were accurate in your description of Trojan Horses, but I understand for the sake of length, you could not go into full detail. As you indicated, there are many more functions of a trojan horse.

Worms can infect a computer without any human interaction. They depend on file and information transport features in a computer to spread. When one worm infects your computer, it will replicate potentially into hundreds or thousands, and all will be sent out simultaneously. This will undoubtedly cause computers to freeze and networks to crash because so much data is going through at one time. A classic example of a worm is one that sends infected messages to everyone in your e-mail contacts list, then it infects the contacts list of each of your recipients, and the process inevitably continues at an exponential rate.

August 3, 2010 at 5:26 pm
(45) antivirus says:

Hi Mike,

I appreciate your desire for more clarity. However, it is untrue that viruses require human intervention or that worms do not. Also, replicate means to “do again” so is a perfectly good term to describe a file-infecting virus.

Few worms today create mass copies of themselves. Most use their propagation ability to spread a single copy to additional drives / shares. The goal is not to disrupt systems, but rather to steal data. With the exception of scareware, few malware cause any noticeable system crashes/freezes. Many actually improve performance in order to remain undetected.

September 8, 2010 at 11:12 am
(46) Aaron says:

I have a trojan/virus thing and at first it had 9 alerts and i Rebooted and then it went to 24 alerts. The only bad thing (to me) that happened was it did something with my World of Warcraft files, now it wont let me even get to the log-in screen. someone please help!

December 4, 2010 at 5:04 am
(47) Rose says:

I’m using AVAST and Facebook. I belong to several groups. I went to one of the groups and AVAST message came up as Trojan Horse blocked. Its NOT in the virus chest, and its ONLY on a facebook group. It did NOT affect my computer. Now I don’t know how to fix that in order to go to that group.

January 4, 2011 at 2:32 pm
(48) why me says:

i recently got the trojan virus. so then i asked a friend who i got a strange email from pharmacy from canada. well it turns out my friend got the same email then used her name and got sent to me so now my computer will not connect to internet since I unpluged it since it was going to bad websites automaticly,like every 5 minutes.i then when i went online it will not allow me to connect even though i am connected to the wifi. so i do belive that that this is a very dangerous virus and its hard to stop even with norton 360 and windows defender and it wont seperate the virus but it says its resolved. what will work?

January 11, 2011 at 2:51 pm
(49) Spencer says:

You should not install more than one anti-virus software as most use up a lot of CPU and RAM. Only install more than one if you have a high end dual core or better unless you are ok with lagging. Usually if one antivirus software does not work well, it is unlikely that two will. Just try and find a good, software that is known to prevent viruses. The best free one I found so far is COMODO.

March 1, 2011 at 5:37 am
(50) Claire says:

I got a Trojan virus last night that was probably lurking around my computer. It activated itself, or I did, dunno how, and began modifying my window files so I was unable to scan and get rid of it. When I realised this, a ‘system tool’ proection software popped up and asked me to get an activation code via the Internet, knowing that there was some bloke somewhere in the world going through my files with a comb to get my data and send it off to whoever knows who I just shut the whole thing down and disconnected the Internet for good measure. The Trojan even put a pretty new background on my screen with a big red warning and some stuff about spyware and family and friends at risk or something.

So yeah, it’s at the compute shop now, who has my permission to wipe the whole computer if need be. In just an hour it did that because I have never realised or been told how dangerous Trojans and viruses are. It should be taught in school!

If they did take some financial or password information should I change my passwords and notify the bank? Or is that not necessary? :)

Thats my story anyway. Bloody Trojans.

March 26, 2011 at 10:08 am
(51) Jay Smythe says:

I have a Trojans on my computer called “System Defender” and “Internet Defender” -look like MS Windows products, but are not.

Any advice on how to remove them? I’ve tried numerous programs, but none remove the trojans. Many thanks for any assistance.

May 30, 2011 at 12:46 am
(52) bob says:

i cant get rid of this virus any help for me?

July 3, 2011 at 2:01 am
(53) fairy says:

Trojan Horse is not the virus it does not affect your files

September 28, 2011 at 6:25 pm
(54) Frances says:

Thank you thank you thank you

October 26, 2011 at 12:49 am
(55) Chenna says:

My computer has the Trojan and Recycle Bin Virus. how should I remove it from my computer? Please, help me to solve this problem!!

January 14, 2012 at 3:07 pm
(56) James says:

To get rid of a trojan “note:works on most with internet access.” Boot into safe mode and download malewarebytes it is free by the way. Run a full scan after it is done it should ask you what you want done with it click remove and your trojan should be gone. Hope this helped.

February 13, 2012 at 10:49 am
(57) jane says:

tomorow i have to teach class on viruses . so say some few points on trojan horse virus.

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