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Computer Trojan Horses / written by, R a v e N & tHe mAnIac
(blacksun.box.sk)

version 1.0, 30/11/1999
Converted to HTML by Penguin

Contents
1. What is a trojan?
2. Why the name 'trojan horse'?
3. Remote Administration Trojans
4. How RATs work
5. Using RATs for legitimate purposes
6. Password Trojans
7. Priviledges-Elevating Trojans
8. Keyloggers
9. Destructive Trojans
10. Joke Programs
11. Protecting Yourself Against Trojans
    - Under Unix
    - Under Windows

What is a trojan?
A trojan horse could be either:
a) Unauthorized instructions contained within a legitimate program. These instrcutions perform functions unknown to (and probably unwanted by) the user.
b) A legitimate program that has been altered by the placement of anauthorized instructions within it. These instructions perform functions unknown to (and probably unwanted by) the user.
c) Any program that appears to perform a desirable and necessary function but that (because of unauthorized instructions within it) performs functions unknown to (and probably unwanted by) the user.
Under a restricted environment (a restricted Unix shell or a restricted Windows computer), malicious trojans can't do much, since they are restricted in their actions. But on a home PC, trojans can be lethal and quite destructive.
Why the name 'trojan horse'?
In the 12th century B.C., Greece declared war on the city of Troy. The dispute erupted when the prince of Troy abducted the queen of Sparta and declared that he wanted to make her his wife, which made the Greeks and especially the queen of Sparta quite furious.
The Greeks gave chase and engaged Troy in a 10-year war, but unfortunately for them, all of their efforts went down the drain. Troy was simply too well fortified.
In a last effort, the Greek army pretended to be retreating, leaving behind a hude wooden horse. The people of Troy saw the horse, and, thinking it was some kind of a present from the Greeks, pulled the horse into their city, without knowing that the finest soldiers of Greece were sitting inside it, since the horse was hollow.
Under the cover of night, the soldiers snuck out and opened the gates of the city, and later, together with the rest of the army, killed the entire army of Troy.
This is why such a program is called a trojan horse - it pretends to do something while it does something completely different, or does what it is supposed to be and hides it's malicious actions from the user's prying eyes.
During the rest of this text, we will explain about the most common types of trojan horses.
Remote Administration Trojans
These trojans are the most popular trojans now. Everyone wants to have them trojan because they let you have access to your victim's hard drive, and also perform many functions on his computer (open and close his CD-ROM drive, put message boxes on his computer etc'), which will scare off most computer users and are also a hell lot of fun to run on your friends or enemies.
Modern RAT'S (remote administration trojans) are very simple to use. They come packaged with two files - the server file and the client file (if you don't know which is which, look for a help file, a FAQ, a readme or instructions on the trojan's homepage). Just fool someone into runnig the server file and get his IP and you have FULL control over his/her computer (some trojans are limited by their functions, but more functions also mean larger server files. Some trojans are merely ment for the attacker to use them to upload another trojan to his target's computer and run it, hence they take very little disk space). You can also bind trojans into other programs which appear to be legitimate.
RAT'S have the common remote access trojan functions like:
keylogging (logging the target's keystrokes (keyboard functions) and sometimes even interfering with them, thus being able to use your keyboard to type instead of the target and say weird things in chatrooms or scare the hell out of people), upload and download function, make a screenshot of the target's monitor and so on.
Some people use the trojans for malicious purposes. They either use them to irritate, scare or harm their enemies, scare the hell out of their friends or enemies and seem like a "super hacker" to them, getting information about people and spying on them or just get into people's computers and delete stuff. This is considered very lame.
There are many programs out there that detects the most common trojans (such as Nemesis at blacksun.box.sk, which also detects people trying to access your computer), but new trojans are released every day and it's pretty hard to keep track of things.
Trojans would usually want to automatically start whenever you boot-up your computer. If you use Windows, you can get b00tm0n from blacksun.box.sk (note: at the time this tutrial was released, b00tm0n was not ready yet, but it should be ready some time before year 2,000, so if you're reading this after Y2K, b00tm0n should probably be available at blacksun.box.sk). Under Unix, we suggest getting some sort of an IDS (Intrusion Detection System) programs to monitor your system.
Most Windows trojans hide from the Alt+Ctrl+Del menu (we havn't seen any Unix program that had the ability to hide itself from the processes list yet, but you can never know - one day someone might discover a way to do so. Hell, someone might have already did). This is bad because there are people who use the task list to see which process are running. There are programs that will tell me you exactly what processes are running on your computer (such as Wintop, which is the Windows version of the popular Unix program called top). Some trojans, however, use fake names and it's a little harder for certain people to realize that they are infected.
Also, some trojans might simply open an FTP server on your computer (usually NOT on port 21, the default FTP port, in order to be less noticable). The FTP server is, of course, unpassworded, or has a password which the attacker has determined, and allows the attacker to download, upload and execute files quickly and easily. For more info about FTP servers and FTP security, read our FTP security tutorial at blacksun.box.sk.
How RATs work
Remote administration trojans open a port on your computer and bind themselves to it (make the server file listen to incoming connections and data going through these ports). Then, once someone runs his client program and enters the victim's IP, the trojan starts receiving commands from the attacker and runs them on the victim's computer.
Some trojans let you change this port into any other port and also put a password so only the person that infect this specific computer will be able to use the trojan. However, some of these password protections can be cracked due to bugs in the trojan (people who program RATs usually don't have much knowledge in the field of programming), and in some cases the creator of the trojan would also put a backdoor (which can be sometimes detected, under certain conditions) within the server file itself so he'll be able to access any computer running his trojan without the need to enter a password. This is called "a backdoor within a backdoor".
The most popular RATs are Netbus (because of it's simplicity), BO (has many functions and hides itself pretty good) and Sub7 (lots of functions and easy to use). These are all Windows RATs.
If you havn't done so already, it is advised to get some RAT and play around with it, just to see how the whole thing works. Using RATs for legitimate purposes
Some people use RATs to remotely administer computers they are allowed to have access to. This is all good and fine, but anyway, you should always be careful while working with RATs. Make sure you have legal access and the right to remotely administer a computer before using a RAT on it.
Password Trojans
Yes, password trojans. Password trojans scour your computer for password and then send them to the attacker or the author of the trojan. Whether it's your Internet password, your Hotmail password, your ICQ password or your IRC passwords, there is a trojan for every passsword. These trojans usually send the information back to the attacker via Email.
Priviledges-Elevating Trojans
These trojans would usually be used to fool system administrators. They can either be binded into a common system utility or pretend to be something unharmful and even quite useful and appealing. Once the administrator runs it, the trojan will give the attacker more priviledges on the system. These trojans can also be sent to less-priviledges users and give the attacker access to their account.
Keyloggers
These trojans are very simple. They log all of your keystrokes (including passwords), and then either save them on a file or Email them to the attacker once in a while.
Keyloggers usually don't take much disk space and can masquerade as important utilities, thus making them very hard to detect. Some keyloggers can also highlight passwords found in text boxes with titles such as 'enter password' or just the word password somewhere within the title text.
Destructive Trojans
These little fellows do nothing but damaging your computer. These trojans can destroy your entire hard drive, encrypt or just scramble important files and basically make you feel very unpleasent. I wouldn't want to bump into one in a dark alley.
Some might seem like joke programs, while they are actually tearing every file they encounter to pieces.
Joke Programs
Joke programs are nice, cute and unharmful. They can either pretend to be formatting your hard drive, sending all of your passwords to some evil cracker, self-destructing your computer, turning in all information about illegal and pirated software you might have on your computer to the FBI etc'. They are certainly no reason to worry about (except if you work in tech support, since unexperienced computer users tend to get scared off pretty easily by joke programs.
Protecting Yourself Against Trojans
Under Unix
If you are working on your PC, DO NOT work as root! If you run a trojan as root, you can endanger your entire system! The whole point in multi-users on a single-user system is limiting yourself in such cases (or in case you want to prevent yourself from doing anything stupid). Switch to root only when you NEED root, and when you know what you're running. Also, remember that even if you're working on a restricted environment, you still put the passwords and files you still have access to to risk. Also, if someone has a keylogger on your system, and you type in some passwords (especially the root password), they will be logged!
Also, DO NOT download any files from untrusted sources (small websites, underground websites, Usenet newsgroups, IRC etc'), even if it comes in the form of source code.
Under Windows
Windows is a whole lot different in this aspect. Limiting yourself under Windows is quite an annoyance. It is almost impossible to work like that, in comparison to Unix.
Also, make sure you don't run any untrusted software. There are much more evil Windows trojans for Windows than Unix, since people are more motivated to write trojans for Unix (because of all the security Unix imposes). Also, when running on a restricted Windows environment, you cannot just act like you're so protected and all. Remember that people can still steal passwords owned by the restricted user, and also, some trojans can break into administrator priviledges and then compromise your entire system, since Windows imposes such lame security.
Oh, and one last tip - you should try to download and use at least some of the types of trojans listed above, so you could get to know them better and be able to remove them in case you get infected.