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# Writing your own high-level control kit

Written by: ShellExecute

Some of you are probably spending your days in school, sitting by computer and finding bugs just for fun. The next step for this whould be for you to take advantage of a bug to gain a higher level of access to the network.

This article will act as an introduction for writing useful trojans, backdoors or rootkits for, say, a school network. Sure, you can download Bifrost, nRAT or one of the many public & popular trojans out there. But they're designed to work on as many systems as possible, and therefore they can't dig as deep as a custom coded trojan can. And, because they're public, signature based anti-virus programs detects them as simple as an elephant in a kitchen.

The downside of a costum-coded trojan is of course that it's usually made to fit a particular system and therefore not very portable. But this is as I already explained earlier a good thing, too.

You don't have to know any programming to understand this article, but it's recommended if you want to test the examples.

OK, here we go.

## The story of Nantes

Nantes is a basic downloader, featuring a download function and cmd.exe command execution function. The only problem with it is this: it's not made yet. So, in order to create Nantes we should look at some basic things:

- Program type
- Infestation of host
- Functions
- Communication protocol

## Program type

The first thing we should look at is what kind of program we want. Do we want a costumized trojan or do we want a keylogger with an upload function? Do we want both? In order to find out what we want, we have to find out what we need. Since networking is such a great deal in the world of today we should probably have some sort of remote control of the program.

Remote control is often made possible by two forms of networking, one being the reverse connect form where the program connects to a server program that steers it. In the other form, called direct connect, a client program connects to the server program, that runs on the computer of the victim. These days that's very unusual, since firewalls block any attempts to reach the listening port.

However there's a third form of connection, and I really don't know what to call it, but it uses a third-party computer to relay control information. Since this form of connection doesn't involve direct contact between the attacker and the victim, it can be made very safe and hard to track even without the use of proxys.

(How about 3-way UDP handshaking, like Skype does? --ed.)

The downside with this type of third-party connection is the fact that it's hard to get it working without delay. But for programs such as keyloggers or downloaders this is used often with good results. The third-party server can be a SMTP-relay server, a web server or a FTP server.

More about this in the section "communication protocol".

So, we've already decided to make a downloader with some control capability over the host. Therefore the type of third-party connection described above is a very good choice.

## Infestation of the host

This part is kinda tricky at the start, because in a large network, the workstations are often very restricted when it comes to a regular user trying to change values in the registry.

Of course the registry isn't the only place where you can make your program start automaticly, but it's without doubt the leanest way. Other ways of making a file start automatically is by appending it to a file used often, for example iexplore.exe. You can also add it to the Autostart folder or start it at a certain time using at.

But let's start with the registry. The most common way of making the program start at boot-up is by adding a value in the "HKLM\Software\Microsoft\Windows\CurrentVersion\Run" path. This can be done through system() or ShellExecute calling reg.exe with the needed values, or through an API call (three, to be precise). RegCreateKeyEx(), RegSetValueEx() and last but not least, RegCloseKey().

If we don't have the permission to do this, we could inject it into an already running process. But since this is a basic tutorial, you have to learn that from someone else. I recommend Phrack issue 62. RIP.

If your school have programs running in the "All users" Autostart folder and you've got rwx permission to it, then you could make a .lnk camouflaged as that particular program in the same folder.

## Functions

OK, we've already decided to have a binary download function and a cmd.exe command execution function. But this is not everything we can have. If we want to use our network for something nice, we probably want a function for that. Lets say we make a function for something as basic as a net send storm function that uses mailslots to spoof the senders name, or a spam function that functions as a SMTP relay server with a reverse connection.

The possibilities are by the restrictions of the system. I take for granted that you've already aquired enough access to install your trojan. Of course, we can make these functions into small programs that we fetch with the already existing binary download function.

One important feature that also should be included in the main .exe file is the self-kill function. There's many neat killfunctions already out there, so we will use a pre-written version, made by someone else, for Nantes.

## Communication protocol

Without the ability to communicate with the world, our program will be pretty much worthless. But you might want to consider if you really need networking capability for your application.

Nantes will use the FTP protocol for communicating. It won't be RFC compliant but it will have the basic calls. Actually, I think we settle for the USER, PASS, TYPE, PASV, RETR and QUIT commands. Now is a good time for you to read RFC959 if you don't know what I'm talking about.

## The Program

Now that we have the basic behavior of Nantes specified it's time to get busy. I like to define my program's behavior in text before I code it rather than making a flowchart, but that's very individual. I agree on the fact that a flowchart is easier to look at.

## Start

Nantes starts from a file located in the Autostart folder, maybe made to look like Microsoft Office or something. The program doesn't spawn a window, if it's a console app it uses FreeConsole().

## Infestation

Nantes look for itself in the Autostart folder, if it's missing then Nantes creates it. It also checks for a copy of itself in another folder. If that copy doesn't exist Nantes tries to copy itself to that folder and adding a value to the at service to start the copy on a time when the computer is likely to be turned on.

## Receiving orders

Nantes calls a function that connects to the FTP server sending the USER and PASS, TYPE I PASV and RETR commands and then receives the predefined global and local orderfiles. When the files have been received, Nantes sends the QUIT command.

## Interpreting orders

The magic interpreter function is called and directed to interpret the orderfiles and then carry out the orders or call other functions to do that.

## Cleaning up

Removes the temporary files created.

## Shutting down

Yea, shutting down.

No threading will be needed. We could use the "receiving orders" function to receive binary files aswell and using the same code more than once.

We could also make the Nantes code a bit object-oriented by using classes to handle things as sockets. However this will make the code harder to understand for the untrained eye, and because this is a beginner's tutorial I will skip that. The code is pretty thin as it is, classes only make a difference in larger projects.

The language of choice is C++ (you probably figured that out already), since it's my langue maternelle. The program will be of console type, mainly because of the fact that this is a tutorial.

All the addresses and the things that vary from network to network will be hard-coded, and we will use a lot of global variables. Sorry.

Since the password will be stored in a vector and sent unencrypted it's not so wise to use your home computer's FTP server with the account having full rwx permission to your hard-drive. Use your ex-girlfriend's trojan-infested workstation as the orderhost instead.

The order language will be pretty simple and will only contain three phrases:

    !C "the cmd.exe command streching over many words but only one line"

    !B binary_file_name_on_server.exe c:\path\on\HD\filename.exe!

    !K

The !K phrase is without any arguments and simply deletes the main .exe file. This and the use of a global orderfile will make the removal of the evil network very quick if someone is on to you.

The !B ends with a !.

You can use the phrases how you want, but they can't be longer than a line.

Ex.

    !B msiexec16.exe c:\windows\temp\msiexec16.exe!

    !C c:\windows\temp\msiexec16.exe

    !C "msg * muahahahahaha"

    !K

Is a a good way to use the program (in terms of the fact that it works) while

    !B msiexec16.exe     c:\windows\temp\msiexec16.exe!

    !B msiexec16.exe c:\windows\temp\msiexec16.exe

    !B msiexec16.exe

    c:\windows\temp\msiexec16.exe!

    !b msiexec16.exe c:\windows\temp\msiexec16.exe!

Won't work at all. You can of course modify the sourcecode and make it work, but that's up to you.

## The Source code

The source code is here.

This is one of the many uses for our program. Small programs carrying a payload with one single purpose decided by you. Of course you can use a downloader as a backdoor since its pretty stealthy (well, not Nantes but the priciple is the same). It's up to you.

Copyright 2005, EuroHacker Magazine