Introduction To SHELLS And How It Works For Web Hacking

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I am sure many have you have read about “Hacked / Defaced with shells”, So I am pretty sure that the first thing that comes to your mind is “What the heck are these shells?” . So this article would give you complete idea about shells and its use.


Difference between Bind Shell and Reverse Shell


Let us see the basic differences between a bind shell and a reverse shell..

What is a Shell ?



A shell is a software that acts as a intermediary between user and the kernel. It provides the user an interface which provides access to the services of kernel.

Eg : Bash shell etc..

Code:
+-----------------+               _______________           +----------------+
| Aneesh | Behind NAT / / | Shabbir |
| With Private ip | ----> ----> / Internet /----> ----> | with Public IP |
+-----------------+ /______________/ +----------------+

Ok.. So in this scenario.. Aneesh has a computer connected to the internet with a private ip..(no hosting) while Shabbir is connected to the internet with a Public IP (Hosted)..It basically means Shabbir’s system can be accessed by any one connected on the internet but this does’nt go for Aneesh.. Aneesh’s system being behing the NAT cannot be directly connected by other Machines on the internet..

Bind Shell

Lets suppose Shabbir has encountered some problem with his system and need some help from Aneesh.. He simply binds his shell (cmd.exe or /bin/bash) to a specific port and sends Aneesh its port no and other details.. In this scenario Aneesh can simply connect to the Shabbir’s Machine and Get the Shell!!So in this case :-

Aneesh’s End :-Connect to shabbir (Acts as a client)


Shabbir’s End :-Listen for connections (listen / act as a server and bind his command shell on the network..)

Reverse Shell
Now lets suppose after some days Aneesh screwed up his system and now he asks Shabbir for his help..But in this case the bind shell cannot be used as ‘Aneesh’ doesn’t have a Public IP and his system is not available publicly!! Now to conquer this problem. Aneesh sends his command prompt to Shabbir.. So , in this case :-

Aneesh’s End :-Would bind his shell and send it to Shabbir through the network..(Connect)


Shabbir’s End :-Listen for connections , Respond to them (listen / act as a server)

Difference between FTP & Shells:

Many times I see that some of us know how to use the shell but once they have uploaded they get confused. So to start with, Let me give you some information about FTP:

File Transfer Protocol

Whenever you want to open your website, the first thing you will do is to get some web hosting for your self. That cud be either free or paid. When your get your hosting services, you create a website on your computer first and then upload it to your hosting server so it becomes a World Wide Web. This process of uploading the documents from your computer to your hosting server is done through FTP [File Transfer Protocol]. It basically looks like a program with 2 columns, one column shows your computer files and another shows your servers files. Just like when you copy the stuffs from some USB drive to your computer. So here, I will show you an example is how you would connect if you own go4expert. So when you want to connect your self to your web hosting server, following information is required in order to authenticate yourself:

Server : ftp.go4expert.com
Username: shabbir
password: whatever

So, once you put in this information, server understands that you are shabbir and gives you access to all the files on the server so you can work on it.

Shells:

Since you understand the FTP now, we know that none of us will get access to Go4expert’s server because we don’t have the username and password authenticate yourself. Somehow we can manage to get the access to G4E’s FTP we can easily remove/edit/replace files. So we can destroy this entire forum and upload our own stuffs. That is when shells comes into the picture. Shells are a malicious PHP files which you will need to upload to any website, and once you execute it you will get access to its server directly WITHOUT authenticating your self.

Moral of the Story:



I wrote the difference between FTP and shells so that you guyz can understand it, because lots of people tends to get confused between them. So again to make it clear, you can following thing:

Quote:
FTP is a protocol that lets you connect your computer to your hosting server so that you can upload/edit/delete/replace your files. Since we wouldn’t have the username & password to connect to any website’s ftp, thats why we will use the SHELL to get access. SO SHELL IS NOT FTP BUT IT GIVES YOU ACCESS TO THE HOSTING SERVER.

Funny Incidents:

Let me tell you guyz why i gave time to write this much about FTP in this article. In my forum, i reelased a couple of videos about “hacking with shells” etc .etc. So some guyz saw it and just copied what I was doing without understanding the logic.

I remember i saw couple of threads which said following thing:
” Hi guyz, i managed to hack my 1st website today! YAY, I am really happy! But theres only 1 problem, i uploaded the shell and ran it and it worked fine. The only problem is i dont have access to FTP.”

Y0, i hacked a website today, uploaded a shell and it worked fine, now i am trying to get access to FTP 

Logic



Shell is not a tool that you can run and complete your work. As I said, its just a normal “.php” file, you have to find a way in any website to upload that shell. The Idea is, you upload the shell to any website so it will be saved on their server and it will give you the access to it.

Phase 1 : Uploading a shell:

Suppose you want to hack “something.com”. So the first thing that you will do is, open up “something.com”, and try to find some place from where you can upload the files on the website. There are many such places for example, “file uploads, avatars, resume upload, cooking recipe uploads, upload your photo”. So these are the places which will give you an opportunity to upload your shell. All you have to do is, try to upload the shell.php which is located in your computer and click on submit. So suppose you went to the webpage “something.com/submit_resume.php” and you uploaded your resume.

Phase 2 : Executing your uploaded shelll:

Once we have uploaded the shell as shown in “Phase:1”, we know that its sitting on the server. The only thing we need to do now is to execute the shell from a browser so we get access to it.

Example:

So suppose i uploaded my shell as an attachment in THIS THREAD. SO now taht attachment is sitting on G4E’s server. Now if we want to executive it, we will use following URL:

Code:
http://www.go4expert.com/attachment.php?attachmentid=456&d=1249607339

So that is the DIRECT url to the attachment which is called EXECUTION. In the same way if you execute your shell, it will take you to a webpage where you will see everything thats on the server. And you will have FULL ACCESS to remove/edit/replace/delete the files. So you are another shabbir!

Phase 3 : Defacing:

Defacign is a word which means “replacing the current index file with our own index with our motive and slogan on it”. So once you have access to the server, you are the king.

Different types of shells:

There are many shells available, most of them are public and some of them are private. Most of them does the samething to give you the access of the server. “c99, r57, b0yzone, j32” are some very common and easily available shells.

Where do I get them from?:

I’d have uploaded them here, but then it might have marked G4E as “Harmful web” on Google. So the best way is Google search with “inurl:c99.txt”. You can replace c99 with r57, j32 or anything else.

Conclusion:



Now that you guyz know what are shells and how it works, i will start covering other method in coming articles. I will soon write about “RFI, LFI” which are somewhat connected with shells. Meanwhile, keep playing with it and learn more.

Important Piece of advice:

I would suggest you to download WAMP SERVER, which lets you make your own server on your comptuer. And then try to use shells on it. Which will help you avoid hacking in live environment. Because, if webmaster is smart then, he can simply check the logs for that shell fine and track down your IP which executed the shell. Then you might be in problem.




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