Shellcoding for Linux and Windows Tutorial with example windows and linux shellcode.
Created - July 2. Updated Faq regarding stack randomization. In computer security, shellcoding in its most literal sense, means writing code that will return a remote shell when executed.
The meaning of shellcode has evolved, it now represents any byte code that will be inserted into an exploit to accomplish a desired task. There are tons of shellcode repositories all around the internet, why should I write my own? Yes, you are correct, there are tons of repositories all around the internet for shellcoding.
Namely, the metasploit project seems to be the best. Writing an exploit can be difficult, what happens when all of the prewritten blocks of code cease to work?
You need to write your own! Hopefully this tutorial will give you a good head start. What do I need to know before I begin? A decent understanding of x86 assembly, C, and knowledge of the Linux and Windows operating systems. What are the differences between windows shellcode and Linux shellcode? Linux, unlike windows, provides a direct way to interface with the kernel through the int 0x80 interface. A complete listing of the Linux syscall table can be found here.
Windows on the other hand, does not have a direct kernel interface. The system must be interfaced by loading the address of the function that needs to be executed from a DLL Dynamic Link Library. The key difference between the two is the fact that the address of the functions found in windows will vary from OS version to OS version while the int 0x80 syscall numbers will remain constant.
Windows programmers did this so that they could make any change needed to the kernel without any hassle; Linux on the contrary has fixed numbering system for all kernel level functions, and if they were to change, there would be a million angry programmers and a lot of broken code. So, what about windows? How do I find the addresses of my needed DLL functions?
Don't these addresses change with every service pack upgrade? There are multitudes of ways to find the addresses of the functions that you need to use in your shellcode.
There are two methods for addressing functions; you can find the desired function at runtime or use hard coded addresses. This tutorial will mostly discuss the hard coded method. The only DLL that is guaranteed to be mapped into the shellcode's address space is kernel This DLL will hold LoadLibrary and GetProcAddress, the two functions needed to obtain any functions address that can be mapped into the exploits process space.
There is a problem with this method though, the address offsets will change with every new release of Windows service packs, patches etc. Further dynamic addressing will be referenced at the end of the paper in the Further Reading section.
What's the hype with making sure the shellcode won't have any NULL bytes in it? Normal programs have lots of NULL bytes! Well this isn't a normal program! The main problem arises in the fact that when the exploit is inserted it will be a string.
If we have a NULL byte in our shellcode things won't work correctly. Well, in most shellcode the assembly contained within has some sort of self modifying qualities.
Useful Online Tools
Since we are working in protected mode operating systems the. That is why the shell program needs to copy itself to the stack before attempting execution. Sure, just email shanna uiuc. Feel free to ask questions, comments, or correct something that is wrong in this tutorial.
I don't know!
I am really sorry! Why does my program keep segfaulting?
You probably are using an operating system with randomized stack and address space and possibly a protection mechanism that prevents you from executing code on the stack. All Linux based operating systems are not the same, so I present a solution for Fedora that should adapt easily.
When testing shellcode, it is nice to just plop it into a program and let it run.
Shellcode linux pdf to text
The C program below will be used to test all of our code. The easiest way to begin would be to demonstrate the exit syscall due to it's simplicity. Here is some simple asm code to call exit. Now, run the program.
We have a successful piece of shellcode! One can strace the program to ensure that it is calling exit. Example 2 - Saying Hello For this next piece, let's ease our way into something useful. In this block of code one will find an example on how to load the address of a string in a piece of our code at runtime.
This is important because while running shellcode in an unknown environment, the address of the string will be unknown because the program is not running in its normal address space.
This code combines what we have been doing so far.
Shellcode - File Reader Linux x86
This code attempts to set root privileges if they are dropped and then spawns a shell. Well the only problem with that approach is the fact that system always drops privileges.
In order to write successful code, we first need to decide what functions we wish to use for this shellcode and then find their absolute addresses. For this example we just want a thread to sleep for an allotted amount of time.
Let's load up arwin found above and get started. Remember, the only module guaranteed to be mapped into the processes address space is kernel So for this example, Sleep seems to be the simplest function, accepting the amount of time the thread should suspend as its only argument.
Shellcodes database for study cases
When this code is inserted it will cause the parent thread to suspend for five seconds note: it will then probably crash because the stack is smashed at this point :-D. This second example is useful in the fact that it will show a shellcoder how to do several things within the bounds of windows shellcoding.
Although this example does nothing more than pop up a message box and say "hey", it demonstrates absolute addressing as well as the dynamic addressing using LoadLibrary and GetProcAddress.
The library functions we will be using are LoadLibraryA, GetProcAddress, MessageBoxA, and ExitProcess note: the A after the function name specifies we will be using a normal character set, as opposed to a W which would signify a wide character set; such as unicode. Let's load up arwin and find the addresses we need to use. We will not retrieve the address of MessageBoxA at this time, we will dynamically load that address.
This example, while not useful in the fact that it only pops up a message box, illustrates several important concepts when using windows shellcoding. Static addressing as used in most of the example above can be a powerful and easy way to whip up working shellcode within minutes. This example shows the process of ensuring that certain DLLs are loaded into a process space.
Once the address of the MessageBoxA function is obtained ExitProcess is called to make sure that the program ends without crashing. This third example is actually quite a bit simpler than the previous shellcode, but this code allows the exploiter to add a user to the remote system and give that user administrative privileges. This code does not require the loading of extra libraries into the process space because the only functions we will be using are WinExec and ExitProcess.
Note: the idea for this code was taken from the Metasploit project mentioned above. The difference between the shellcode is that this code is quite a bit smaller than its counterpart, and it can be made even smaller by removing the ExitProcess function! When this code is executed it will add a user to the system with the specified password, then adds that user to the local Administrators group. After that code is done executing, the parent process is exited by calling ExitProcess.
This section covers some more advanced topics in shellcoding. Over time I hope to add quite a bit more content here but for the time being I am very busy. If you have any specific requests for topics in this section, please do not hesitate to email me.
PDF Text/Character Recognition with Multiple Pages on Linux - OCR with gscan2pdf
The basis for this section is the fact that many Intrustion Detection Systems detect shellcode because of the non-printable characters that are common to all binary data. The IDS observes that a packet containts some binary data with for instance a NOP sled within this binary data and as a result may drop the packet. In addition to this, many programs filter input unless it is alpha-numeric. The motivation behind printable alpha-numeric shellcode should be quite obvious.
By increasing the size of our shellcode we can implement a method in which our entire shellcode block in in printable characters.
This section will differ a bit from the others presented in this paper. This section will simply demonstrate the tactic with small examples without an all encompassing final example. Our first discussion starts with obfuscating the ever blatant NOP sled. This is due to the fact that whenever we use a register in our shellcode we wither move a value into it or we xor it. Incrementing or decrementing the register before our code executes will not change the desired operation.
So, the next portion of this printable shellcode section will discuss a method for making one's entire block of shellcode alpha-numeric-- by means of some major tomfoolery.
We must first discuss the few opcodes that fall in the printable ascii range 0x33 through 0x7e. Surprisingly, we can actually do whatever we want with these instructions. Below you can find a diagram of the basic plan for constructing the shellcode. The plan works as follows: -make space on stack for shellcode and loader -execute loader code to construct shellcode -use a NOP bridge to ensure that there aren't any extraneous bytes that will crash our code.
Settle down, have I got a solution for you! Now you're wondering why I said subtract to put values into EAX, the problem is we can't use add, and we can't directly assign nonprintable bytes.