I'm going to show one technique I learned for writing basic shellcode. The shellcode in this example I will be writing shellcode for x86 (if you don't know what that is then it's your first sign you will struggle with shellcode) processors running *nix OS's. Don't just copy what i've written here... create your own ASM code and play about with it. Also when learning ASM, if your a beginner it is VERY important to find a syntax that you are comfortable in. If your comfy in masm or intel then do the same in windows, although this tutorial will not help you. Don't be put off learning ASM in windows if you prefer. I prefer coding in linux so GNU ASM was a natural choice and I enjoy it.

This shellcode will basically exit the program... simple but demonstrates the method of creating shellcode.

First we create our ASM program that does nothing else but exit.



The above is said code I wrote in GNU ASM. You can use intel syntax, nasm, masm or any other syntax. This only works with linux systems due to the syntax I've used.

We then assemble and link the file:

as exit.s -o exit.o

ld exit.o -o exit

We then run the program, "./exit" and the program will immediately exit. We use objdump to get the relevant machine code we need to create the shellcode.

objdump -d exit

Output:


We encorporate the machine code (centre column above) into a C program to produce and check our shellcode. Of course remembering \x as a prefix.



Compile:

gcc -o exit_shellcode exit_shellcode.c

We then trace the progression through the program and we see that the program exit's using the exit syscall. To view this use:

strace ./exit_shellcode

At the bottom of your output you will see exit(0) ? or exit_group(0) ?

Congrats you've made your first piece of shellcode :)