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Writing Self-Modifying Code

Utilizing Advanced Assembly techniques

(by russell sanford)

Creating Alpha-Numeric shellcode

This article is online from 3885 days and has been seen 5884 times

(*** download for full text ***)

Writing Self-Modifying Code and
Utilizing Advanced Assembly techniques
Article 2: Advanced Filters, Creating
Alpha-Numeric shellcode
By: XORt aka Russell Sanford

- --INTRO-- -
Here we are again. I could not end this "project" having just said what I had covered in the first article. I felt it necessary to move onto this next and more advanced topic. In this article I'm going to show you how to conquer a rather tedious - and ALMOST impossible task: creating shellcode completely comprised of alphanumeric characters. "Why on earth would we want to do this?" you may be asking yourself right about now. The answer is simple.
There are several filtering schemes out there being employed by programs that ONLY allow alphanumeric characters to be passed into their buffers. Many programmers/hackers will tell you these are impossible to exploit. In a great deal of ways they are right, but for the most part they are wrong. Creating alphanumeric shellcode is time-consuming, annoying, and tiring. Most people simply give up due to the complexity of its creation and just assume it to be an impossible task. This is why there is little other documentation like this available to you out there on the net.
But, we will go onto cover this topic in thorough detail. By the end of this article you will not only be able to create your own code with little effort. But you will know exactly what obstacles your code will be facing and how to overcome them when possible. There is one last note I should add before beginning this article. Due to the complexity of this type of attack and the ratio of shellcode/original-shellcode, this type of attack will almost never work against a Windows host. The code covered in this article is intended only for a Linux box running under an IA32 Intel processor.
Ok, so lets get started.

[1] What Information We Need To Know Before beginning
[2] The Plan
[3] The Blueprint
[4] Code #1: Alpha-Numeric Shellcode with XOR
[5] Code #2: Alpha-Numeric Shellcode with IMUL
[6] The Code

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