The Linux Virus Writing HOWTO
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# 1. Introduction

| You cannot have a science without measurement.
| R. W. Hamming

Writing a program that inserts code into another program file is one thing. Writing that program so that it can be injected itself is a very different art. Although this document shows a lot of code and technique, it is far from being a "Construction Kit For Dummies". You can't build a working virus just by copying whole lines from this text. Instead I'll try to show how things work. Translation of infecting code to a assembly is left as a (non-trivial) exercise to the reader.

An astonishing number of people think that viruses require secret black magic. Here you will find simple code that patches other executables. It is not hard to write a virus (once you have a good understanding of assembler, compiler, linker and operating system). It's just hard to let it make any impact.

Regular users can't overwrite system files (at least under serious operating systems). So you need root permissions. You can either trick the super user to run your virus. Or combine it with a root-exploit. But since all popular distributions come with checksum mechanisms, a single command can detect any modification. Unless you implement kernel-level stealth functionality…

I do believe that free software is superior, at least in regard to security. And I strongly oppose the argument that Linux viruses will flourish once it reaches a critical mass of popularity. On the contrary I question the credibility of people whose income relies on widespread use of ridiculously insecure operating systems.

This document is my way to fight the FUD. Use the information presented here in any way you like. I bet that Linux will only grow stronger.

## 1.1. Behind the stages

All sections titled "Output" are real product of source code and shell scripts included in this document. Most numbers and calculations are processed by a `Perl` script parsing these output files. The document itself is written in DocBook, a SGML document type definition. Conversion to HTML is the last step of a Makefile that builds and runs all examples.

I used an installation of RedHat 7.2 for development All required tools are contained on the freely downloadable CDs.

This document is not yet officially accepted by LDP. You might want to look at the discussion on LDP-discuss. For the time being you will find complete source of this release and all previous versions here.

## 1.2. Copyright & trademarks

Copyright (c) 2002 Alexander Bartolich

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts.

All accompanying source code, build scripts and makefiles is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

All copyrights are held by their respective owners, unless specifically noted otherwise. Use of a term in this document should not be regarded as affecting the validity of any trademark or service mark.

Naming of particular products or brands should not be seen as endorsements.

## 1.3. Disclaimer

No liability for the contents of this documents can be accepted. Use the concepts, examples and other content at your own risk. As this is a new edition of this document, there may be errors and inaccuracies, that may of course be damaging to your system. Proceed with caution, the author does not take any responsibility.

All software is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose. See the GNU General Public License for more details.

Who am I kidding? This is dangerous stuff! Stop reading immediately or risk lethal pollution of your systems!

You are strongly recommended to take a backup of your system before major installation and backups at regular intervals.

## 1.4. Credits

Everything in this document is either plain obvious or has been written by someone else long time ago. My meager contribution is nice formatting, reproducibility and the idea to take the subject to mainstream media. But I'm certainly not innovative.

Silvio Cesare. `<silvio@big.net.au>` Founder of the trade. Keeper of the source. Check out http://www.big.net.au/~silvio and admire the release date.

John Reiser. `<jreiser@BitWagon.com>` Found one bug and two superfluous bytes in In the language of evil. Proved that I can't code a straight 23 byte "Hello World".

paddingx. Contributed technical details and implementation for Additional code segments and Doing it in C.

A lot of people helped me shape language and ethical position of this document (sorted by perl(1), blame Larry): Charles Curley, Dave Wreski, David Merrill, Gary Lawrence Murphy, Greg Ferguson, Harald Wagener, Ian Turner, Marinho Paiva Duarte, Martin Wheeler, QuickFox of kuro5hin, Steve Sanfratello.

## 1.5. Feedback

Feedback is most certainly welcome for this document. Please send your additions, comments and criticisms to the following email address: `<alexander.bartolich@gmx.at>`

A few people sent me executables for that other operating system. I am very grateful for this kindness. But you should know that this document is about Linux only.

German speaking readers might find this exchange of emails between Network Associates, Inc. and the admin of my site interesting.

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