All files are for educational and/or historic purposes only. [back to library]

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DISCLAIMER: The author hereby disclaims himself
DEDICATION: This was written to make the lives
  of scum such as Patty Hoffman, John McAffee,
  and Ross Greenberg a living hell.
OTHER STUFF:  Thanks go to The Shade of Sorrow,
  Demogorgon, and Orion Rouge on their comments
  (which I occasionally listened to!).   Thanks
  also to Hellraiser, who gave me an example of
  some virus source code (his own, of course).

Dark Angel's Phunky Virus Writing Guide
---- ------- ------ ----- ------- -----
Virii are  wondrous creations written for the sole purpose of spreading and
destroying the  systems of unsuspecting fools.  This eliminates the systems
of simpletons  who can't  tell that there is a problem when a 100 byte file
suddenly blossoms  into a  1,000 byte  file.   Duh.  These low-lifes do not
deserve to  exist, so  it is  our sacred duty to wipe their hard drives off
the face of the Earth.  It is a simple matter of speeding along survival of
the fittest.

Why did  I create  this guide?  After writing several virii, I have noticed
that virus  writers generally  learn how to write virii either on their own
or by  examining the  disassembled code  of  other  virii.    There  is  an
incredible lack  of information  on the  subject.   Even books published by
morons such as Burger are, at best, sketchy on how to create a virus.  This
guide will show you what it takes to write a virus and also will give you a
plethora of source code to include in your own virii.

Virus writing  is not  as hard  as you  might first  imagine.   To write an
effective virus,  however, you  *must*  know  assembly  language.    Short,
compact code  are hallmarks  of assembly  language and  these are desirable
characteristics of  virii.  However, it is *not* necessary to write in pure
assembly.   C may  also be  used, as  it allows almost total control of the
system while  generating relatively compact code (if you stay away from the
library functions).   However,  you still  must access  the interrupts,  so
assembly knowledge  is still  required.  However, it is still best to stick
with pure  assembly,  since  most  operations  are  more  easily  coded  in
assembly.  If you do not know assembly, I would recommend picking up a copy
of The Microsoft Macro Assembler Bible (Nabajyoti Barkakati, ISBN #: 0-672-
22659-6).   It is an easy-to-follow book covering assembly in great detail.
Also get yourself a copy of Undocumented DOS (Schulman, et al, ISBN #0-201-
57064-5), as it is very helpful.

The question  of which  compiler to  use arises  often.   I  suggest  using
Borland Turbo  Assembler and/or  Borland C++.   I  do not  have a  copy  of
Zortech C  (it was  too large  to download), but I would suspect that it is
also a good choice.  Stay away from Microsoft compilers, as they are not as
flexible nor as efficient as those of other vendors.

A few more items round out the list of tools helpful in constructing virii.
The latest version of Norton Utilities is one of the most powerful programs
available, and  is immeasurably  helpful.   MAKE SURE YOU HAVE A COPY!  You
can find  it on  any decent board.  It can be used during every step of the
process, from  the writing  to the testing.  A good debugger helps.  Memory
management  utilities   such  as   MAPMEM,  PMAP,   and  MARK/RELEASE,  are
invaluable, especially  when coding  TSR virii.   Sourcer,  the  commenting
disassembler, is  useful when  you wish  to examine the code of other virii
(this is a good place to get ideas/techniques for your virus).

Now that  you have  your tools,  you are  ready to  create a  work  of  art
designed to smash the systems of cretins.  There are three types of virii:

     1) Tiny virii (under 500 bytes) which are designed to be  undetectable
        due to their small size.   TINY  is  one  such  virus.    They  are
        generally very simple because their code length is so limited.
     2) Large  virii  (over 1,500 bytes)   which   are   designed   to   be
        undetectable because they cover their tracks very  well  (all  that
        code DOES have a use!).  The best example  of  this  is  the  Whale
        virus, which is perhaps the best 'Stealth' virus in existence.
     3) Other virii which are not designed to be hidden at all (the writers
        don't give  a  shit).    The  common  virus  is  like  this.    All
        overwriting virii are in this category.

You must  decide which  kind of  virus you wish to write.  I will mostly be
discussing  the  second  type  (Stealth  virii).    However,  many  of  the
techniques discribed  may be easily applied to the first type (tiny virii).
However, tiny  virii generally do not have many of the "features" of larger
virii, such  as  directory  traversal.    The  third  type  is  more  of  a
replicating trojan-type,  and will  warrant a  brief  (very,  very  brief!)
discussion later.

A virus may be divided into three parts: the replicator, the concealer, and
the bomb.   The  replicator part  controls the spread of the virus to other
files, the concealer keeps the virus from being detected, and the bomb only
executes when  the activation  conditions of the virus (more on that later)
are satisfied.

The job  of the  replicator is to spread the virus throughout the system of
the clod  who has caught the virus.  How does it do this without destroying
the file it infects?  The easiest type of replicator infects COM files.  It
first saves  the first  few bytes  of the  infected file.  It then copies a
small portion of its code to the beginning of the file, and the rest to the

  +----------------+      +------------+
  | P1 | P2        |      | V1 | V2    |
  +----------------+      +------------+
 The uninfected file     The virus code

In the  diagram, P1 is part 1 of the file, P2 is part 2 of the file, and V1
and V2  are parts 1 and 2 of the virus.  Note that the size of P1 should be
the same  as the size of V1, but the size of P2 doesn't necessarily have to
be the  same size  as V2.   The  virus first  saves P1 and copies it to the
either 1)  the end  of the  file or 2) inside the code of the virus.  Let's
assume it copies the code to the end of the file.  The file now looks like:

  | P1 | P2        | P1 |

Then, the  virus copies  the first  part of  itself to the beginning of the

  | V1 | P2        | P1 |

Finally, the virus copies the second part of itself to the end of the file.
The final, infected file looks like this:

  | V1 | P2        | P1 | V2    |

The question  is: What  the fuck  do V1 and V2 do?  V1 transfers control of
the program to V2.  The code to do this is simple.

     JMP FAR PTR Duh       ; Takes four bytes
Duh  DW  V2_Start          ; Takes two bytes

Duh is  a far pointer (Segment:Offset) pointing to the first instruction of
V2.   Note that  the value  of Duh must be changed to reflect the length of
the file  that is  infected.   For example,  if the  original size  of  the
program is  79 bytes,  Duh must  be changed  so  that  the  instruction  at
CS:[155h] is  executed.   The value of Duh is obtained by adding the length
of V1,  the original size of the infected file, and 256 (to account for the
PSP).  In this case, V1 = 6 and P1 + P2 = 79, so 6 + 79 + 256 = 341 decimal
(155 hex).

An alternate, albeit more difficult to understand, method follows:

     DB 1101001b              ; Code for JMP (2 byte-displacement)
Duh  DW V2_Start - OFFSET Duh ; 2 byte displacement

This inserts  the jump  offset directly  into the  code following  the jump
instruction.  You could also replace the second line with

     DW V2_Start - $

which accomplishes the same task.

V2 contains the rest of the code, i.e. the stuff that does everything else.
The last  part of  V2 copies  P1 over  V1 (in memory, not on disk) and then
transfers control  to the  beginning of the file (in memory).  The original
program will  then run happily as if nothing happened.  The code to do this
is also very simple.

     MOV SI, V2_START      ; V2_START is a LABEL marking where V2 starts
     SUB SI, V1_LENGTH     ; Go back to where P1 is stored
     MOV DI, 0100h         ; All COM files are loaded @ CS:[100h] in memory
     MOV CX, V1_LENGTH     ; Move CX bytes
     REP MOVSB             ; DS:[SI] -> ES:[DI]

     MOV DI, 0100h
     JMP DI

This code assumes that P1 is located just before V2, as in:


It also  assumes ES  equals CS.  If these assumptions are false, change the
code accordingly.  Here is an example:

     PUSH CS               ; Store CS
     POP  ES               ;  and move it to ES
                           ; Note MOV ES, CS is not a valid instruction
     MOV SI, P1_START      ; Move from whereever P1 is stored
     MOV DI, 0100h         ;  to CS:[100h]

     MOV DI, 0100h
     JMP DI

This code  first moves CS into ES and then sets the source pointer of MOVSB
to where  P1 is located.  Remember that this is all taking place in memory,
so you  need the  OFFSET of P1, not just the physical location in the file.
The offset  of P1  is 100h  higher than  the physical file location, as COM
files are loaded starting from CS:[100h].

So here's a summary of the parts of the virus and location labels:

     JMP FAR PTR Duh
Duh  DW  V2_Start


  ; First part of the program stored here for future use

  ; Real Stuff

V1_Length EQU V1_End - V1_Start

Alternatively, you could store P1 in V2 as follows:




That's all there is to infecting a COM file without destroying it!  Simple,
no?   EXE files,  however, are a little tougher to infect without rendering
them inexecutable - I will cover this topic in a later file.

Now let  us turn our attention back to the replicator portion of the virus.
The steps are outlined below:

     1) Find a file to infect
     2) Check if it is already infected
     3) If so, go back to 1
     4) Infect it
     5) If infected enough, quit
     6) Otherwise, go back to 1

Finding a  file to  infect is  a  simple  matter  of  writing  a  directory
traversal procedure  and issuing  FINDFIRST  and  FINDNEXT  calls  to  find
possible files  to infect.   Once  you find  the file, open it and read the
first few  bytes.   If they are the same as the first few bytes of V1, then
the file  is already  infected.  If the first bytes of V1 are not unique to
your virus,  change it  so that they are.  It is *extremely* important that
your virus  doesn't reinfect  the same  files, since that was how Jerusalem
was first  detected.   If the file wasn't already infected, then infect it!
Infection should take the following steps:

     1) Change the file attributes to nothing.
     2) Save the file date/time stamps.
     3) Close the file.
     4) Open it again in read/write mode.
     5) Save P1 and append it to the end of the file.
     6) Copy V1 to the beginning, but change the offset which it JMPs to so
        it transfers control correctly. See the previous part on infection.
     7) Append V2 to the end of the file.
     8) Restore file attributes/date/time.

You should  keep a counter of the number of files infected during this run.
If the number exceeds, say three, then stop.  It is better to infect slowly
then to give yourself away by infecting the entire drive at once.

You must  be sure  to cover  your tracks  when you infect a file.  Save the
file's  original   date/time/attributes  and  restore  them  when  you  are
finished.   THIS IS VERY IMPORTANT!  It takes about 50 to 75 bytes of code,
probably less,  to do  these few simple things which can do wonders for the
concealment of your program.

I will  include code for the directory traversal function, as well as other
parts of the replicator in the next installment of my phunky guide.

This is  the part  which conceals  the program  from notice by the everyday
user and virus scanner.  The simplest form of concealment is the encryptor.
The code for a simple XOR encryption system follows:

encrypt_val   db   ?

     mov ah, encrypt_val

     mov cx, part_to_encrypt_end - part_to_encrypt_start
     mov si, part_to_encrypt_start
     mov di, si

     lodsb                 ; DS:[SI] -> AL
     xor al, ah
     stosb                 ; AL -> ES:[DI]
     loop xor_loop

Note the encryption and decryption procedures are the same.  This is due to
the weird  nature of  XOR.   You can CALL these procedures from anywhere in
the program,  but make sure you do not call it from a place within the area
to be  encrypted, as  the program  will crash.  When writing the virus, set
the encryption  value to  0.  part_to_encrypt_start and part_to_encrypt_end
sandwich the area you wish to encrypt.  Use a CALL decrypt in the beginning
of V2  to unencrypt  the file  so your  program can  run.  When infecting a
file, first change the encrypt_val, then CALL encrypt, then write V2 to the
end of the file, and CALL decrypt.  MAKE SURE THIS PART DOES NOT LIE IN THE

This is how V2 would look with the concealer:





Alternatively, you  could move  parts  of  the  unencrypted  stuff  between
Part_To_Encrypt_End and V2_End.

The value  of encryption  is readily  apparent.  Encryption makes it harder
for virus  scanners to  locate your virus.  It also hides some text strings
located in  your program.   It is the easiest and shortest way to hide your

Encryption is only one form of concealment.  At least one other virus hooks
into the  DOS interrupts  and alters  the output  of DIR  so the file sizes
appear normal.   Another  concealment scheme  (for TSR virii) alters DOS so
memory utilities  do not  detect the  virus.   Loading the virus in certain
parts of  memory allow  it to survive warm reboots.  There are many stealth
techniques, limited only by the virus writer's imagination.

So now all the boring stuff is over.  The nastiness is contained here.  The
bomb part  of the virus does all the deletion/slowdown/etc which make virii
so annoying.   Set  some activation  conditions of  the virus.  This can be
anything, ranging  from when  it's your  birthday to  when  the  virus  has
infected 100  files.   When these  conditions are met, then your virus does
the good stuff.  Some suggestions of possible bombs:

     1) System slowdown - easily  handled  by  trapping  an  interrupt  and
        causing a delay when it activates.
     2) File deletion - Delete all ZIP files on the drive.
     3) Message display - Display a nice message saying  something  to  the
        effect of "You are fucked."
     4) Killing/Replacing the Partition Table/Boot Sector/FAT of  the  hard
        drive - This is very nasty, as most dimwits cannot fix this.

This is, of course, the fun part of writing a virus, so be original!

There is  one caveat  regarding calculation of offsets.  After you infect a
file, the  locations of  variables change.  You MUST account for this.  All
relative offsets  can stay  the same, but you must add the file size to the
absolute offsets  or your  program will  not work.  This is the most tricky
part of  writing virii  and taking  these into  account can  often  greatly
increase the  size of  a virus.   THIS  IS VERY IMPORTANT AND YOU SHOULD BE
If you  don't, you'll  get fucked  over and  your virus WILL NOT WORK!  One
entire part of the guide will be devoted to this subject.

Testing virii  is a  dangerous yet  essential part  of the  virus  creation
process.   This is  to make  certain that people *will* be hit by the virus
and, hopefully,  wiped out.   Test  thoroughly and  make sure  it activates
under the  conditions.  It would be great if everyone had a second computer
to test  their virii  out, but,  of course, this is not the case.  So it is
ESSENTIAL that  you keep BACKUPS of your files, partition, boot record, and
FAT.   Norton is  handy in  this doing  this.  Do NOT disregard this advice
(even though  I know  that you will anyway) because you WILL be hit by your
own virii.   When  I wrote my first virus, my system was taken down for two
days because I didn't have good backups.  Luckily, the virus was not overly
PIRATE BOARD!   I find a RamDrive is often helpful in testing virii, as the
damage is  not permanent.   RamDrives  are also useful for testing trojans,
but that is the topic of another file...

This is  another fun  part of  virus writing.   It  involves  sending  your
brilliantly-written  program   through  the  phone  lines  to  your  local,
unsuspecting bulletin  boards.   What you  should do  is infect a file that
actually does something (leech a useful utility from another board), infect
it, and upload it to a place where it will be downloaded by users all over.
The best  thing is  that it  won't be detected by puny scanner-wanna-bes by
McAffee, since it is new!  Oh yeah, make sure you are using a false account
(duh).   Better yet,  make a  false account  with the  name/phone number of
someone you  don't like  and upload  the infected  file under the his name.
You can  call back  from time to time and use a door such as ZDoor to check
the spread  of the virus.  The more who download, the more who share in the
experience of your virus!

I promised a brief section on overwriting virii, so here it is...
All these  virii do  is spread  throughout the  system.   They  render  the
infected files  inexecutable, so they are easily detected.  It is simple to
write one:

   +-------------+   +-----+   +-------------+
   | Program     | + |Virus| = |Virus|am     |
   +-------------+   +-----+   +-------------+

These virii are simple little hacks, but pretty worthless because of their
easy detectability.  Enuff said!

wraps it  up for  this installment  of Dark  Angel's Phunky  virus  writing
guide.   There will (hopefully) be future issues where I discuss more about
virii and  include much  more source  code (mo' source!).  Till then, happy