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Tuesday, November 11, 2008

Dark Angel's Virus Writing

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 **** 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

MOV DI, 0100h

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

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:

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


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