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| OllyDbg Instruction Prefix Detection |
Debugging |
ap0x |
prefixes-OllyDBG.zip |
March 11 2006 |
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.386
.model flat, stdcall
option casemap :none ; case sensitive
include \masm32\include\windows.inc
include \masm32\include\user32.inc
include \masm32\include\kernel32.inc
includelib \masm32\lib\user32.lib
includelib \masm32\lib\kernel32.lib
.data
DbgNotFoundTitle db "Debugger status:",0h
DbgFoundTitle db "Debugger status:",0h
DbgNotFoundText db "Debugger not found!",0h
DbgFoundText db "Debugger found!",0h
.data?
SavedESP dd ?
.code
start:
; MASM32 antiOlly example
; coded by ap0x
; Reversing Labs: http://ap0x.headcoders.net
; This example can detect Olly because OllyDBG does not handle
; prefixes well.
; If we insert prefix before one byte instruction that fires-up
; SEH OllyDBG will ignore it and walk right over it.
; If debugger is not present SEH will fire and continue code execution
; in SEH handler.
ASSUME FS:NOTHING
PUSHAD
MOV DWORD PTR[SavedESP],ESP ;Save ESP
PUSH offset SehContinue
PUSH DWORD PTR FS:[0]
MOV DWORD PTR FS:[0],ESP
db 0F3h,64h ;Prefix
db 0F1h ;1 byte INT 1h
POP DWORD PTR FS:[0]
ADD ESP,4
POPAD
PUSH 30h
PUSH offset DbgFoundTitle
PUSH offset DbgFoundText
PUSH 0
CALL MessageBox
RET
SehContinue:
POP DWORD PTR FS:[0]
MOV ESP,DWORD PTR[SavedESP] ;Restore ESP
POPAD
PUSH 40h
PUSH offset DbgNotFoundTitle
PUSH offset DbgNotFoundText
PUSH 0
CALL MessageBox
RET
end start
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