Modify dll function call in compiled exe

StackOverflow https://stackoverflow.com/questions/18987718

  •  29-06-2022
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Question

I have an exe file that I had written a while back and cannot find the source code for it (it was written in C++).

It calls the MessageBoxA function in user32.dll and passes necessary parameters to it. I want to modify the flags parameter to include the MB_ICONERROR (0x10) flag.

How do I go about finding which bytes in the exe file need to be modified to accomplish this?

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Solution

You need a disassembler like ICE or IDA. https://www.hex-rays.com/products/ida/support/download.shtml. Load the executable. Find the Win32 API call on Names Window, to find it, just type the function name. Then double click CODE XREF to go to referenced caller.

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Then you get what you want: enter image description here

Just select the line and click on Hex-View to get the address.

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