Understanding the manipulation of the stack
To understand what we are trying to do with the writing of the exploit, you must understand what is happening in memory. We are going to inject data into an area of memory where there was no bound checking. This usually means that a variable was declared a specific size, and when data was copied into that variable there was no verification that the data would fit in it before copying.
This means that more data can be placed in a variable than what was intended. When that happens, the excess data spills into the stack and overwrites saved values. One of those saved values includes the EIP. The image below highlights how the injected data is pushed onto the stack and can move to overwrite the saved values. ...
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