Understanding stack adjustments

We showed that the code execution failed in mid-exploit because our stage two clobbered our stage one code in memory. So, we need more stack space to complete this exploit. We can either split our code up in memory if necessary or we can simply expand the space in the stack.

This is done by telling the system to add space to the ESP. You can do this in one of two ways: by adding negative space or subtracting positive space. The reason for this is because the stack grows from high address to low addresses as we mentioned earlier.

So, we see that we are clobbering the shellcode with this exploit, so we can compensate instead by telling the ESP to move to accommodate the necessary space.

To do this, we need to add a hexadecimal adjustment to the front of the shellcode. We are going to do this in two different ways. The first way we will highlight in this section. We will then explain the second manner of doing it as we reverse Metasploit payloads. First we need...