C# creating buffer overflow

Question

I'm trying to create a buffer overflow with C# for a school project:

unsafe
{
    fixed (char* ptr_str = new char[6] {'H', 'a', 'l', 'l', 'o', ','})
    {
        fixed (char* ptr_str2 = new char[6] {'W', 'e', 'r', 'e', 'l', 'd'})
        {
            fixed (char* ptr_str3 = new char[6] {'!', '!', '!', '!', '!', '!'})
            {
                for (int i = 0; i < 8; i++)
                {
                    ptr_str2[i] = 'a';
                }

                for (int i = 0; i < 6; i++)
                {
                    this.Label2.Text += ptr_str[i];
                    this.Label3.Text += ptr_str2[i];
                    this.Label4.Text += ptr_str3[i];
                }
            }
        }
    }
}

I thought this would flood ptr_str2 and thereby overwriting chars in ptr_str. However that does not seem to happen. It does execute but the values in ptr_str are not overwritten.

Can anyone help with achieving this? I don't understand what I'm doing wrong.

Answer 1

You are missing the fact that arrays are objects themselves. They have an object header like any managed reference type and a private field that stores the array size. You have to overwrite those first before you start overwriting the array elements. On a 32-bit machine, you'll start overwriting the first element of ptr_str2 with this:

                        for (int i = 0; i < 13; i++) {
                            ptr_str[i] = 'a';
                        }

Of course, it had to be 13.

Observe this by setting a breakpoint on the for loop. Debug + Windows + Memory + Memory 1, type "ptr_str" in the Address box. Step the code to see the memory getting changed. You'll see ptr_str2 right after that, 4 bytes for the syncblk, 4 bytes for the method table pointer and 4 bytes for the array length. 12 bytes total, 6 chars.

Answer 2

The traditional attack that exploits a buffer overflow overflows a stack buffer; you are overflowing a heap buffer. It is a lot easier to see a write to one buffer smashing another buffer when they're both on the stack. Try using stackalloc instead of new char to force the allocation into the stack.

Answer 3

Stack overflow is an overflow of calling stack. It is done much easier:

int Test ()
{
    return Test ();
}

Console.WriteLine (Test ());

If you meant buffer overflow, there is a similar question.

Answer 4

You do not seem to be making a Stackoverflow here - you aren't really using the stack at all. You seem instead to be trying to create a buffer overflow, I assume thinking that unsafe C# is like C, which it is similar but with several important differences.

A stackoverflow can be made simply:

public void Stackoverflow()
{
    Stackoverflow();
}

and then by calling Stackoverflow() somewhere.