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Re: Understanding Windows Heap Overflows

From: Dave Aitel <dave () immunitysec com>
Date: Tue, 04 Oct 2005 11:43:14 -0400
Well, don't learn heap overflows on SP2 before you're good at win2k sp4, is one important note. :> Another good note is that Nico and Sinan are teaching a 1 day class: 
http://www.pacsec.jp/dojoheap.html

This should overcome your problems with this sort of thing.

-dave

pbb wrote:
If you remember from Blackhats, the one I showed you, was a management app (also had 7 threads) and had a 4byte overwrite but I couldn't get it consistantly to where I wanted (there seemed to be many pointer fix ups in the heap that made it crash before a control structure overwrite).
With the example given, I couldn't get it to do anything, no 4 byte overwrite. I seem not to be able to step through a overwrite of the UEF in visual studio, I read somewhere it was because the debugger overwrites the Exception handler already so the original pointer isn't called thus the overflow overwrites the wrong address.
I was able to get the SP2 one to work out of visual studio but not within, does anyone have a way around this issue. 

Paul.

halvar () gmx de wrote:


hey paul,

have you gotten to the point of being able to write arbitrary data ?

----- Original Message ----- From: "pbb" <pbb () 65535 com>
To: <dailydave () lists immunitysec com>
Sent: Tuesday, October 04, 2005 2:04 AM
Subject: [Dailydave] Understanding Windows Heap Overflows



Hi everyone,
I've been a long time lurker but never posted. I know Dave suggested to me to post about Buffy ;) but I really would like to get to grips with Heap overflows. I have been trying to understand the Heap Overflow in windows and have been fumbling with IDAPro and Visual Studio to try and understand the concept for a while now (in between real life). I have been reading as many papers as I could and have read the following and assumed I had some understanding of them(I listed them at the bottom). I have managed to get the example code from Defeating Microsoft Windows XP SP2 Heap protection and DEP bypass by Alexander Anisimov to work but not in Visual Studio. I read somewhere (long time ago) that the debugger can ruin the overflow as it intercepts or re-writes the exception handler which you are trying to overflow. I tried to get David Litchfields example code from his blackhats presentation in 2004 to work (on a sp1 XP box, so no heap protection) but inisde or outside a debugger it wouldn't work.
I thought I understood the theory of the overwrite of the heap control structure but struggle to be able to see it in practice. Is there a way to step through the overflow in a debugger, can anyone give me example code and a suggested platform to help me see it in action. I realise there are a couple of different ways to gain the EIP whether it's through the UEF or PEB or SEH but how do I know which one to use. I also realise that with a 4 byte overwrite you may need to somewhere that calls or jmps to a register that points to your heap but I haven't managed to step through it with a debugger. As it's abusing the heap management of the OS is it possible to step through in a debugger.
I have been on Halvar's "Analyzing Software for Security Vulnerabilities" blackhat course (not that I've had time to put much of that in practice.
Need more time :)) And would like to start reversing some applications that I think have heap overflows in them and attempt to write an overflow but I'm not confident enough that I know what I'm doing.
I've Read these papers, can anyone suggest any others? (probably need to re-read them again though.) 
blackhats-win-04-litchfield-code.rtf
blackhats-win-04-litchfield.ppt
phrack 61-6 Advanced Doug lea malloc exploits
Managing Heap Memory in Win32 -MSDN
defeating-xpsp2-heap-protection - Alexander Anisimov
Practical-SEH-exploitation.pdf - Johnny Cyberpunk
msrpcheap.pdf - Of course Dave Aitel
msrpcheap2.pdf - Of course Dave Aitel
Practical Win32 and Unicode exploitation - Phenoelit
If I had a simple program like below could I overflow it and learn the theory? (stolen from I think the shellcoder's handbook) What am I looking for and how can I see this somewhere else.
Thanks Guys for your time and hope this newbie questions doesn't anony anyone. 

Paul.

Here's one I was trying to step through in a debugger.

#include <stdio.h>
#include <windows.h>

DWORD MyExceptionHandler(void);
int foo(char *buf);

int main(int argc, char *argv[])
{
  char *filename = NULL; // filename of the data to overflow with.
  HMODULE l; // library handle
  FILE *fp_overflowFile = NULL; // pointer to datafile
  char *buffer = NULL;
  int count = 0;
  int check = 0;

  l = LoadLibrary("mscvrt.dll");
  l = LoadLibrary("netapi32.dll");

  printf("\n\nHeap overflow program.\n");
  if( argc != 2)
  {
      return printf("ARGS!");
  }

  foo(argv[1]);
  return 0;
}

DWORD MyExceptionHandler(void)
{
  printf("In exception handler ...");
  ExitProcess(1);
  return 0;
}

int foo(char *buf)
{
  HLOCAL h1 =0, h2 = 0;
  HANDLE hp;
__try{ 
          hp = HeapCreate(0,0x1000,0x10000);
          if(!hp)
              return printf("Failed to create heap.\n");
                   h1 = HeapAlloc(hp,HEAP_ZERO_MEMORY,26);
                   printf("HEAP: %.8x %.8x\n", h1, &h1);
                   // Heap overflow occurs here:
          strcpy(h1, buf);
// The second call to HeapAlloc() is when we gain control 
                   h2 = HeapAlloc(hp,HEAP_ZERO_MEMORY,26);
  }
  __except(MyExceptionHandler()){
          printf("Exception occured...");
  }
  return 0;
}
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