full-disclosure - Heap overflow problem----Help

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Date: Sun May  7 11:34:57 2006
From: ad at heapoverflow.com (ad@...poverflow.com)
Subject: Heap overflow problem----Help

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if I remember you need to setup back the UEF to its previous state, look
it previous state and add a small line of assembly at start of your
shellcode wich will mod back the UEF.

Tauqeer Ahmad wrote:
> Hi all
> 
> I am exploiting a heap-based buffer overflow in one of the ftp server
on window 2000 advanced server with no SP. The problem that I face is
that when using UEF(unhandled exception filter) method it doesn?t work.
The following is the data:
> 
> EAX  ?  77E4FB7A -----  Address of CALL DWORD PTR [ESI + 4C]
> ECX  ?  77EE044C  -----  pointer to UnhandeledExceptionFilter
> 
> When program executes the following instruction what happens is
explained beside the instruction:
> 
> MOV DWORD PTR DS:[ECX], EAX -----THIS IS OK ADDRESS IS COPIED AT UEF
> MOV DWORD PTR DS:[EAX+4], ECX --- THIS ACCESS VIOLATES
> 
> The reason it access violates is that [EAX + 4] is pointing to code
segment which is readable. When it?s trying to write at it the program
crashes.
> 
> What I want to ask is that where am i going wrong? Every thing seems to
be right but logic says that it must crash at MOV DWORD PTR DS:[EAX+4],
ECX. What I am getting from all this is that I am missing the
UEF(However it is unlikely since i have disassembled
SetUnhandledExceptionFilter function and got the address from there)
because when the instruction access violated UEF should have been
executed and control should have been transferred to CALL DWORD PTR [ESI
+ 4C]. Please correct me if I am wrong or if I am using the wrong method
on wrong OS. Furthermore, when I run the server without debugger and
exploit it the EAX and ECX ends up some where else. I mean to say that
provided data don?t get copied on the registers. Advance thanks for the
help.
> 
> Regards,
> 
> Tauqeer Ahmad
> 
> 
> 
> 
> 
>
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