Heap overflow problem----Help

[Tauqeer Ahmad <ahmadtauqeer () yahoo com>]

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  à  77E4FB7A -----  Address of CALL DWORD PTR [ESI + 4C]
  ECX  à  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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