[Full-disclosure] Heap based overflow Problem--Help

[Tauqeer Ahmad <ahmadtauqeer@xxxxxxxxx>]

  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 I am 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 the SetUnhandledExceptionFilter function and 
get 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
   
   



                
---------------------------------
New Yahoo! Messenger with Voice. Call regular phones from your PC and save big.

_______________________________________________
Full-Disclosure - We believe in it.
Charter: http://lists.grok.org.uk/full-disclosure-charter.html
Hosted and sponsored by Secunia - http://secunia.com/