[Full-disclosure] Exploiting heap overflows in W2K

Hi list,

I am trying to exploit a Heap buffer overflow vulnerability and facing some problems, hope you could help.
I run the vulnerable program in a VMWare, attached with Olly.

These are my problems:

1. I control both EAX and ESI, when the program goes to

mov [esi], eax
mov [eax + 4], esi

First of all, I tried gainig control of execution through PEB but, according to Halvar's presentation, there are some restrictions to what you can write in the header of the overflowed buffer.
Quoting:

" Properties our block must have:

Bit 0 of Flags must be set
Bit 3 of Flags must be set
Field_4 must be smaller than 0x40
The first field (own size) must be larger than 0x80

The block ?XXXX99XX? meets all requirements"

So, supposing PEB pointer to overwrite is 0x7FFDF020 I would need to specify for example: XXXX20f0fd7f, but this is not matching required properties and so RtlFreeHeap exits.
I am sure I must be missing something here, but can't find it.

2. An aditional problem I am facing, due to the fact that this is my first heap overflowing session, is that when I trigger the vulnerability as soon as the programs comes back from "revert snapshot" then I get to rtlHeapFree ok, but if some other request are performed to the program before, then I cannot reproduce that behaviour again and different behaviours and situation arise.
It is obvious that my exploit won't be the first request the program receives so, how can I manage this?


Hope you could help!
Regards

IvaN!


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