Re: Some help With BOF Exploits Writing.

["deepcode ." <pondermate () hotmail com>]

The return address should be before your shellcode, inside the nop's.

[NNNNNNNNNSSSSSSSSSSSSSRET]    buffer stored on stack.
5    1              2                    3     4
0xFFFFA        0xFFFFD       0xFFFFE        grows upwards.

1. Bunch of nop instructions: 0x90, that do nothing, so execution goes to 
the right until your code
  is executed.

2. shellcode.

3. return address, which is calculated to point somewhere within the nop 
operations, this is calculated
  locally, by using the stack pointer esp. see 4.

4. Esp stack pointer points to the top of stack, which is usually here, 
unless there is other data
  on the stack, to calculate the address of the NOP's, you'd get the esp 
address and subtract
 an offset from it depending on the size of the data within the stack.

5. Ebp, the current location inside the stack, so if anything gets pushed, 
it'll get pushed here and
  ebp will continue to shift to the left as more things are added to the 
stack.

most unix code does this like this:
-----------------------------
get_esp()
{
__asm__("movl %esp, %eax");   puts the esp (current stack top) into eax. Eax 
is the return value
}                                             of most function calls in C.

//calculate ret, using offset supplied by user.

offset = atoi(argv[1]);               will crash if there was no input 
however. Should check first.
RET = get_esp() - offset;

----------------------------------

[NNNNNNNNNNNNNSSSSSSSSSSSSSSSRET]
10         20         30         40        50      60

simple decimal example.

/exploit 20

RET = Getesp() - offset
RET =  60 - 20
RET = 40

crash .. middle of shellcode

/exploit 40

RET = getesp() - offset
RET = 60 - 40
RET = 20

Bingo, right in the nops, execution moves to the right until shellcode 
hits.. thats the basic way of
doing it anyway.

Another method is by putting the shellcode, and alot more nops inside an 
environment variable, to
increase the size of the padding(NOPs) to increase chances of success and 
have less guesswork.

Anyone want to add to this?

And a question of my own, how does remote exploits accomplish this?? Thats 
been on my mind for
quite some time.

deepcode

> From: "theetabond" <theetabond () rediffmail com>
> Reply-To: "theetabond" <theetabond () rediffmail com>
> To: pondermate () hotmail com
> Subject: Some help With BOF Exploits Writing.
> Date: 25 Jul 2003 06:56:15 -0000
>
> Hi there DeepCode,
>                   I've been reading u'r recent posts on Vul-Dev, and they 
> were very informative and useful for me. I had some questions in my mind 
> regarding writing buffer overflows on Win32 platform, and i hope may be you 
> cud help me with that.
> I had written some exploits ( stack overflow ) for win98 successfully. But 
> now i want to do the same thing at win2k/winxp platforms. My problem in 
> this is - in calculating the return address which u write over the previous 
> RET instruction. On win98 i had a util called getcode.exe , which will scan 
> the memory and list out the jmp eax, ret eax, call eax, call ebx and 
> similar useful addresses which u can use to write at return addresses. 
> Unfortunately this particular tool deosn't work on win2k/Xp. So how can i 
> calculate the return address on 2k/Xp platform?? Dissembling the DLLs/EXEs 
> and searching them all for such instances is kinda hard to do.
>  So is there any way/tool which can give me the desired output ??
> Thank You Very Much
> theeta.

_________________________________________________________________
Add photos to your e-mail with MSN 8. Get 2 months FREE*.  
http://join.msn.com/?page=features/featuredemail