Starting in Exploit Development – Day 02

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Using Kali and Virtual Box Guest Additions

Hopeful that I’ll waste a lot less time than yesterday, I’ve setup a Kali virtual machine. I had one problem while installing: as soon as the actual installation started, I had a “The failing step is ‘Install the System'” error message. This was solved when I created a 15GB Virtual Hard Drive (VHD) rather than a 10GB Virtual Box Drive. I also had to setup Kali so it works with VirtualBox Guest Additions. You’ll first need to update the sources in /etc/apt/sources.list by including the second repositorty(http://http.kali.org);

Once done, update the package lists by running apt-get update and then install the the linux-headers for Kali:

Finally, install the guest additions as described in the Kali FAQ [1]….and realize it doesn’t work. According to the log file, 2 errors occured:

This can be solved by using the latest version of VirtualBox. Once I’ve upgraded to 4.3.8, I was able to compile the Guest additions with no trouble.

The Fun Finally Begins…Exploitin’

Continuing Part 2 of Fuzzy Security exploit tutorial. So now, I can finally use the pattern_create.rb script to find out at which position in the payload I need to put the return address in EIP. In the latest version of Metasploit, pattern_create.rb is in the following directory:

After creating the pattern and putting it in the python script, the EIP registers overflows with value 0x69413269, which correspond to offset 247 of the payload.  So far so good.

Metasploit Pattern in EIP
Metasploit Pattern in EIP
Offset Required to Overwrite EIP
Using Mona.py to find the offset of the Metasploit pattern required to overwrite EIP

Once confirmed, then we need to redirect the program’s execution flow to the ESP, the stack pointer, according to tutorial of Fuzzy Security. Good enough, but why ? Well the ESP always points to the top of the stack, which contains an address. If you look at the screenshot above, you’ll notice that the address contained in ESP, 0x00C7FC2C contains our “C”s, i.e. a value we control as well.

ESP Overwritten with "C"s
ESP Overwritten with “C”s, indicating we can place our shellcode at ESP

So if we replace those “C”s with our shellcode, our objective will be to find a way to jump to the address contained in ESP. To do so, we will find an “jmp esp” instruction in memory, and put the address of the instruction in EIP – the register that contains the address of the next instruction to execute.Use “mona jmp -r esp” to locate the JMP instruction. Once Mona is finished, select View -> Log to see the result.

Pointers to a "jmp esp" instruction
Results of the search for pointers to a “jmp esp” instruction

The results I have differs from the one of the tutorials. Shouldn’t be an issue, so I’ll take the one in “ntdll.dll” (0x7C91FCD8) as it seems to be a stable DLL. In little endian form, it becomes “\xD8\xFC\x91\x7C”. To test if it works, you’ll need a breakpoint at whatever address you are pointing, otherwise the execution flow will just land somewhere in memory. To add a breakpoint, right click in the CPU view, select “Go to” -> “Expression” and in the window, type your address (big endian). After you clicked OK, you should land on your address. Then press F2 to toggle the breakpoint at this location.

Following an address in memory using Immunity Debugger
Right click on the main CPU view, select “Go To”, “Expression” and type the address.

So we have verified it worked, we can now move on. I won’t fray to far from the tutorial and just generate a local shell bind payload on port 13373. To do, we’ll need the following Metasploit tool:

Basically what this command does is generate the shellcode for binding a shell on port 13373. It then encodes it in bytes without using bytes 00, 0A and 0C. “\x00” is a terminator value for strings. If included in the shellcode, it will break the code. The same goes with 0A and 0D, which are the “New Line” and “Carriage Return” values (i.e. chr(10), chr(13)). The “-t py” option specifies that the output will be formatted in Python. I thought I could almost conclude this session, but of course, I had to hit one more hiccup. After running the exploit, the debugger throw an “Illegal Instruction” exception at 0x00C7FC28, where the first byte of the shell code is.

I’ve wonder many minutes about this issue. I’ve decided to just plain follow the tutorial and choose the same port, 9988 and regenerate the shell code. To my astonishment, the new shell code worked! The first byte is different, but I’m still not sure why the new code works while the other don’t. More to follow…

In any case, the exploit worked and opened port 9988 on the target machine, which can be connected with netcat.

[1] “Kali Linux Virtual Box Guest.” Kali Linux Official Documentation. http://docs.kali.org/general-use/kali-linux-virtual-box-guest (accessed March 14, 2014).

1 thought on “Starting in Exploit Development – Day 02”

  1. I was curious if you ever considered changing
    the layout of your blog? Its very well written; I love what youve got to say.
    But maybe you could a little more in the way of content so people could connect
    with it better. Youve got an awful lot of text for only having one or
    2 pictures. Maybe you could space it out better?

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