Placing Backdoors Through Firewalls version 1.5 - Hackers often want to retain access to systems they have penetrated even in the face of obstacles such as new firewalls and patched vulnerabilities. To accomplish this the attackers must install a backdoor which does its job is not easily detectable. The kind of backdoor needed depends on the firewall architecture used. As a gimmick and proof-of-concept, a nice backdoor for any kind of intrusion is included.
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---[ Placing Backdoors Through Firewalls ]---
<BR>
v1.5
</H1>
<BR><BR>
<TABLE CELLSPACING="3" BORDER="3" CELLPADDING="7">
<TR><TD>Author: <I><A HREF="mailto:vh@reptile.rug.ac.be">van Hauser</A> / THC</I><BR>
</TD></TR>
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<BR><BR><BR>
----[ Introduction
<P>
This article describes possible backdoors through different firewall
architectures. However, the material can also be applied to other
environments to describe how hackers (you?) cover their access to a system.
<P>
Hackers often want to retain access to systems they have penetrated
even in the face of obstacles such as new firewalls and patched
vulnerabilities. To accomplish this the attackers must install a
backdoor which a) does it's job and b) is not easily detectable. The
kind of backdoor needed depends on the firewall architecture used.
<P>
As a gimmick and proof-of-concept, a nice backdoor for any kind of
intrusion is included, so have fun.
<P>
<BR><BR><BR>
----[ Firewall Architectures
<P>
There are two basic firewall architectures and each has an enhanced version.
<P>
Packet Filters:<UL><UL>
This is a host or router which checks each packet against an
allow/deny ruletable before routing it through the correct
interface. There are very simple ones which can only filter
from the origin host, destination host and destination port, as
well as good ones which can also decide based on incoming interface,
source port, day/time and some tcp or ip flags.<BR>
This could be a simple router, f.e. any Cisco, or a Linux
machine with firewalling activated (ipfwadm).
</UL></UL><P>
Stateful Filters:
<UL><UL> This is the enhanced version of a packet filter. It
still does the same checking against a rule table and only
routes if permitted, but it also keeps track of the state
information such as TCP sequence numbers. Some pay attention
to application protocols which allows tricks such as only
opening ports to the interiour network for ftp-data channels
which were specified in a permitted ftp session. These
filters can (more or less) get UDP packets (f.e. for DNS and
RPC) securely through the firewall. (Thats because UDP is a
stateless protocol. And it's more difficult for RPC services.)<BR>
This could be a great OpenBSD machine with the ip-filter software,
a Cisco Pix, Watchguard, or the (in)famous Checkpoint FW-1.
</UL></UL><P>
Proxies / Circuit Level Gateways:
<UL><UL> A proxy as a firewall host is simply
any server which has no routing activated and instead has
proxy software installe. <BR>Examples of proxy servers which may
be used are squid for WWW, a sendmail relay configuration
and/or just a sockd.
</UL></UL><P>
Application Gateways:
<UL><UL> This is the enhanced version of a proxy. Like a proxy, for every
application which should get through the firewall a software must
be installed and running to proxy it. However, the application
gateway is smart and checks every request and answer, f.e. that
an outgoing ftp only may download data but not upload any, and that
the data has got no virus, no buffer overflows are generated in
answers etc. One can argue that squid is an application
gateway, because it does many sanity checks and let you filter
stuff but it was not programmed for the installation in a secure
environment and still has/had security bugs.<BR>
A good example for a freeware kit for this kind is the TIS firewall
toolkit (fwtk).
</UL></UL><P>
Most firewalls that vendors sell on the market are hybrid firwalls,
which means they've got more than just one type implemented; for
example the IBM Firewall is a simple packet filter with socks and a
few proxies. I won't discuss which firewall product is the best,
because this is not a how-to-by-a-firewall paper, but I will say this:
application gateways are by far the most secure firewalls,
although money, speed, special protocols, open network policies,
stupidity, marketing hype and bad management might rule them out.
<BR><BR><BR>
----[ Getting in
<P>
Before we talk about what backdoors are the best for which firewall
architecture we should shed a light on how to get through a firewall
the first time. Note that getting through a firewall is not a plug-n-play
thing for script-kiddies, this has to be carefully planned and done.
<P>
The four main possibilities:
<P>
Insider:
<UL><UL> There's someone inside the company (you, girl/boy-friend, chummer)
who installs the backdoor. This is the easiest way of course.
</UL></UL><P>
Vulnerable Services:
<UL><UL> Nearly all networks offer some kind of services,
such as incoming email, WWW, or DNS. These may be on the
firewall host itself, a host in the DMZ (here: the zone in front
of the firewall, often not protected by a firewall) or on an internal
machine. If an attacker can find a hole in one of those services,
he's got good chances to get in. You'd laugh if you'd see how many
"firewalls" run sendmail for mail relaying ...
</UL></UL><P>
Vulnerable External Server:
<UL><UL> People behind a firewall sometimes work on
external machines. If an attacker can hack these, he can
cause serious mischief such as the many X attacks if the
victim uses it via an X-relay or sshd. The attacker could
also send fake ftp answers
to overflow a buffer in the ftp client software, replace a gif
picture on a web server with one which crashs netscape and
executes a command (I never checked if this actually works, it
crashs, yeah, but I didn't look through this if this is really
an exploitable overflow). There are many possibilities with
this but it needs some knowledge about the company. However,
an external web server of the company is usually a good start.
Some firewalls are configured to allow incoming telnet from
some machines, so anyone can sniff these and get it. This is
particulary true for the US, where academic environments and
industry/military work close together.
</UL></UL><P>
Hijacking Connections:
<UL><UL> Many companies think that if they allow incoming telnet with
some kind of secure authentication like SecureID (secure algo?, he)
they are safe. Anyone can hijack these after the authentication and
get in ... Another way of using hijacked connections is to modify
replies in the protocol implementation to generate a buffer
overflow (f.e. with X).
</UL></UL><P>
Trojans:
<UL><UL> Many things can be done with a trojan horse.
This could be a gzip file which generates a buffer overflow
(well, needs an old gzip to be installed), a tar file which
tampers f.e. ~/.logout to execute something, or an executable
or source code which was modified to get the hacker in somehow.
To get someone running this, mail spoofing could be used or
replacing originals on an external server which internal employees
access to update their software regulary (ftp xfer files and www
logs can be checked to get to know which files these are).
</UL></UL><P>
<BR><BR><BR>
----[ Placing the Backdoors
<P>
An intelligent hacker will not try to put the backdoors on machines in
the firewall segment, because these machines are usually monitored and
checked regulary. It's the internal machines which are usually unprotected
and without much administration and security checks.
<P>
I will now talk about some ideas of backdoors which could be implemented.
Note that programs which will/would run on an stateful filter will of course
work with a normal packet filter too, same for the proxy. Ideas for an
application gateway backdoor will work for any architecture.<BR>
Some of them are "active" and others "passive". "Active" backdoors are those
which can be used by a hacker anytime he wishes, a "passive" one triggers
itself by time/event so an attacker has to wait for this to happen.
<P>
Packet Filters:
<UL><UL> It's hard to find a backdoor which gets through this one but does
not work for any other. The few ones which comes into my mind<BR>
is a) the ack-telnet. It works like a normal telnet/telnetd except
it does not work with the normal tcp handshake/protocol but uses
TCP ACK packets only. Because they look like they belong to an
already established (and allowed) connection, they are permitted.
This can be easily coded with the spoofit.h of Coder's Spoofit
project (<A HREF="http://reptile.rug.ac.be/~coder">http://reptile.rug.ac.be/~coder</A>).<BR>
b) Loki from Phrack 49/51 could be used too to establish a tunnel
with icmp echo/reply packets. But some coding would be needed to
to be done.<BR>
c) daemonshell-udp is a backdoor shell via UDP<BR>
(<A HREF="http://r3wt.base.org/">http://r3wt.base.org</A> look for thc-uht1.tgz)<BR>
d) Last but not least, most "firewall systems" with only a screening
router/firewall let any incoming tcp connection from the source port
20 to a highport (>1023) through to allow the (non-passive) ftp
protocol to work. "netcat -p 20 target port-of-bindshell" is the
fastest solution for this one.
</UL></UL><P>
Stateful Filters:
<UL><UL> Here a hacker must use programs which initiates the connection from
the secure network to his external 0wned server.
There are many out there which could be used:<BR>
active:<UL>
tunnel from Phrack 52.<BR>
ssh with the -R option (much better than tunnel ... it's
a legtimitate program on a computer and it encrypts the
datastream).
</UL>
passive:<UL>
netcat compiled with the execute option and run with a
time option to connect to the hacker machine (<A HREF="ftp://ftp.avian.org/">ftp.avian.org</A>).<BR>
reverse_shell from the thc-uht1.tgz package (see above) does the same.
</UL></UL><P>
Proxies / Circuit Level Gateways:
<UL><UL> If socks is used on the firewall, someone can use all those stuff
for the stateful filter and "socksify" them. (<A HREF="http://thc.pimmel.com/files/thc/www.socks.net.com">www.socks.nec.com</A>)
For more advanced tools you'd should take a look at the application
gateway section.
</UL></UL><P>
Application Gateways:
<UL><UL> Now we get down to the interesting stuff. These beasts can be
intelligent so some brain is needed.<BR>
active:<UL>
(re-)placing a cgi-script on the webserver of the company,
which allows remote access. This is unlikely because it's
rare that the webserver is in the network, not monitored/
checked/audited and accessible from the internet. I hope
nobody needs an example on such a thing ;-)<BR>
(re-placing) a service/binary on the firewall. This is
dangerous because those are audited regulary and sometimes
even sniffed on permanent ...<BR>
Loading a loadable module into the firewall kernel wich
hides itself and gives access to it's master. The best
solution for an active backdoor but still dangerous.
</UL>
passive:<UL>
E@mail - an email account/mailer/reader is configured in a
way to extract hidden commands in an email (X-Headers with
weird stuff) and send them back with output if wanted/needed.<BR>
WWW - this is hard stuff. A daemon on an internal machine
does http requests to the internet, but the requests are
in real the answers of commands which were issued by a
rogue www server in a http reply. This nice and easy beast
is presented below (-><A HREF="http://thc.pimmel.com/files/thc/fw-backd.htm#example">Backdoor Example: The Reverse WWW Shell</A>)<BR>
DNS - same concept as above but with dns queries and
replies. Disadvantage is that it can not carry much data.
(<A HREF="http://www.icon.co.za/~wosp/wosp.dns-tunnel.tar.gz">http://www.icon.co.za/~wosp/wosp.dns-tunnel.tar.gz</A>, this
example needs still much coding to be any effective)
</UL>
</UL></UL><P>
<BR><BR><BR><A NAME="example"></A>
----[ Backdoor Example: The Reverse WWW Shell
<P>
This backdoor should work through any firewall which has got the security
policy to allow users to surf the WWW (World Wide Waste) for information
for the sake and profit of the company.<BR>
For a better understanding take a look at the following picture and try
to remember it onwards in the text:
<PRE>
+--------+ +------------+ +-------------+
|internal|--------------------| FIREWALL |--------------|server owned |
| host | internal network +------------+ internet |by the hacker|
+--------+ +-------------+
SLAVE MASTER
</PRE>
Well, a program is run on the internal host, which spawns a child every day
at a special time. For the firewall, this child acts like a user, using his
netscape client to surf on the internet. In reality, this child executes
a local shell and connects to the www server owned by the hacker on the
internet via a legitimate looking http request and sends it ready signal.
The legitimate looking answer of the www server owned by the hacker are
in reality the commands the child will execute on it's machine it the
local shell. All traffic will be converted (I'll not call this "encrypted",
I'm not Micro$oft) in a Base64 like structure and given as a value for
a cgi-string to prevent caching.
<PRE>
Example of a connection:
Slave
GET /cgi-bin/order?M5mAejTgZdgYOdgIO0BqFfVYTgjFLdgxEdb1He7krj HTTP/1.0
Master replies with
g5mAlfbknz
</PRE><P>
The GET of the internal host (SLAVE) is just the command prompt of the
shell, the answer is an encoded "ls" command from the hacker on the
external server (MASTER).
Some gimmicks:<P>
The SLAVE tries to connect daily at a specified time to the MASTER if
wanted; the child is spawned because if the shell hangs for whatever
reason you can check & fix the next day; if an administrator sees connects
to the hacker's server and connects to it himself he will just see a
broken webserver because there's a Token (Password) in the encoded
cgi GET request; WWW Proxies (f.e. squid) are supported; program masks
it's name in the process listing ...
<P>
Best of all: master & slave program are just one 260-lines perl file ...
Usage is simple: edit rwwwshell.pl for the correct values,
execute "rwwwshell.pl slave" on the SLAVE, and just run "rwwwshell.pl"
on the MASTER just before it's time that the slave tries to connect.
<P>
Well, why coding it in perl? a) it was very fast to code, b) it's highly
portable and c) I like it.
If you want to use it on a system which hasn't got perl installed, search
for a similar machine with perl install, get the a3 compiler from the perl
CPAN archives and compile it to a binary. Transfer this to your target
machine and run that one.
<P>
The code for this nice and easy tool is appended in the section THE CODE
after my last words. If you've got updates/ideas/critics for it drop me an
email. If you think this text or program is lame, write me at root@localhost.
Check out <A HREF="http://r3wt.base.org/">http://r3wt.base.org</A> for updates.
<BR><BR><BR>
----[ The Source
<P>
Grab it here ...<P>
<UL><A HREF="http://thc.pimmel.com/files/thc/rwwwshell-1.6.perl">rwwwshell v1.6</A></UL>
<BR><BR><BR>
----[ Security
<P>
Now it's an interesting question how to secure a firewall to deny/detect
this. It should be clear that you need a tight application gateway firewall
with a strict policy. email should be put on a centralized mail server,
and DNS resolving only done on the WWW/FTP proxies and access to WWW only
prior proxy authentication. However, this is not enough. An attacker can
tamper the mailreader to execute the commands extracted from the crypted
X-Headers or implement the http authentication into the reverse www-shell
(it's simple). Also checking the DNS and WWW logs/caches regulary with good
tools can be defeated by switching the external servers every 3-20 calls
or use aliases.
<P>
A secure solution would be to set up a second network which is
connected to the internet, and the real one kept seperated - but tell
this the employees ...
A good firewall is a big improvement, and also an Intrusion Detection
Systems can help. But nothing can stop a dedicated attacker.
<P>
<PRE>
----[ Last Words
Have fun hacking/securing the systems ...
Greets to all guys who like + know me ;-) and especially to those good
chummers I've got, you know who you are.
Ciao...
van Hauser / [THC] - The Hacker's Choice
For further interesting discussions you can email me at
<A HREF="mailto:vh@reptile.rug.be">vh@reptile.rug.ac.be</A> with my public pgp key blow:
Type Bits/KeyID Date User ID
pub 2048/CDD6A571 1998/04/27 van Hauser / THC <VH@REPTILE.RUG.AC.BE>
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</PRE>
----[ THE END
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