Date: Mon, 29 Dec 2008 12:00:13 +0100
From: Marc Ruef <marc.ruef@...putec.ch>
To: full-disclosure@...ts.grok.org.uk, pen-test@...urityfocus.com, 
	news@...uriteam.com, submissions@...ketstormsecurity.org
Subject: telnetrecon project

I would like to present a new project of the recon framework. The new 
release is called telnetrecon:

    http://www.computec.ch/projekte/telnetrecon/

The telnetrecon project is doing some research in the field of telnet 
server fingerprinting. The goal is the highly accurate identification of 
given telnetd implementations. This is very important within 
professional vulnerability analysis.

Besides the discussion of different approaches and the documentation of 
gathered results also an implementation for automated analysis is 
provided. This software shall improve the easyness and efficiency of 
this kind of enumeration. The main approach is the fingerprinting of the 
telnet options negotiation initiated by the server part. Some basic 
ideas of application fingerprinting were discussed in the book "Die 
Kunst des Penetration Testing" (Chapter 9, Application Fingerprinting, 
http://www.amazon.de/dp/3936546495/).

Telnet is a traditional tcp service which is served by default on port 
23. The initial specification is defined in RFC 854. Telnet stands for 
terminal emulation over a network. This means an user will be able to 
connect to a terminal remotely. This makes it possible to remote-conrol 
a server within the command-line.

The given implementations rose the need for further possibilities. This 
required the introduction of telnet options. The server and the client 
should be able to negotiate which techniques and features should be used 
and which should not. The negotiation of options are handled by the 
keywords WILL, WONT, DO and DONT.

telnetrecon uses the following technique of fingerprinting the given 
telnetd implementation. After connecting to a host the server responds 
with the option demands and requests. These are dissected and compared 
to the values within the fingerprinting database. As more matches could 
be found as higher is the accuracy of the mapped fingerprint.

For example the following is the negotiaton the telnet server 
implementation a Microsoft Windows XP sends back:

    ÿý%ÿûÿûÿý'ÿýÿýÿû

Those characters will be translated to their ASCII representation which 
is easier to analyze and compare them. This will generate the following 
fingerprint string:

    255-253-37-255-251-255-251-255-253-92-39-255-253-255-253-255-251

The different demands are dissected by the IAC data byte 255. Then 
follows the requirement. The first requirement is introduced with the 
symbol 253 which stands for the option code DO. The requirement itself 
is 37 which stands for "Authentication Option" as it is discussed in RFC 
2941. Afterwards follows another 255 which introduces 251 which stands 
for the option code WILL. This indicates the desire to begin performing, 
or confirmation that you are now performing, the indicated option. And 
so on.

The currently known implementations of telnet fingerprinting, primarily 
telnetfp by Team Teso, is using a strong identification mechanism. This 
means the tool is gathering the telnet option negotiation and compare it 
to the known strings. The identification is only successful if the 
collected strings are identical. This is the easiest approach which does 
not require real measurement of fingerprint hits.

However, this introduces the possibility of missing some partially known 
implementations. For example if a well-known server has been configured 
to announce RSA (authentication type 6) instead of KERBEROS_V5 (type 2). 
This is the reason why telnetrecon uses a more modular approach which 
was already introduced in httprecon 
(http://www.computec.ch/projekte/httprecon/) and later in browserrecon 
(http://www.computec.ch/projekte/browserrecon/). The different 
negotiation aspects are handled seperately. This makes it possible to 
provide the accuracy of not exactly matching fingerprint scans.

The first release of telnetrecon is 0.1 which is not a major release 
because many features are missing. Especially the fingerprint database 
is very small and contain two example fingerprints only. Help to improve 
the project and upload new fingerprints of known telnet daemons.

For further details on telnet fingerprinting see the following documents:
* http://www.iana.org/assignments/telnet-options
* http://www.sans.org/resources/idfaq/fingerp_telnet.php
* http://md.hudora.de/presentations/2002-12-29-fingerprinting-21c3.pdf
* http://www.securiteam.com/tools/6J00L0K06U.html
* http://www.computec.ch/projekte/telnetrecon/?s=documentation

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