Rule:
  
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Rule:
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Sid:
492

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Summary:
This event is generated when an unsuccessful login attempt was made via telnet.

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Impact:
Possible unauthorized access via password brute-forcing

An attacker may have attempted to gain access to a valid user's account 
via the telnet service, but did not succeed.  The telnet service is 
running, which uses insecure authentication mechanisms.

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Detailed Information:
A user tried to log on to a system via telnet, but has been rejected,
either due to invalid username, password, or both. This could mean 
someone is trying to log on without proper password (if there are 
multiple unsuccessful logins) or they may have just mistyped the 
username or the password.

The telnet server typically runs on TCP port 23.  Upon access to the
server, account access is granted based on an unencrypted user name and
password.  Upon a failed login (resulting from either an invalid account
or an incorrect password), a login failure message will be returned.
This rule matches the common text "Login failed".

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Affected Systems:
Any system running a telnet server.

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Attack Scenarios:
Attackers can, particularly when armed with a valid account name,
attempt to use guessing attacks or brute-force means to gain access via
the telnet service.  Many successive events of this type would likely be
indicative of such an attack.

The use of a telnet server allows the passive attack of traffic
sniffing, which can extract a username and password from any valid
login.

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Ease of Attack:
Simple.

This event indicates it is possible to perform a brute-force attack; the
ease of such an attack is dependent upon the strength of passwords, and
rate-limiting techniques employed by the telnet server in question.

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False Positives:
This event will match any badly-typed or -remembered password, and will
therefore generate a false positive.  Look for rapid successive events.

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False Negatives:
If a password is correctly guessed, no failure will be noted.

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Corrective Action:
Check how many invalid attempts occurred, change the password of the 
user that tried to log in.

It is best to avoid using telnet whenever possible; its authentication
system is lacking, and encryption is generally unavailable.  If your
telnet server can be configured to temporarily disable access after
rapid successive failures, it as advised that you do so.

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Contributors:
Original Rule Writer Unknown
Snort documentation contributed by Chaos <c@aufbix.org> and Nick Black, Reflex Security <dank@reflexsecurity.com>
Sourcefire Research Team
Nigel Houghton <nigel.houghton@sourcefire.com>

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Additional References:

Telnet RFC:
http://www.faqs.org/rfcs/rfc854.html

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