VULNERABILITIES IN THE S/KEY ONE TIME PASSWORD SYSTEM
                                       
   Author: Mudge
   Organization: L0pht Heavy Industries (aka l0pht.com)
   
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   This may be redistributed as long as proper credit is maintained. In
   other words... if you like it don't try to take credit for it. If you
   don't like it don't take credit for it.
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   This paper is still under revision and modification. If you have any
   comments or feedback please e-mail me.
   
   
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Intro

   
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   [note: This is not intended to be an extensive introduction on how to
   use the s/key package. For this there are several good resources
   available on the net. What this paper attempts to do is to quickly
   summarize what some of s/key's vulnerabilities are.]
   
   Being sniffed is the sys-admin / security specialists worst nightmare.
   For those that aren't privy, sniffing is slang for placing a network
   card into promiscuous mode so that it actually looks at all of the
   traffic coming along the line and not just the packets that are
   addressed to it. By doing this one can catch passwords, login names,
   confidential information, etc. Of course there are good sides to being
   able to place a card into promiscuous mode such as traffic analysis,
   debugging drivers and network applications, and testing router
   configurations to name just a few. In promiscuous mode anything that
   goes by in plaintext is fair game. Each time you telnet to a machine,
   ftp to another machine, connect to the smtp port or POP port, read
   news, off of a different machine, or issue Remote Procedure Calls you
   hand your password and other private information to anyone who wants
   to sit on the lines and listen in. There are several ways of
   protecting oneself. DESLogin provides completely encrypted telnet
   sessions, there is a kerberized POP server available, and there are
   hardware and software utilities to do encryption on different OSI
   layers. Many of these suffer from either being commercial products or
   simply being too difficult to administer.
   
   A wonderful security tool was presented to the network community that
   provides a seeming solution to having your password sniffed. The
   theory behind it is to never use the same password. This is
   accomplished by a challenge response system. The system you are
   contacting issues you a unique challenge. You go off and compute your
   response and then send back that unique response to the host. The next
   time you are presented with a different challenge and thus come back
   with a different response. Sounds great doesn't it? What's even better
   is that the software for the server side and the client side are free
   and widely available. Here's an example of what it looks like:
   

madwand.l0pht.com> telnet some.host.somewhere
Trying 199.99.99.99... Connected to some.host.somewhere.
Escape character is '^]'.

login: jdoe
s/key 99 k113355
Password: WELD GUY CHIMP SWING GONE

   
   
   What has happened here is a telnet to a machine where S/KEY is in use.
   After logging in with the username of jdoe, a challenge is issued.
   jdoe computes his response on a local machine and uses that as input
   for the password prompt. Let's take a look at this and figure:
   

s/key 99 k113355
s/key - text so the user knows what is going on.
99 - number of iterations through MD4
k113355 - seed

   Assuming jdoe provided a valid response, the next time he would try to
   log in the iterations counter would be decremented [i.e. s/key 98
   k113355] and the response that would be computed would be different.
   Thus anybody who sniffed the above [i.e. user: jdoe - Password: WELD
   GUY CHIMP SWING GONE] would not be able to gain access to the machine
   with this information as the password is no longer valid.
   
   
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Dictionary Attacks

   
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  A
  
   
   One of the first vulnerabilities is that all of the information is
   broadcast in plaintext. What this means is that it is trivial to take
   the challenge and response and compare this with the result of the
   challenge applied to words out of a dictionary.
   

username: jdoe
challenge: 99 k113355
response: WELD GUY CHIMP SWING GONE
jdoe's real password: ????

dictionary word 1: love
challenge: 99 k113355
response: BAD LOST CRUMB HIDE KNOT
(well that's not it...)

dictionary word 2: sex
challenge: 99 k113355
response: FORT HARD BIKE HIT SWING
(not it either...)

dictionary word 3: secret
challenge 99 k113355
response: WELD GUY CHIMP SWING GONE
(bingo! )

   
   
   We now know that jdoe's real password is 'secret'. With this
   information we can generate a valid response no matter what the
   challenge.
   
   This is particularly important since in the current implementations of
   the skey package neither the client or server side does any sort of
   sanity check on the security of the chosen password. This means there
   is no failsafe to try to prevent you from choosing your login name as
   your password or other silliness.
   
  B
  
   
   Another source for dictionary attacks come from the /etc/skeykeys
   file. The number of sites that have skey set up that have the improper
   permissions set on this file is quite surprising. This file should not
   be set to world readable. While the 'keyinfo' program would like it to
   be so, the harm outweighs the good. The skeykeys file looks like this:
   
   

root 0072 k113357          12afaa8be65f0502  Jun 29,1995 12:40:48
jdoe 0099 k113355       c7f42dfd84914af3  May 30,1995 16:20:19
[etc...]

   
   What we have here is the username, iteration counter, seed, and a hex
   representation of the five word response we saw before. The other
   three fields are simply date and time information which isn't of much
   interest right now. The exact same sort of dictionary hack can be made
   with this information. Just to bring the point home, let's pretend you
   have a large user base of say, 200 users, and since you are security
   conscious you have a shadow password system and are using s/key. While
   the average account will not be able to look at the real password file
   since it is shadowed they will not be able to do a standard dictionary
   attack on it. They will, however, be able to see the skeykeys file and
   do a dictionary attack on it if the permissions are set improperly,
   thus defeating the benefits of a shadowed password file.
   
   
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Spoofing Attacks

   
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  A
  
   
   Since the iteration counter is transmitted along with the seed, one
   possibility is to masquerade as the server. This could be done by
   setting up a bogus gateway or whatever. Who we are really spoofing is
   the user. Let's take the following scenario...
   

login: jdoe
[jdoe telnet's to his machine]

s/key 55 k113355
password:
[what jdoe should have seen was a challenge of 98 k113355 but instead we have s
ent a lower challenge of 55 k113355.]

password: MY SPIT LOFT HEAD TEAR
[jdoe calculates the response for 55 k113355 based upon his secret password]

Login incorrect
login:
[we tell jdoe that his login was incorrect and forward the rest of the connecti
on to the actual machine he really wanted to talk to in the first place]

   
   With the response we have for the 55 k113355 challenge all we have to
   do is run it through more iterations of md4 for the subsequent
   responses. For example, with the information we have now, if we want
   to figure out the response for the challenge 60 k113355 we need to run
   it through 5 more iterations of md4. If we were looking for the
   response to the challenge of 97 k113355 we would need to run it
   through 42 more iterations of md4. etc. etc.
   
   We can now telnet to the machine and, as long as the iteration counter
   in the challenge is above 55 k113355, we can compute the proper
   response without ever having to know the secret password.
   
   There are many variations on the above theme...
   
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Trojan Attacks

   
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  A
  
   
   This is not a problem with s/key but simply a reminder.
   
   The same vulnerability for trojanized login and su programs for the
   s/key replacements exists as the regulars.
   
   
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Race Attacks

   
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  A
  
   
   There seems to be a problem with s/key that allows two simultaneous
   logins to occur with the same key. If I am in a position to
   capture-look at-modify jdoe's responses as they go bye (i.e. we're a
   bogus gateway or something again), we can open up another telnet
   session to the same machine and, since he hasn't logged in yet the
   iteration counter hasn't been decremented, get the same challenge. As
   we receive jdoe's response we simply send the same information over to
   our other processes as well as his original login. With a little luck
   a locking problem will occur with the skeykeys file and we will both
   be let in under the same challenge and response. This should be easily
   fixable in the source but I have not yet investigated this fully.
   
   
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Hi-Jacking Attacks

   
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  A
  
   
   This is not a problem with s/key but simply a reminder.
   
   Remember that once a connection has been established and jdoe has
   successfully answered the challenge with the appropriate response
   there are no more checks done on the s/key side of things. It is not
   designed to. It is not kerberos. People are too often lulled into
   false senses of security, as in the choosing of easily guessed
   passwords, when they use the s/key system. Once in, the same IP
   Hi-jacking and monitoring tricks can be used on jdoe's session as can
   be used on non-s/key sessions.
   
   
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Outro

   
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   I like s/key. I think s/key is a very good utility and a great asset
   to the community in general. This paper should not dissuade anyone
   from using s/key. It is simply meant to point out certain
   vulnerabilities. The worst thing that can happen to a the security
   conscious is a false sense of security. While s/key does provide added
   security, neither it nor any other product currently out on the market
   is the be-all end-all to security. If this paper has helped to remind
   anyone of this then it has done it's job.
   
   
    -Mudge
    mudge@l0pht.com