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Cult of the Dead Cow 327
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...presents... Vulnerabilities in the S/KEY One-Time
Password System by Mudge
01/01/1997-#327
__//////\ -cDc- CULT OF THE DEAD COW -cDc- /\\\\\\__
Est. 1984 \\\\\\/ xXx BOW to the COW xXx \////// Est. 1984
__ _ _ __ _ _ __ _ _ __ _ _ __
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This text attempts to touch upon several vulnerabilities found in One-
Time Password (OTP) implementations such as S/KEY. I'm writing specifically
of the free version of S/KEY from Bellcore.
Being sniffed is the sys-admin's/security analyst's worst nightmare
(well, not really but humor me). For those that aren't aware, 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:
evil.guy.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:
[At this point the user either issues the following in another window or else
suspends the current session]
^]
telnet>^Z
evil.guy.com> key 99 k113356
Reminder - Do not use key while logged in via telnet or dial-in.
Enter secret password:[passwd doesn't echo]
FLY ACE TIDY BURR CON CADY
evil.guy.com> fg
Password:FLY ACE TIDY BURR CON CADY
Welcome back jdoe!
bash%
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: FLY ACE TIDY BURR CON CADY)
would not be able to gain access to the machine with this information as the
same password is not valid for the next session.
* Dictionary Attacks
One of the first vulnerabilities we find 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.
Remember in the above example, the user escaped to a local shell and
entered the following:
evil.guy.com> key 99 k113356
Reminder - Do not use key while loged in via telnet or dial-in.
Enter secret password:[passwd doesn't echo]
FLY ACE TIDY BURR CON CADY
Since people will pick easy-to-remember words or phrases this works to
the cracker's advantage. Granted, users can now choose phrases instead of
a single word... however, how many people are likely to choose "k35jsX/
O0l3f ffdg99999d" as opposed to "mary had a little lamb"?
The dictionaries used for this sort of attack will simply start to
encompass phrases. For example:
username: jdoe # here we have the information
challenge: 99 k113355 # that we captured
response: WELD GUY CHIMP SWING GONE
jdoe's real password: ????
dictionary word 1: love # here we start the dictionary
challenge: 99 k113355 # attack
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
S/KEY 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.
Another source for dictionary attacks come from the /etc/skeykeys file.
The number of sites that have S/KEY set up that have the improper permissions
set on this file is quite surprising (although this is to be expected as the
code from both Bellcore and Weitse Venema's logdaemon set it up this way. A
quick fix is to simply change the mode to 600). This file should not be set
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. The average user 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. He 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.
* Spoofing Attacks
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 sent 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 connection 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.
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.
* Race Attacks
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 by (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. As the program has to work this way as to avoid denial of
service attacks we get the same challenge. When 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 (but wait you say, there can't be a locking problem as S/KEY does
not do real file locking. Instead it jumps through a bunch of hoops to do a
similar thing... get the idea?). This should be easily fixable in the source
but I have not yet investigated this fully.
* Hijacking Attacks
This is not a problem with S/KEY but simply a reminder of your
vulnerability. 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
re-check the validity and authorization of a user once he is logged in. It
is not kerberos. People are too often lulled into a false sense of security,
as in the choosing of easily guessed passwords, when they use the S/KEY
system. Once in, the same IP hijacking and monitoring tricks can be used on
jdoe's session as can be used on non-S/KEY sessions.
Along with hijacking, think of the administrator who logs in with his
OTP and the proceeds to enter new accounts. The passive sniffer will still
see all of the passwords and sensitive information that the administrator is
entering.
* Conclusion
I like S/KEY. I think S/KEY is a very useful utility and a great asset
to the community in general. This article should not dissuade anyone from
using S/KEY. It is simply meant to point out certain vulnerabilities. The
worst thing that can happen to the security conscious is thinking they are
secure when they really aren't. 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 article has helped to remind anyone of this
then it has done its job.
-Mudge mudge@l0pht.com
Organizations: L0pht Heavy Industries (aka l0pht.com)
cDc communications
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