_ | \ | \ | | \ __ | |\ \ __ _____________ _/_/ | | \ \ _/_/ _____________ | ___________ _/_/ | | \ \ _/_/ ___________ | | | _/_/_____ | | > > _/_/_____ | | | | /________/ | | / / /________/ | | | | | | / / | | | | | |/ / | | | | | | / | | | | | / | | | | |_/ | | | | | | | | c o m m u n i c a t i o n s | | | |________________________________________________________________| | |____________________________________________________________________| ...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 __ _ _ __ _ _ __ _ _ __ _ _ __ |__heal_the_sick__raise_the_dead__cleanse_the_lepers__cast_out_demons__| 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 .-. _ _ .-. / \ .-. ((___)) .-. / \ /.ooM \ / \ .-. [ x x ] .-. / \ /.ooM \ -/-------\-------/-----\-----/---\--\ /--/---\-----/-----\-------/-------\- /lucky 13\ / \ / `-(' ')-' \ / \ /lucky 13\ \ / `-' (U) `-' \ / `-' the original e-zine `-' _ Oooo eastside westside / ) __ /)(\ ( \ WORLDWIDE / ( / \ \__/ ) / Copyright (c) 1997 cDc communications and the author. \ ) \)(/ (_/ Award-winning CULT OF THE DEAD COW is a trademark of oooO cDc communications, PO Box 53011, Lubbock, TX, 79453, USA. _ oooO All rights reserved. Edited by Swamp Ratte'. __ ( \ / ) /)(\ / \ ) \ \ ( \__/ Save yourself! Go outside! Do something! \)(/ ( / \_) "THE COW WALKS AMONGST US" Oooo