289 lines
12 KiB
Text
289 lines
12 KiB
Text
==Phrack Inc.==
|
|
|
|
Volume 0x0b, Issue 0x3b, Phile #0x0b of 0x12
|
|
|
|
|
|
|=-----------------=[ It cuts like a knife. SSHarp. ]=-------------------=|
|
|
|=-----------------------------------------------------------------------=|
|
|
|=----------------=[ stealth <stealth@segfault.net> ]=------------------=|
|
|
|
|
--[ Contents
|
|
|
|
- Intoduction
|
|
|
|
1 - Playing with the banner
|
|
|
|
2 - Playing with the keys
|
|
|
|
3 - Countermeasures
|
|
|
|
4 - An Implementation
|
|
|
|
5 - Discussion
|
|
|
|
6 - Acknowledgments
|
|
|
|
7 - References
|
|
|
|
|
|
--[ Introduction
|
|
|
|
The Secure Shell (SSH) protocol which itself is considered strong is often
|
|
weakly implemented. Especially the SSH1/SSH2 interoperability as
|
|
implemented in most SSH clients suffers from certain weak points as
|
|
described below. Additionally the SSH2 protocol itself is also flexible
|
|
enough to contain some interesting parts for attackers.
|
|
|
|
For disclaimer see the pdf-version of this article available [here].
|
|
|
|
The described mim-program will be made available one week after releasing
|
|
this article to give vendors time for fixes (which are rather trivial) to
|
|
limit the possibility of abuse.
|
|
|
|
In this article I will describe how SSH clients can be tricked into
|
|
thinking they are missing the host-key for the host they connected to even
|
|
though they already have it in their list of known hosts. This is possible
|
|
due to some points in the SSH drafts which makes life of SSH developers
|
|
harder but which was ment to offer special protection or more flexibility.
|
|
|
|
I assume you have a basic understanding of how SSH works. However it is
|
|
not necessary to understand it all in detail because the attacks succeeds
|
|
in the handshake where only a few packets have been exchanged. I also
|
|
assume you are familiar with the common attacking scenarios in networks
|
|
like Man in the Middle attacks, hijacking attacks against plaintext
|
|
protocols, replay attacks and so on.
|
|
|
|
|
|
--[ 1 - Playing with the banner
|
|
|
|
The SSH draft demands that both, client and server, exchange a banner
|
|
before negotiating the key used for encrypting the communication channel.
|
|
This is indeed needed for both sides to see which version of the protocol
|
|
they have to speak. A banner commonly looks like
|
|
|
|
|
|
SSH-1.99-OpenSSH_2.2.0p1
|
|
|
|
|
|
A client obtaining such a banner reads this as "speak SSH1 or SSH2 to me".
|
|
This is due to the "1" after the dash, the so called remote major version.
|
|
It allows the client to choose SSH1 for key negotiation and further
|
|
encryption. However it is also possible for the client to continue with
|
|
SSH2 packets as the "99" tells him which is also called the remote minor
|
|
version. (It is a convention that a remote-minor version of 99 with a
|
|
remote-major version of 1 means both protocols.)
|
|
|
|
Depending on the clients configuration files and command-line options he
|
|
decides to choose one of both protocols. Assuming the user does not force a
|
|
protocol with either of the "-1" or "-2" switch most clients should behave
|
|
the same way. This is due to the configuration files which do not differ
|
|
that much across the various SSH vendors and often contain the line
|
|
|
|
|
|
Protocol 1,2
|
|
|
|
|
|
which makes the client choose SSH protocol version 1. It is obvious what
|
|
follows now. Since the SSH client used to use SSH1 to talk to the server it
|
|
is likely that he never spoke SSH2 before. This may be exploited by
|
|
attackers to prompt a banner like
|
|
|
|
|
|
SSH-2.00-TESO-SSH
|
|
|
|
|
|
to the client. The client looks up his database of known hosts and misses
|
|
the host-key because it only finds the SSH1 key of the server which does
|
|
not help much because according to the banner he is not allowed to speak
|
|
SSH1 anymore (since the remote major version number is 2). Instead of
|
|
presenting a warning like
|
|
|
|
|
|
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|
|
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
|
|
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|
|
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
|
|
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
|
|
It is also possible that the RSA1 host key has just been changed.
|
|
The fingerprint for the RSA1 key sent by the remote host is
|
|
f3:cd:d9:fa:c4:c8:b2:3b:68:c5:38:4e:d4:b1:42:4f.
|
|
Please contact your system administrator.
|
|
|
|
|
|
if someone tries MiM attacks against it without the banner-hack, it asks
|
|
the user to just accept the new key:
|
|
|
|
|
|
Enabling compatibility mode for protocol 2.0
|
|
The authenticity of host 'lucifer (192.168.0.2)' can't be established.
|
|
DSA key fingerprint is ab:8a:18:15:67:04:18:34:ec:c9:ee:9b:89:b0:da:e6.
|
|
Are you sure you want to continue connecting (yes/no)?
|
|
|
|
|
|
It is much easier now for the user to type "yes" instead of editing the
|
|
known_hosts file and restarting the SSH client. Once accepted, the
|
|
attackers SSH server would record the login and password and would forward
|
|
the SSH connection so the user does not notice his account was just
|
|
compromised.
|
|
|
|
The described attack is not just an upgrade attack. It also works to
|
|
downgrade SSH2 speaking clients to SSH1. If the banner would contain "2.0"
|
|
the client only spoke SSH2 to the original server and usually can not know
|
|
the SSH1 key of the server because he does not speak SSH1 at all. However
|
|
our MiM server speaks SSH1 and prompts the client once again with a key he
|
|
cannot know.
|
|
|
|
This attack will not work for clients which just support one protocol
|
|
(likely to be SSH1) because they only implement one of them. These clients
|
|
should be very seldom and most if not all SSH clients support both
|
|
versions, indeed it is even a marketing-pusher to support both versions.
|
|
|
|
If the client uses RSA authentication there is no way for the attacker to
|
|
get in between since he cannot use the RSA challenges presented to him by
|
|
the server because he is talking a different protocol to the client. In
|
|
other words, the attacker is never speaking the same version of the
|
|
protocol to both parties and thus cannot forward or intercept RSA
|
|
authentication.
|
|
|
|
A sample MiM program (ssharp) which mounts the banner-hack and records
|
|
logins can be found at [ssharp].
|
|
|
|
|
|
--[ 2 - Playing with the keys
|
|
|
|
It would be nice to have a similar attack against SSH without a version
|
|
switch. This is because the version switch makes it impossible to break the
|
|
RSA authentication.
|
|
|
|
Reading the SSH2 draft shows that SSH2 does not use the host-key for
|
|
encryption anymore (as with SSH1 where the host and server-key was sent to
|
|
the client which sent back the session-key encrypted with these keys).
|
|
Instead the client obtains the host-key to check whether any of the
|
|
exchanged packets have been tampered with by comparing the server sent MAC
|
|
(Message Authentication Code; the server computes a hash of the packets
|
|
exchanged and signs it using the negotiated algorithm) with his own
|
|
computed hash. The SSH2 draft is flexible enough to offer more than just
|
|
one static algorithm to allow MAC computation. Rather it specifies that
|
|
during key exchange the client and the server exchange a list of preferred
|
|
algorithms they use to ensure packet integrity. Commonly DSA and RSA are
|
|
used:
|
|
|
|
|
|
stealth@liane:~> telnet 192.168.0.2 22
|
|
Trying 192.168.0.2...
|
|
Connected to 192.168.0.2.
|
|
Escape character is '^]'.
|
|
SSH-1.99-OpenSSH_2.2.0p1
|
|
SSH-2.0-client
|
|
`$es??%9?2?4D=?)??ydiffie-hellman-group1-sha1ssh-dss...
|
|
|
|
|
|
I deleted a lot of characters and replaced it with "..." because the
|
|
interesting part is the "ssh-dss" which denotes the servers favorite
|
|
algorithm used for MAC computation. Clients connecting to 192.168.0.2
|
|
cannot have a RSA key for computation because the server does not have one!
|
|
Of course the attackers MiM program has a RSA key and offers only RSA to
|
|
ensure integrity:
|
|
|
|
|
|
stealth@liane:~> telnet 192.168.0.2 22
|
|
Trying 192.168.0.2...
|
|
Connected to 192.168.0.2.
|
|
Escape character is '^]'.
|
|
SSH-2.0-OpenSSH_2.9p1
|
|
SSH-2.0-client
|
|
at s?eu??>vM??E=diffie-hellman-group-exchange-sha1,
|
|
diffie-hellman-group1-sha1ssh-rsa...
|
|
|
|
|
|
A SSH client connecting to our MiM server will once again prompt the user
|
|
to accept the new key instead of issuing the MiM warning.
|
|
|
|
The MiM server connected to the original server and got to know that he
|
|
is using DSA. He then decided to face the user with a RSA key. If the
|
|
original server offers DSA and RSA the MiM server will wait until the
|
|
client sends his preferred algorithms and will choose an algorithm the
|
|
client is naming for his second choice. A RFC compliant SSH2 server has to
|
|
choose the first algorithm he is supporting from the client list, our MiM
|
|
server will choose the next one and thus produces a key-miss on
|
|
client-side. This will again produce a yes/no prompt instead of the warning
|
|
message. "ssharp" also supports this key-hack mode.
|
|
|
|
|
|
--[ 3 - Countermeasures
|
|
|
|
Having the RSA host-key for a server offering a DSA host-key means nothing
|
|
for todays clients. They ignore the fact that they have a valid host-key
|
|
for that host but in a different key-type. SSH clients should also issue
|
|
the MiM warning if they find host-keys for the server where either the
|
|
version or type does not match. Its very likely someone in playing MiM
|
|
games. In my eyes it is definitely a bug in the SSH client software.
|
|
|
|
|
|
--[ 4 - An Implementation
|
|
|
|
There already exist some MiM implementations for SSH1 such as [dsniff] or
|
|
[ettercap]. Usually they understand the SSH protocol and put much effort
|
|
into packet assembling and reassembling or forwarding. Things are much
|
|
simpler. ssharp is based on a normal OpenSSH daemon which was modified to
|
|
accept any login/password pair and starts a special shell for these
|
|
connections: a SSH client which is given the username/password and the real
|
|
destination IP. It logs into the remote host without user-interaction and
|
|
since it is bound to the mim servers pty it looks for the user like he
|
|
enters his normal shell. This way it is not needed to mess with SSH1 or
|
|
SSH2 protocol or to replace keys etc. We just play with the banner or the
|
|
signature algorithm negotiation the way described above.
|
|
|
|
If compiled with USE_MSS option enabled, ssharp will slip the SSH client
|
|
through a screen-like session which allows attaching of third parties to
|
|
existing (mimed) SSH1 or SSH2 connections. It is also possible to kick out
|
|
the legitimate user and completely take control over the session.
|
|
|
|
|
|
--[ 5 - Discussion
|
|
|
|
I know I know; a lot of people will ask "thats all?" now. As with every
|
|
discovery plenty of folks will claim that this is "standard UNIX semantics"
|
|
or it is feature and not a bug or that the vulnerability is completely
|
|
Theo...cal. Neither of them is the case here, and the folks only looking
|
|
for weaknesses in the crypto-algorithms such as key-stream-reuse and
|
|
possibilities to inject 2^64 ;-) adaptive choosen plain-texts will
|
|
hopefully acknowledge that crypto-analysis in 2002 welcomes laziness and
|
|
misunderstanding of drafs on board. Laziness already broke Enigma, but next
|
|
years will show how much impact it has when people are not able to
|
|
completely understand protocols or put too much trust in crypto and do not
|
|
think about the impact of violating the simple MUST in section
|
|
1.1.70.3.3.1.9.78. of the super-crypto draft.
|
|
|
|
|
|
--[ 6 - Acknowledgments
|
|
|
|
Folks from the segfault dot net consortium ;-) for discussing and offering
|
|
test environments. If you like to donate some hardware or money to these
|
|
folks let me know. It would definitely help to let continue research on
|
|
this and similar topics.
|
|
|
|
Also thanks to various other folks for discussing SSH with me.
|
|
|
|
This article is also available [here] as pdf paper with some screen-shots
|
|
to demonstrate the power of ssharp.
|
|
|
|
|
|
--[ 7. References
|
|
|
|
[dsniff] as far as I know the first SSH1 MiM implementation "monkey in the
|
|
middle" part of dsniff package.
|
|
http://www.monkey.org/~dugsong/dsniff
|
|
|
|
[ettercap] good sniffer/mim combo program for lazy hackers ;-)
|
|
http://ettercap.sourceforge.net
|
|
|
|
[ssharp] an implementation of the attacks described in this article
|
|
http://stealth.7350.org/7350ssharp.tgz
|
|
|
|
[here] this article as pdf with screenshots
|
|
http://stealth.7350.org/ssharp.pdf
|
|
|
|
|=[ EOF ]=---------------------------------------------------------------=|
|
|
|
|
|