SSH Protocol Trick

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SSH Protocol Weakness Advisory
Monday, July 22 2002
- - rtm

OK, here it is guys... I saw this today when I was looking at the newest issue of phrack (59)
and I discovered that an old little technique of SSH man in the middle attacks I had been working
on was now part of a Phrack article....
Luckily, source code hadn't been disclosed yet, and neither will mine. I just wanted to get this
issue out in the open so people could secure themselves while they can.
Remember, that the ssh daemon

So far, all vendors are vulnerable to this little trick, including commercial based SSH and OpenSSH.
http://www.ssh.com
http://www.openssh.com

You can find more details about the attack at http://www.sekurityfocus.com/phrack59/
(Note: this is a leaked copy of phrack magazine which is not endorsed by phrack.org)

Basically, ssh daemons advertise one of two major versions, depending on what is supported by the
software /configuration files, for SSH protocol version 1, or 2. Compatibility mode is enabled with a
version of 1.99. It is servers which advertise this compatibility mode of 1.99 which are vulnerable to
the attack. Servers in compatability mode have both protocols 1 and 2 enabled.
If the client has a key enabled for say, only SSH protocol 1 or 2, the malicious interloper, "Mallory,"
using ssh mitm arp techniques which are available in say, ettercap or dsniff, can advertise the opposite
protocol in the fake sshd version string used in the banner handshake.
If a client has only used say, SSH 1 authentication in the past, it will not contain a SSH2 key, so
no "Host Identification has changed" message will be present when the fake server advertises its public
host key. The targeted victim will only see a "KEY NOT PRESENT" prompt and will be asked if they want
to add the key.
Obviously, this removes some of the fear paranoid users would feel when facing a real mitm attack.
Remember, this is not a direct vulnerability in the SSH 1 or 2 protocols, but rather a slight trick that
can be abused.


POTENTIAL SOLUTIONS
- -------------------
Client-Based:
Check known_hosts and known_hosts2 in your ~/.ssh directory, and check to see which keys you use for each host.
If you only use ssh1, force the ssh protocol to protocol 1, by specifying the -1 option.
Or if you use ssh2, force the ssh protocol 2 with the -2 option.
If you receive a hostkey change identification, you know something must be up!

Server-Based:
Disable sshd? <- Isn't this always the best security approach, especially now? :)
Really, there isn't anything to do except mail users and tell them which protocol to force, and to ensure
that strict host key checking is always on!

I guess we'll have to wait for the vendors to release an inventive patch for this one...
Now, what I want to know is why somebody who works at SuSE has not been publishing these details openly!?!


Thanks
A concerned citizen,
- -Robert
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