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sshd1 allows unencrypted sessions regardless of server policy
From: Markus.Friedl () INFORMATIK UNI-ERLANGEN DE (Markus Friedl)
Date: Tue, 14 Dec 1999 16:43:32 +0100

[I am posting this here since nobody seems to take care of ssh-1.2.27]

While working on OpenSSH I discovered the following defect in
ssh-1.2.27, OpenSSH and other related implementations of SSH1:

        A malicious ssh-client can force a server to use the so
        called cipher "none" even if the server-policy does not
        permit this.

In the SSH1 protocol, during connection setup, the server sends a
list of supported ciphers to the client.  This list represents the
server policy and includes the ciphers the server is going to accept.
Usually the client chooses one cipher from this list and sends its
choice back to the server.

However, in all these implementations, the server does _not_ check
whether the cipher chosen by the client is included in the list of
previously offered ciphers.

According to README.CIPHERS from recent ssh-1.2.2x releases login
sessions 'encrypted' with cipher "none" are disabled by default:

        "This cipher is intended only for testing, and should not
        be enabled for normal use. Using no encryption makes SSH
        vulnerable to network-level attacks (such as connection
        hijacking).  There are also more subtle ways to exploit
        using no encryption, and servers should not allow such
        connections at all except when testing the protocol.


        You can allow "none" encryption by giving the --with-none
        option to configure. Using no encryption is not allowed by

This is wrong.

Because passphrase-less hostkeys are 'encrypted' with cipher "none"
the code for this cipher is always compiled into the programs.  This
way the client is free to choose "none" and no server will complain.

The current version OpenSSH-1.2.1 is not vulnerable.  The obvious
fix can be found below.  A patch for the versions of OpenSSH shipped
with OpenBSD-2.6 is available from

Information on OpenSSH can be found at http://www.openssh.com/

Index: sshd.c
--- sshd.c      1999/12/06 20:15:30     1.68
+++ sshd.c      1999/12/07 13:38:05
@@ -869,8 +869,11 @@
         /* Read clients reply (cipher type and session key). */
         packet_read_expect(&plen, SSH_CMSG_SESSION_KEY);

-       /* Get cipher type. */
+       /* Get cipher type and check whether we accept this. */
         cipher_type = packet_get_char();
+        if (!(cipher_mask() & (1 << cipher_type)))
+               packet_disconnect("Warning: client selects unsupported cipher.");

         /* Get check bytes from the packet.  These must match those we
            sent earlier with the public key packet. */

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