Portalbe OpenSSH's don't know agent's keys after authenticion failure with partial success

[Tóth, László Attila]

Hi,

Yes, it seems that there's a case that's missing, but I think not from
regress/multipubkey.sh, because in this case there is only one public key,
which is in ~/.ssh/id_dsa and in the agent

I try to describe it:

Topology:
-------------

1) client: OpenSSH 7.1p2 with running ssh-agent

the client uses ssh-agent, and there is a passphrase-protected (e.g. DSA)
key, that's loaded into the agent, but can be accessed via its standard
file name (~/.ssh/id_dsa in the example).
No more public key is available either in the agent or in ~/.ssh

ssh -vp 2222 -o PubkeyAcceptedKeyTypes=+ssh-dss localhost -l gu=a at panther


2) the server: OpenSSH, but it doesn't really matter

3) between the client and the server there is a proxy that requires
additional authentication not related to the server's authentication


Communication without the patch, with output of 'ssh -v'
--------------------------------------------------------------------------------

debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,keyboard-interactive

1) client sends that public key to the proxy without signature.
(SSH_MSG_USERAUTH_REQUEST)
debug1: Next authentication method: publickey
debug1: Offering DSA public key: /home/panther/.ssh/id_dsa

2) the proxy accepts it, but requres the signed one (SSH_MSG_USERAUTH_PK_OK)
debug1: Server accepts key: pkalg ssh-dss blen 434

3) client sends the public key with signature (USERAUTH_REQUEST)

Here comes the tricky part, because the client already sent the public key,
therefore it won't resend it:

4) the proxy tries this public key on the server side (USERAUTH_REQUEST, to
the server, without signature)
5) the server doesn't know this key - missing from authorized_keys
(USERAUTH_FAILURE, partial_succes=0)
6) the proxy sends this failure to the client (USERAUTH_FAILURE,
partial_success=1 because it is alredy authenticated to the proxy)
Authenticated with partial success
debug1: Authentications that can continue: publickey,password

>From now on the proxy doesn't really matter, just forwards packets:

7) the client retries the same public key from the agent (USERAUTH_REQUEST,
without signature)
debug1: Next authentication method: publickey
debug1: Offering DSA public key: /home/panther/.ssh/id_dsa

8) the server receives this, and rejects it again (USERAUTH_FAILURE,
partial_succes=0)

Here the client goes wrong, I think:

9) the client is confused, finds the same public key, but not in the agent,
therefore it tries to read from the file system, but it's encrypted
debug1: Authentications that can continue: publickey,password
debug1: Trying private key: /home/panther/.ssh/id_rsa
debug1: Trying private key: /home/panther/.ssh/id_dsa
Enter passphrase for key '/home/panther/.ssh/id_dsa':

10) I presss enter, and it falls back to password
debug1: Trying private key: /home/panther/.ssh/id_ecdsa
debug1: Trying private key: /home/panther/.ssh/id_ed25519
debug1: Next authentication method: password
....panther at localhost's password:



Communication with the patch, with output of 'ssh -v'
--------------------------------------------------------------------------------
1-8) the same
9) the password method immediately (as it should be)
debug1: Authentications that can continue: publickey,password
debug1: Trying private key: /home/panther/.ssh/id_rsa
debug1: Trying private key: /home/panther/.ssh/id_ecdsa
debug1: Trying private key: /home/panther/.ssh/id_ed25519
debug1: Next authentication method: password
...panther at localhost's password:


Shortly from the client's view, with step numbers:
----------------------------------------------------------------------
-1) USERATUH_REQUEST(none)
 0) USERAUTH_FAILURE(publickey,keyboard-interactive, partial_success=0)
 1) USERAUTH_REQUEST(publickey, no signature)
 2) USERAUTH_PK_OK
 3) USERAUTH_REQUEST(publickey, signature)
 6) USERAUTH_FAILURE(publickey,password, partial_success=1)
 7) USERAUTH_REQUEST(publickey, no signature)
 8) USERAUTH_FAILURE(publickey,password, partial_success=0)
and here the ~/.ssh/id_dsa file is read directly if OpenSSH is not patched


Regards,
Laszlo Attila Toth



2016-02-18 23:47 GMT+01:00 Damien Miller <djm at mindrot.org>:

> On Thu, 18 Feb 2016, Tóth, László Attila wrote:
>
> > Hi,
> >
> > I created a patch on the top of git repository git://
> > anongit.mindrot.org/openssh.git
> > commit 292a8dee14e5e67dcd1b49ba5c7b9023e8420d59 djm at mindrot.org upstream
> > commit
> >
> > which fixes the issue, and OpenSSH client seems to be working fine in
> this
> > case, too. The patch is attached.
>
> I'm not exactly clear on what the problem is here - authentication using
> multiple public keys is tested in the regress/multipubkey.sh test at the
> moment. Is there a case that is missing?
>
> -d