Password authentication problem with 6.4p1 (and later) clients: An analysis

[Damien Miller]

On Mon, 4 Aug 2014, JCA wrote:

>   if (partial != 0) {
>     logit("Authenticated with partial success.");
>     /* reset state */
>     pubkey_cleanup(authctxt);
>     pubkey_prepare(authctxt);
>   }
>   debug("Authentications that can continue: %s", authlist);
> 
> Before 6.4p1 the 'pubkey_cleanup' and 'pubkey_prepare' invocation were
> missing. The undesirable behavior is introduced by 'pubkey_prepare'.
> When invoked here, it will modify the OpenSSH authentication context
> 'authctxt' so that the client code will try public key authentication
> again - rather than falling back to the next authentication mechanism
> (password authentication, in this case) as it should. An infinite loop
> ensues. Before the 'pubkey_*' lines were introduced OpenSSH would
> still do the right thing, despite of the fact that it was getting a
> misleading message from the embedded server.

The idea here is that if a server supports multiple authentications,
then it should be managing the list of authentications that can
continue. In particular, once key based authentication is complete
and the server won't accept any more keys then the server should
no longer offer publickey in the list of accepted auth methods.

I agree the client should be a bit smarter here about not trying
things that have already succeeded or failed.

> Thus, although OpenSSH 6.4p1 and later clients are proceeding
> correctly according to the standards, because of the way the code is
> organized a malicious server could potentially cause problems to
> clients. This would probably be no more than a nuisance for actual
> interactive sessions, but might be a real issue for batch ones.

I don't think there is any new DoS here. A broken/malicious server
could just hang after authentication and achieve the same result.

-d