Nginx mail proxy module - unexpected behavior re: authentication

[Maxim Dounin]

Hello!

On Wed, Mar 24, 2010 at 12:45:13PM +0530, Naresh V wrote:

> I have a setup with me where I'm using nginx to talk to a redirector
> (embedded perl script) which redirects IMAP/POP communication to the
> appropriate IMAP/POP server based on a simple DB query that says "this
> user's emails are on that IMAP server".
> I'm not doing any authentication at this level since according to
> this[1] request-timeline diagram, the authentication request/response
> is done again with the actual IMAP server anyway.
> 
> When my IMAP client is trying to authenticate with legitimate username
> and password, nginx - as expected according to the diagram - delegates
> the ". login username at domain password" to the real IMAP server whose
> ". OK" is relayed back to the IMAP client by nginx.
> The problem I'm facing right now is when I provide the wrong password,
> IMAP server returns AUTHENTICATIONFAILED, but nginx thinks it's an
> invalid response and cuts off the session with the IMAP client.
> 
> In nginx error log:
> 
> > 2010/03/23 14:43:53 [info] 12550#0: *29 upstream sent invalid response: "* BAD internal server error" while reading response from upstream, client: xx.xx.xx.xx, server: 0.0.0.0:143, login: "username at domain", upstream: yy.yy.yy.yy:143

Well, you didn't check password as auth_http was expected to do, 
and this resulted in nginx complaining about internal error.  
Looks like expected behaviour for me.

> an example telnet session:
> 
> [root at nginx ~]# telnet localhost 143
> Trying 127.0.0.1...
> Connected to 127.0.0.1.
> Escape character is '^]'.
> * OK IMAP4 ready
> + login username at domain wrongpassword
> * BAD internal server error
> Connection closed by foreign host.
> [root at nginx ~]#
> 
> The behaviour I expect is:
> 
> [root at nginx ~]# telnet localhost 143
> Trying 127.0.0.1...
> Connected to 127.0.0.1.
> Escape character is '^]'.
> * OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE
> AUTH=PLAIN AUTH=LOGIN] Dovecot ready.
> + login username at domain wrongpassword
> + NO [AUTHENTICATIONFAILED] Authentication failed.
> <telnet session doesn't close>
> 
> How do I go about configuring nginx to support this?
> 'proxy_pass_error_message on' didn't help.
> Where exactly do I look in the source to make the appropriate changes?

In the scenario you suggest nginx does 2 "internal requests" to 
authenticate user: the one to find out it's backend and one to 
check it's password.  As username client will use after failed 
authentication isn't known it has to close backend connection 
anyway on failed authentication.

Moreover, in this mode it would be trivial to find out valid 
usernames via timing attack (even if you make sure that messages 
returned by auth_http and backend match).

Not even talking about the fact that in this scenario you won't be 
able to use CRAM-MD5/APOP authentication at all.

I believe checking password in auth_http is fairy trivial and this 
is what should be done instead of an attempt to change the way how 
it works.

Maxim Dounin