Authentication tests, and plain 'password' authentication with a SCRAM verifier

On Tue, Mar 14, 2017 at 9:36 PM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:

While looking at the test, I noticed that the SCRAM patch didn't include
support for logging in with plain 'password' authentication, when the user
has a SCRAM verifier stored in pg_authid. That was an oversight. If the
client gives the server the plain password, it's easy for the server to
verify that it matches the SCRAM verifier.

Right. I forgot about that..

Attached patches add the TAP test suite, and implement plain 'password'
authentication for users with SCRAM verifier. Any comments?

+       /*
+        * The password looked like a SCRAM verifier, but could not be
+        * parsed.
+        */
+       elog(LOG, "invalid SCRAM verifier for user \"%s\"", username);
This would be sent back to the client, no? I think that you should use
*logdetail as well in scram_verify_plain_password.

+# This test cannot run on Windows as Postgres cannot be set up with Unix
+# sockets and needs to go through SSPI.
Yes, true. Having that in its own folder is fine for me.
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Michael

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On Tue, Mar 14, 2017 at 5:36 AM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:

Hi,

I didn't include the authentication TAP tests that Michael wrote in the
main SCRAM commit last week. The main issue was that the new test was
tacked on the src/test/recovery test suite, for lack of a better place. I
propose that we add a whole new src/test/authentication directory for it.
It would also be logical to merge src/test/ssl into it, but the SSL test
suite has some complicated setup steps, to create the certificates, and it
cannot be safely run on a multi-user system. So probably best to keep it
separate, after all.

While looking at the test, I noticed that the SCRAM patch didn't include
support for logging in with plain 'password' authentication, when the user
has a SCRAM verifier stored in pg_authid. That was an oversight. If the
client gives the server the plain password, it's easy for the server to
verify that it matches the SCRAM verifier.

I noticed the asymmetry over plain-text passwords, and didn't know if it
was intentional or not. It is somewhat disconcerting that the client will
send a plain-text password to a mis-configured (or mal-configured) server,
but I don't think there is anything this patch series can reasonably do
about that.

Attached patches add the TAP test suite, and implement plain 'password'
authentication for users with SCRAM verifier. Any comments?

Does what it says, says what it does. There is no installcheck target,
which makes sense because it inherently has to muck around with
pg_hba.conf. The test should be updated to test the syntax for
0001-Add-clause-PASSWORD-val-USING-protocol-to-CREATE-ALT.patch if that
gets committed.

The message returned to the client for the wrong password differs between
pg_hba-set scram and pg_hba-set md5/password methods. Is that OK?

psql: error received from server in SASL exchange: invalid-proof

psql: FATAL: password authentication failed for user "test"

Cheers,

Jeff

On 03/14/2017 09:02 PM, Jeff Janes wrote:

It is somewhat disconcerting that the client will send a plain-text
password to a mis-configured (or mal-configured) server, but I don't
think there is anything this patch series can reasonably do about
that.

Yeah. That's one pretty glaring hole with libpq: there's no option to
disallow insecure authentication mechanisms. That's not new, but now
that we have SCRAM that's otherwise more secure, it looks worse in
comparison.

SCRAM also has a nice feature that it provides proof to the client, that
the server knew the password (or rather, had a verifier for that
password). In other words, the client knows that it connected to the
correct server, not a fake one. But currently nothing prevents a fake
server from simply not doing SCRAM authentication.

We clearly need something similar to sslmode=require in libpq, to
tighten that up.

The message returned to the client for the wrong password differs between
pg_hba-set scram and pg_hba-set md5/password methods. Is that OK?

psql: error received from server in SASL exchange: invalid-proof

psql: FATAL: password authentication failed for user "test"

Ah yeah, I was on the fence on that one. Currently, the server returns
the invalid-proof error to the client, as defined in RFC5802. That
results in that error message from libpq. Alternatively, the server
could elog(FATAL), like the other authentication mechanisms do, with the
same message. The RFC allows that behavior too but returning the
invalid-proof error code is potentially more friendly to 3rd party SCRAM
implementations.

One option would be to recognize the "invalid-proof" message in libpq,
and construct a more informative error message in libpq. Could use the
same wording, "password authentication failed", but it would behave
differently wrt. translation, at least.

- Heikki

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On 03/14/2017 03:43 PM, Michael Paquier wrote:

+       /*
+        * The password looked like a SCRAM verifier, but could not be
+        * parsed.
+        */
+       elog(LOG, "invalid SCRAM verifier for user \"%s\"", username);
This would be sent back to the client, no? I think that you should use
*logdetail as well in scram_verify_plain_password.

No, LOG messages are never sent to the client. Well, unless you have
client_min_messages='log', but then all the LOG messages with details
would be sent to the clients anyway. (We don't process the GUCs from the
startup packet until after authentication, so an unauthenticated user
cannot set client_min_messages='log').

Committed, thanks.

- Heikki

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On Fri, Mar 17, 2017 at 6:40 PM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:

Committed, thanks.

Thanks for the commit.
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On 03/14/2017 09:25 PM, Heikki Linnakangas wrote:

On 03/14/2017 09:02 PM, Jeff Janes wrote:

The message returned to the client for the wrong password differs between
pg_hba-set scram and pg_hba-set md5/password methods. Is that OK?

psql: error received from server in SASL exchange: invalid-proof

psql: FATAL: password authentication failed for user "test"

Ah yeah, I was on the fence on that one. Currently, the server returns
the invalid-proof error to the client, as defined in RFC5802. That
results in that error message from libpq. Alternatively, the server
could elog(FATAL), like the other authentication mechanisms do, with the
same message. The RFC allows that behavior too but returning the
invalid-proof error code is potentially more friendly to 3rd party SCRAM
implementations.

One option would be to recognize the "invalid-proof" message in libpq,
and construct a more informative error message in libpq. Could use the
same wording, "password authentication failed", but it would behave
differently wrt. translation, at least.

I went ahead and changed the backend code to not send the
"invalid-proof" error. That seemed like the easiest fix for this. You
now get the same "password authentication failed" error as with MD5 and
plain password authentication.

- Heikki

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