From bf6b9e94445610a3d84cf9521032fab993f96fd6 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas
Date: Mon, 7 Aug 2017 17:03:42 +0300
Subject: Don't allow logging in with empty password.

Some authentication methods allowed it, others did not. In the client-side,
libpq does not even try to authenticate with an empty password, which makes
using empty passwords hazardous: an administrator might think that an
account with an empty password cannot be used to log in, because psql
doesn't allow it, and not realize that a different client would in fact
allow it. To clear that confusion and to be be consistent, disallow empty
passwords in all authentication methods.

All the authentication methods that used plaintext authentication over the
wire, except for BSD authentication, already checked that the password
received from the user was not empty. To avoid forgetting it in the future
again, move the check to the recv_password_packet function. That only
forbids using an empty password with plaintext authentication, however.
MD5 and SCRAM need a different fix:

* In stable branches, check that the MD5 hash stored for the user does not
not correspond to an empty string. This adds some overhead to MD5
authentication, because the server needs to compute an extra MD5 hash, but
it is not noticeable in practice.

* In HEAD, modify CREATE and ALTER ROLE to clear the password if an empty
string, or a password hash that corresponds to an empty string, is
specified. The user-visible behavior is the same as in the stable branches,
the user cannot log in, but it seems better to stop the empty password from
entering the system in the first place. Secondly, it is fairly expensive to
check that a SCRAM hash doesn't correspond to an empty string, because
computing a SCRAM hash is much more expensive than an MD5 hash by design,
so better avoid doing that on every authentication.

We could clear the password on CREATE/ALTER ROLE also in stable branches,
but we would still need to check at authentication time, because even if we
prevent empty passwords from being stored in pg_authid, there might be
existing ones there already.

Reported by Jeroen van der Ham, Ben de Graaff and Jelte Fennema.

Security: CVE-2017-7546
---
 doc/src/sgml/ref/create_role.sgml | 11 +++++++++++
 1 file changed, 11 insertions(+)

(limited to 'doc/src')

diff --git a/doc/src/sgml/ref/create_role.sgml b/doc/src/sgml/ref/create_role.sgml
index 43f2303b48..4881e54439 100644
--- a/doc/src/sgml/ref/create_role.sgml
+++ b/doc/src/sgml/ref/create_role.sgml
@@ -219,6 +219,17 @@ CREATE ROLE <replaceable class="PARAMETER">name</replaceable> [ [ WITH ] <replac
         user.  A null password can optionally be written explicitly as
         <literal>PASSWORD NULL</literal>.
        </para>
+       <note>
+         <para>
+           Specifying an empty string will also set the password to null,
+           but that was not the case before <productname>PostgreSQL</>
+           version 10. In earlier versions, an empty string could be used,
+           or not, depending on the authentication method and the exact
+           version, and libpq would refuse to use it in any case.
+           To avoid the ambiguity, specifying an empty string should be
+           avoided.
+         </para>
+       </note>
        <para>
         The password is always stored encrypted in the system catalogs. The
         <literal>ENCRYPTED</> keyword has no effect, but is accepted for
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