qpsmtpd/plugins/auth/auth_checkpassword

#!perl -w

=head1 NAME

auth_checkpassword - Authenticate against a DJB style checkpassword program

=head1 DESCRIPTION

This plugin authenticates users against a DJB style checkpassword
program. Unlike previous checkpassword implementations, this plugin
expects qpsmtpd to be running as the qpsmtpd user. Privilege
escalation can be attained by running the checkpassword binary setuid
or with sudo.

=head1 CONFIGURATION

Configure the path to your checkpassword binary. You can configure this in
config/plugins by defining the checkpw and true arguments as follows:

 auth/auth_checkpassword checkpw /usr/local/vpopmail/bin/vchkpw true /bin/true

or by editing the config file config/smtpauth-checkpassword:

 echo "/usr/local/vpopmail/bin/vchkpw /bin/true" > ~qpsmtpd/config/smtpauth-checkpassword

vchkpw is the checkpassword program provided by vpopmail. Substitute
your own checkpassword app as appropriate.

If you are using vchkpw and this plugin is being executed by a user ID
other than 89 or 0 (as is the default), and the vchkpw binary is not
setuid (as is the default), this plugin will automatically prepend the
vchkpw command with sudo. If that is the case, you must configure sudo
by adding these two lines to your sudoers file:

  Defaults:qpsmtpd	closefrom_override
  qpsmtpd  ALL = (ALL) NOPASSWD: /usr/local/vpopmail/bin/vchkpw

The closefrom_override option is necessary because, by default, sudo
appropriates the first 3 file descriptors. Those descriptors are
necessary to communicate with the checkpassword program. If you run
qpsmtpd as some other user, adjust the sudo lines approriately.

Using sudo is preferable to enabling setuid on the vchkpw binary. If
you reinstall vpopmail and the setuid bit is lost, this plugin will be
broken.

=head1 SEE ALSO

If you are using this plugin with vpopmail, please read the VPOPMAIL
section in docs/authentication.pod

=head1 DIAGNOSTICS

Is the path in the config/smtpauth-checkpassword correct?

Is the path to true in config/smtpauth-checkpassword correct?

Is qpsmtpd running as the qpsmtpd user? If not, did you adjust the
sudo configuration appropriately?

If you are not using sudo, did you remember to make the vchkpw binary
setuid (chmod 4711 ~vpopmail/bin/vchkpw)?

While writing this plugin, I first wrote myself a little test script,
which helped me identify the sudo closefrom_override issue. Here is
that script:

   #!/usr/bin/perl
   use strict;
   my $sudo = "/usr/local/bin/sudo";
   $sudo .= " -C4 -u vpopmail";
   my $vchkpw = "/usr/local/vpopmail/bin/vchkpw";
   my $true   = "/bin/true";

   open(CPW,"|$sudo $vchkpw $true 3<&0");
   printf(CPW "%s\0%s\0Y123456\0",'user@example.com','pa55word');
   close(CPW);

   my $status = $?;
   print "FAIL\n" and exit if ( $status != 0 );
   print "OK\n";

Save that script to vchkpw.pl and then run it as the same user that
qpsmtpd runs as:

  setuidgid qpsmtpd perl vchkpw.pl

If you aren't using sudo, then remove $sudo from the open line.

=head1 ACKNOWLEDGEMENTS

based upon authcheckpassword by Michael Holzt
and adapted by Johan Almqvist 2006-01-18

=head1 AUTHOR

Matt Simerson <msimerson@cpan.org>

=head1 COPYRIGHT AND LICENSE

Copyright (c) 2010 Matt Simerson

This plugin is licensed under the same terms as the qpsmtpd package itself.
Please see the LICENSE file included with qpsmtpd for details.

=cut

sub register {
    my ($self, $qp, %args) = @_;

    my ($checkpw, $true) = $self->get_checkpw(\%args);
    return DECLINED if !$checkpw || !$true;

    $self->connection->notes('auth_checkpassword_bin',  $checkpw);
    $self->connection->notes('auth_checkpassword_true', $true);

    $self->register_hook('auth-plain', 'auth_checkpassword');
    $self->register_hook('auth-login', 'auth_checkpassword');
}

sub auth_checkpassword {
    my ($self, $transaction, $method, $user, $passClear, $passHash, $ticket) =
      @_;

    my $binary = $self->connection->notes('auth_checkpassword_bin');
    my $true   = $self->connection->notes('auth_checkpassword_true');
    chomp($binary, $true);

    my $sudo = get_sudo($binary);

    $self->log(LOGDEBUG, "auth_checkpassword: $sudo $binary $true 3<&0");
    open(CPW, "|$sudo $binary $true 3<&0");
    printf(CPW "%s\0%s\0Y123456\0", $user, $passClear);
    close(CPW);

    my $status = $?;

    if ($status != 0) {
        $self->log(LOGNOTICE, "authentication failed ($status)");
        return (DECLINED);
    }

    $self->connection->notes('authuser', $user);
    return (OK, "auth_checkpassword");
}

sub get_checkpw {
    my ($self, $args) = @_;

    my ($checkpw) = $args->{checkpw} =~ /^(.*)$/ if $args->{checkpw};  # untaint
    my ($true)    = $args->{true} =~ /^(.*)$/    if $args->{true};     # untaint

    return ($checkpw, $true)
      if ($checkpw && $true && -x $checkpw && -x $true);

    my $missing_config =
"disabled due to invalid configuration. See 'perldoc plugins/auth/auth_checkpassword' for how to configure.";

    if (!$self->qp->config('smtpauth-checkpassword')) {
        $self->log(LOGERROR, $missing_config);
        return;
    }

    $self->log(LOGNOTICE, "reading config from smtpauth-checkpassword");
    my $config = $self->qp->config("smtpauth-checkpassword");
    ($checkpw, $true) = $config =~ /^(\S+)\s+(\S+)\s*$/;

    if (!$checkpw || !$true || !-x $checkpw || !-x $true) {
        $self->log(LOGERROR, $missing_config);
        return;
    }
    return ($checkpw, $true);
}

sub get_sudo {
    my $binary = shift;

    return '' if $> == 0;                             # running as root
    return '' if $> == 89 && $binary =~ /vchkpw$/;    # running as vpopmail

    my $mode = (stat($binary))[2];
    $mode = sprintf "%lo", $mode & 07777;
    return '' if $mode eq '4711';                     # $binary is setuid

    my $sudo = `which sudo` || '/usr/local/bin/sudo';
    return '' if !-x $sudo;

    $sudo .= ' -C4';    # prevent sudo from clobbering file descriptor 3

    return $sudo if $binary !~ /vchkpw$/;
    return "$sudo -u vpopmail";
}