IMAP/POP3 proxy authentication script for nginx. See http://www.whatastruggle.com/nginx-as-an-imappop3-proxy-part-2.

auth.pl

#!/usr/bin/perl

use Digest::HMAC_MD5 qw/ hmac_md5_hex /;
use DBI;
use URI::Escape;
use CGI;

print "Content-type: text/html\n";

my $q = CGI->new;
my $auth_shared_secret = $q->http("X-NGX-Auth-Key");

# Shared secret to ensure that the request comes from nginx
if ( $auth_shared_secret ne "your secret" ) {
  print "Auth-Status: Authentication failed.\n\n";
  print STDERR "Wrong X-NGC-Auth-Key $auth_shared_secret";
  exit(0);
}

my $dsn = "DBI:mysql:database=postfix;host=1.2.3.4";
our $dbh =
  DBI->connect_cached( $dsn, 'mailproxy', 'p@ssw0rd',
    { AutoCommit => 1, mysql_auto_reconnect => 1 } );

our $auth_ok;
our $protocol_ports = {};
$protocol_ports->{'pop3'} = 110;
$protocol_ports->{'imap'} = 143;
$protocol_ports->{'smtp'} = 25;

if ( !defined $dbh || !$dbh->ping() ) {
    ( $sec, $min, $hour, $mday, $mon, $year, $wday, $yday, $isdst ) =
      localtime(time);
    $dbh =
      DBI->connect_cached( $dsn, 'mailproxy', 'p@ssw0rd',
        { AutoCommit => 1, mysql_auto_reconnect => 1 } );
    printf STDERR
"%4d/%02d/%02d %02d:%02d:%02d [notice] : MySQL server connection lost. Reconnecting.\n",
      $year + 1900, $mon + 1, $mday, $hour, $min, $sec;
}

my $auth_method = $q->http("Auth-Method");
my $username    = uri_unescape( $q->http("Auth-User") );
my $password    = uri_unescape( $q->http("Auth-Pass") );
my $salt        = $q->http("Auth-Salt");

our $sth = $dbh->prepare("select clear from users where email=? limit 1");
$sth->execute($username);
my $hash          = $sth->fetchrow_hashref();
my $real_password = $hash->{'clear'};

# Authorize user
if (
    ( $auth_method eq "plain" && $password eq $real_password )
    or (   $auth_method eq "cram-md5"
        && $password eq hmac_md5_hex( $salt, $real_password ) )
  )
{

    # Auth OK, find mail server
    our $sth = $dbh->prepare(
        "select destination_mailstore from transport where domain=? limit 1");
    my $domain = $q->http("Auth-User");

    # remove @ and everything before
    $domain =~ s/^.*@//;
    $sth->execute($domain);
    my $hash       = $sth->fetchrow_hashref();
    my $mailserver = $hash->{'destination_mailstore'};
    $mailserver =~ s/smtp://;

    print "Auth-User: $username\n";
    print "Auth-Pass: $real_password\n";
    print "Auth-Status: OK\n";
    print "Auth-Server: $mailserver\n";
    $auth_port = $protocol_ports->{ $q->http("Auth-Protocol") };
    print "Auth-Port: $auth_port\n";
}
else {
    print "Auth-Status: Authentication failed.\n";
}

print "\n";