Cracklib patch for the Roundcube/password plugin. Original version
Thanks to Tony Fung https://notes.sagredo.eu/en/qmail-notes-185/roundcube-plugins-35.html#comment1651

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diff -ruN roundcube-original/plugins/password/drivers/sql.php roundcube/plugins/password/drivers/sql.php
--- roundcube-original/plugins/password/drivers/sql.php	2020-09-27 13:38:37.000000000 +0200
+++ roundcube/plugins/password/drivers/sql.php	2020-10-07 19:24:52.531366744 +0200
@@ -36,6 +36,15 @@
     function save($curpass, $passwd)
     {
         $rcmail = rcmail::get_instance();
+        exec("echo ".$passwd." | /usr/sbin/cracklib-check 2>/dev/null", $output, $return_var);
+
+        if(preg_match("/^.*\: ([^:]+)$/", $output[0], $matches)) {
+            // Check response:
+            if(strtoupper($matches[1])!=="OK") {
+                // Cracklib doesn't like it:
+                return PASSWORD_CONSTRAINT_VIOLATION;
+            }
+        }
 
         if (!($sql = $rcmail->config->get('password_query'))) {
             $sql = 'SELECT update_passwd(%c, %u)';
diff -ruN roundcube-original/plugins/password/drivers/vpopmaild.php roundcube/plugins/password/drivers/vpopmaild.php
--- roundcube-original/plugins/password/drivers/vpopmaild.php	2020-09-27 13:38:37.000000000 +0200
+++ roundcube/plugins/password/drivers/vpopmaild.php	2020-10-07 19:25:26.019048496 +0200
@@ -32,6 +32,15 @@
         $vpopmaild = new Net_Socket();
         $host      = $rcmail->config->get('password_vpopmaild_host');
         $port      = $rcmail->config->get('password_vpopmaild_port');
+        exec("echo ".$passwd." | /usr/sbin/cracklib-check 2>/dev/null", $output, $return_var);
+
+        if(preg_match("/^.*\: ([^:]+)$/", $output[0], $matches)) {
+            // Check response:
+            if(strtoupper($matches[1])!=="OK") {
+                // Cracklib doesn't like it:
+                return PASSWORD_CONSTRAINT_VIOLATION;
+            }
+        }
 
         $result = $vpopmaild->connect($host, $port, null);
         if (is_a($result, 'PEAR_Error')) {