diff --git a/CHANGELOG b/CHANGELOG
index 21eedff5bb5..56d81303b4e 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -4,6 +4,8 @@ CHANGELOG Roundcube Webmail
 - Fix PHP Warning: Use of undefined constant IDNA_DEFAULT on systems without php-intl (#6244)
 - Fix bug where some parts of quota information could have been ignored (#6280)
 - Fix bug where some escape sequences in html styles could bypass security checks
+- Fix bug where some forbidden characters on Cyrus-IMAP were not prevented from use in folder names
+- Fix bug where only attachments with the same name would be ignored on zip download (#6301)
 
 RELEASE 1.3.6
 -------------
diff --git a/plugins/zipdownload/zipdownload.php b/plugins/zipdownload/zipdownload.php
index 4759549f995..ada89ca1418 100644
--- a/plugins/zipdownload/zipdownload.php
+++ b/plugins/zipdownload/zipdownload.php
@@ -18,7 +18,7 @@ class zipdownload extends rcube_plugin
 
     private $charset = 'ASCII';
 
-    private $names = [];
+    private $names = array();
 
     // RFC4155: mbox date format
     const MBOX_DATE_FORMAT = 'D M d H:i:s Y';
@@ -210,7 +210,7 @@ private function _create_displayname($part)
          * Adding a number before dot of extension on a name of file with same name on zip
          * Ext: attach(1).txt on attach filename that has a attach.txt filename on same zip
          */
-        if (isset($this->name[$displayname])) {
+        if (isset($this->names[$displayname])) {
             list($filename, $ext) = preg_split("/\.(?=[^\.]*$)/", $displayname);
             $displayname = $filename . '(' . ($this->names[$displayname]++) . ').' . $ext;
             $this->names[$displayname] = 1;
diff --git a/program/lib/Roundcube/rcube_imap.php b/program/lib/Roundcube/rcube_imap.php
index faf38652667..2faa70b5fbd 100644
--- a/program/lib/Roundcube/rcube_imap.php
+++ b/program/lib/Roundcube/rcube_imap.php
@@ -3727,6 +3727,35 @@ public function folder_sync($folder)
         }
     }
 
+    /**
+     * Check if the folder name is valid
+     *
+     * @param string $folder Folder name (UTF-8)
+     * @param string &$char  First forbidden character found
+     *
+     * @return bool True if the name is valid, False otherwise
+     */
+    public function folder_validate($folder, &$char = null)
+    {
+        if (parent::folder_validate($folder, $char)) {
+            $vendor = $this->get_vendor();
+            $regexp = '\\x00-\\x1F\\x7F%*';
+
+            if ($vendor == 'cyrus') {
+                // List based on testing Kolab's Cyrus-IMAP 2.5
+                $regexp .= '!`@(){}|\\?<;"';
+            }
+
+            if (!preg_match("/[$regexp]/", $folder, $m)) {
+                return true;
+            }
+
+            $char = $m[0];
+        }
+
+        return false;
+    }
+
     /**
      * Get message header names for rcube_imap_generic::fetchHeader(s)
      *
diff --git a/program/lib/Roundcube/rcube_storage.php b/program/lib/Roundcube/rcube_storage.php
index 56703177ca6..faacd4f658e 100644
--- a/program/lib/Roundcube/rcube_storage.php
+++ b/program/lib/Roundcube/rcube_storage.php
@@ -796,6 +796,26 @@ abstract function folder_sync($folder);
      */
     abstract function mod_folder($folder, $mode = 'out');
 
+    /**
+     * Check if the folder name is valid
+     *
+     * @param string $folder Folder name (UTF-8)
+     * @param string &$char  First forbidden character found
+     *
+     * @return bool True if the name is valid, False otherwise
+     */
+    public function folder_validate($folder, &$char = null)
+    {
+        $delim = $this->get_hierarchy_delimiter();
+
+        if (strpos($folder, $delim) !== false) {
+            $char = $delim;
+            return false;
+        }
+
+        return true;
+    }
+
     /**
      * Create all folders specified as default
      */
diff --git a/program/steps/settings/save_folder.inc b/program/steps/settings/save_folder.inc
index ed1b09fc122..0b777798ba9 100644
--- a/program/steps/settings/save_folder.inc
+++ b/program/steps/settings/save_folder.inc
@@ -45,14 +45,8 @@ else if (mb_strlen($name) > 128) {
 else if ($name[0] == '.' && $RCMAIL->config->get('imap_skip_hidden_folders')) {
     $error = $RCMAIL->gettext('namedotforbidden');
 }
-else {
-    // these characters are problematic e.g. when used in LIST/LSUB
-    foreach (array($delimiter, '%', '*') as $char) {
-        if (strpos($name, $char) !== false) {
-            $error = $RCMAIL->gettext('forbiddencharacter') . " ($char)";
-            break;
-        }
-    }
+else if (!$STORAGE->folder_validate($name, $char)) {
+    $error = $RCMAIL->gettext('forbiddencharacter') . " ($char)";
 }
 
 if ($error) {