diff --git a/CHANGELOG b/CHANGELOG
index ff906672a..887de186b 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -79,6 +79,7 @@ CHANGELOG Roundcube Webmail
 - Enigma: Fix key generation in Safari by upgrade to OpenPGP 2.6.2 (#6149)
 - Fix security issue in remote content blocking on HTML image and style tags (#6178)
 - Added 9pt and 11pt to the list of font sizes in HTML editor
+- Fix handling encoding of HTML tags in "inline" JSON output (#6207)
 
 RELEASE 1.3.4
 -------------
diff --git a/program/include/rcmail_output_json.php b/program/include/rcmail_output_json.php
index 5b559e2cd..3a4e76f50 100644
--- a/program/include/rcmail_output_json.php
+++ b/program/include/rcmail_output_json.php
@@ -232,7 +232,7 @@ class rcmail_output_json extends rcmail_output
         $response = $hook['response'];
         unset($hook['response']);
 
-        echo self::json_serialize($response, $this->devel_mode);
+        echo self::json_serialize($response, $this->devel_mode, false);
     }
 
     /**
@@ -245,7 +245,7 @@ class rcmail_output_json extends rcmail_output
         foreach ($this->commands as $i => $args) {
             $method = array_shift($args);
             foreach ($args as $i => $arg) {
-                $args[$i] = self::json_serialize($arg, $this->devel_mode);
+                $args[$i] = self::json_serialize($arg, $this->devel_mode, false);
             }
 
             $out .= sprintf(
diff --git a/program/lib/Roundcube/rcube_output.php b/program/lib/Roundcube/rcube_output.php
index 6c104522e..f52cbee9a 100644
--- a/program/lib/Roundcube/rcube_output.php
+++ b/program/lib/Roundcube/rcube_output.php
@@ -321,14 +321,22 @@ abstract class rcube_output
      *
      * @param mixed   $input  Input value
      * @param boolean $pretty Enable JSON formatting
+     * @param boolean $inline Enable inline mode (generates output safe for use inside HTML)
      *
      * @return string Serialized JSON string
      */
-    public static function json_serialize($input, $pretty = false)
+    public static function json_serialize($input, $pretty = false, $inline = true)
     {
+        // The input need to be valid UTF-8 to use with json_encode()
         $input   = rcube_charset::clean($input);
         $options = JSON_UNESCAPED_SLASHES;
 
+        // JSON_HEX_TAG is needed for inlining JSON inside of the <script> tag
+        // if input contains a html tag it will cause issues (#6207)
+        if ($inline) {
+            $options |= JSON_HEX_TAG;
+        }
+
         // JSON_UNESCAPED_UNICODE in PHP < 7.1.0 does not escape U+2028 and U+2029
         // which causes issues (#6187)
         if (PHP_VERSION_ID >= 70100) {