[5.1] tinymce hardening fix (#43176)

Author: brianteeman

plugins/editors/tinymce/src/PluginTraits/DisplayTrait.php

@@ -459,6 +459,10 @@ public function display(string $name, string $content = '', array $attributes =
                 'branding'  => false,
                 'promotion' => false,
 
+                // Hardened security
+                'sandbox_iframes'       => true,
+                'convert_unsafe_embeds' => true,
+
                 // Specify the attributes to be used when previewing a style. This prevents white text on a white background making the preview invisible.
                 'preview_styles' => 'font-family font-size font-weight font-style text-decoration text-transform background-color border border-radius outline text-shadow',
             ]