diff --git a/wp-includes/class-wp-customize-widgets.php b/wp-includes/class-wp-customize-widgets.php
index c1ed9c8e90..d5cae766b9 100644
--- a/wp-includes/class-wp-customize-widgets.php
+++ b/wp-includes/class-wp-customize-widgets.php
@@ -1419,6 +1419,13 @@ final class WP_Customize_Widgets {
 		if ( isset( $value['raw_instance'] ) && $id_base && wp_use_widgets_block_editor() ) {
 			$widget_object = $wp_widget_factory->get_widget_object( $id_base );
 			if ( ! empty( $widget_object->widget_options['show_instance_in_rest'] ) ) {
+				if ( 'block' === $id_base && ! current_user_can( 'unfiltered_html' ) ) {
+					// The content of the 'block' widget is not filtered on the
+					// fly while editing. Filter the content here to prevent
+					// vulnerabilities.
+					$value['raw_instance']['content'] = wp_kses_post( $value['raw_instance']['content'] );
+				}
+
 				return $value['raw_instance'];
 			}
 		}
diff --git a/wp-includes/version.php b/wp-includes/version.php
index af46fd94f1..33754d11b2 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51413';
+$wp_version = '5.9-alpha-51414';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
diff --git a/wp-includes/widgets/class-wp-widget-block.php b/wp-includes/widgets/class-wp-widget-block.php
index 8d322ef4e7..bf63359e21 100644
--- a/wp-includes/widgets/class-wp-widget-block.php
+++ b/wp-includes/widgets/class-wp-widget-block.php
@@ -178,8 +178,13 @@ class WP_Widget_Block extends WP_Widget {
 	 * @return array Settings to save or bool false to cancel saving.
 	 */
 	public function update( $new_instance, $old_instance ) {
-		$instance            = array_merge( $this->default_instance, $old_instance );
-		$instance['content'] = $new_instance['content'];
+		$instance = array_merge( $this->default_instance, $old_instance );
+
+		if ( current_user_can( 'unfiltered_html' ) ) {
+			$instance['content'] = $new_instance['content'];
+		} else {
+			$instance['content'] = wp_kses_post( $new_instance['content'] );
+		}
 
 		return $instance;
 	}