diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-block-renderer-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-block-renderer-controller.php
index 3388078d05..07a389e794 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-block-renderer-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-block-renderer-controller.php
@@ -70,6 +70,22 @@ class WP_REST_Block_Renderer_Controller extends WP_REST_Controller {
 
 								return rest_validate_value_from_schema( $value, $schema );
 							},
+							'sanitize_callback' => static function ( $value, $request ) {
+								$block = WP_Block_Type_Registry::get_instance()->get_registered( $request['name'] );
+
+								if ( ! $block ) {
+									// This will get rejected in ::get_item().
+									return true;
+								}
+
+								$schema = array(
+									'type'                 => 'object',
+									'properties'           => $block->get_attributes(),
+									'additionalProperties' => false,
+								);
+
+								return rest_sanitize_value_from_schema( $value, $schema );
+							},
 						),
 						'post_id'    => array(
 							'description' => __( 'ID of the post context.' ),
diff --git a/wp-includes/version.php b/wp-includes/version.php
index d4286b9069..ba51c17733 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.5-beta1-48436';
+$wp_version = '5.5-beta1-48437';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.