diff --git a/wp-includes/rest-api.php b/wp-includes/rest-api.php
index 06bb9f9f80..34e1c591f6 100644
--- a/wp-includes/rest-api.php
+++ b/wp-includes/rest-api.php
@@ -1051,6 +1051,28 @@ function rest_validate_value_from_schema( $value, $args, $param = '' ) {
 			}
 		}
 	}
+
+	if ( 'object' === $args['type'] ) {
+		if ( $value instanceof stdClass ) {
+			$value = (array) $value;
+		}
+		if ( ! is_array( $value ) ) {
+			/* translators: 1: parameter, 2: type name */
+			return new WP_Error( 'rest_invalid_param', sprintf( __( '%1$s is not of type %2$s.' ), $param, 'object' ) );
+		}
+
+		foreach ( $value as $property => $v ) {
+			if ( ! isset( $args['properties'][ $property ] ) ) {
+				continue;
+			}
+			$is_valid = rest_validate_value_from_schema( $v, $args['properties'][ $property ], $param . '[' . $property . ']' );
+
+			if ( is_wp_error( $is_valid ) ) {
+				return $is_valid;
+			}
+		}
+	}
+
 	if ( ! empty( $args['enum'] ) ) {
 		if ( ! in_array( $value, $args['enum'], true ) ) {
 			/* translators: 1: parameter, 2: list of valid values */
@@ -1170,6 +1192,26 @@ function rest_sanitize_value_from_schema( $value, $args ) {
 		$value = array_values( $value );
 		return $value;
 	}
+
+	if ( 'object' === $args['type'] ) {
+		if ( $value instanceof stdClass ) {
+			$value = (array) $value;
+		}
+		if ( ! is_array( $value ) ) {
+			return array();
+		}
+
+		foreach ( $value as $property => $v ) {
+			if ( ! isset( $args['properties'][ $property ] ) ) {
+				unset( $value[ $property ] );
+				continue;
+			}
+			$value[ $property ] = rest_sanitize_value_from_schema( $v, $args['properties'][ $property ] );
+		}
+
+		return $value;
+	}
+
 	if ( 'integer' === $args['type'] ) {
 		return (int) $value;
 	}
diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
index 76802fea76..ed101815a1 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
@@ -391,6 +391,7 @@ class WP_REST_Attachments_Controller extends WP_REST_Posts_Controller {
 			'context'     => array( 'view', 'edit', 'embed' ),
 			'arg_options' => array(
 				'sanitize_callback' => null, // Note: sanitization implemented in self::prepare_item_for_database()
+				'validate_callback' => null, // Note: validation implemented in self::prepare_item_for_database()
 			),
 			'properties'  => array(
 				'raw' => array(
@@ -413,6 +414,7 @@ class WP_REST_Attachments_Controller extends WP_REST_Posts_Controller {
 			'context'     => array( 'view', 'edit' ),
 			'arg_options' => array(
 				'sanitize_callback' => null, // Note: sanitization implemented in self::prepare_item_for_database()
+				'validate_callback' => null, // Note: validation implemented in self::prepare_item_for_database()
 			),
 			'properties'  => array(
 				'raw' => array(
diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
index 810a3fb867..415ba8e9bc 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
@@ -1200,6 +1200,7 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 					'context'         => array( 'view', 'edit', 'embed' ),
 					'arg_options'     => array(
 						'sanitize_callback' => null, // Note: sanitization implemented in self::prepare_item_for_database()
+						'validate_callback' => null, // Note: validation implemented in self::prepare_item_for_database()
 					),
 					'properties'      => array(
 						'raw'         => array(
diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
index 018b66ec2d..80027cfea8 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
@@ -1846,6 +1846,7 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 						'context'     => array( 'view', 'edit', 'embed' ),
 						'arg_options' => array(
 							'sanitize_callback' => null, // Note: sanitization implemented in self::prepare_item_for_database()
+							'validate_callback' => null, // Note: validation implemented in self::prepare_item_for_database()
 						),
 						'properties'  => array(
 							'raw' => array(
@@ -1870,6 +1871,7 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 						'context'     => array( 'view', 'edit' ),
 						'arg_options' => array(
 							'sanitize_callback' => null, // Note: sanitization implemented in self::prepare_item_for_database()
+							'validate_callback' => null, // Note: validation implemented in self::prepare_item_for_database()
 						),
 						'properties'  => array(
 							'raw' => array(
@@ -1908,6 +1910,7 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 						'context'     => array( 'view', 'edit', 'embed' ),
 						'arg_options' => array(
 							'sanitize_callback' => null, // Note: sanitization implemented in self::prepare_item_for_database()
+							'validate_callback' => null, // Note: validation implemented in self::prepare_item_for_database()
 						),
 						'properties'  => array(
 							'raw' => array(
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 681d4c23aa..6ebbb27cca 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-41726';
+$wp_version = '4.9-alpha-41727';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.