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 3bdcae5435..3388078d05 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
@@ -34,47 +34,52 @@ class WP_REST_Block_Renderer_Controller extends WP_REST_Controller {
 	 * @see register_rest_route()
 	 */
 	public function register_routes() {
-		$block_types = WP_Block_Type_Registry::get_instance()->get_all_registered();
-
-		foreach ( $block_types as $block_type ) {
-			if ( ! $block_type->is_dynamic() ) {
-				continue;
-			}
-
-			register_rest_route(
-				$this->namespace,
-				'/' . $this->rest_base . '/(?P<name>' . $block_type->name . ')',
+		register_rest_route(
+			$this->namespace,
+			'/' . $this->rest_base . '/(?P<name>[a-z0-9-]+/[a-z0-9-]+)',
+			array(
+				'args'   => array(
+					'name' => array(
+						'description' => __( 'Unique registered name for the block.' ),
+						'type'        => 'string',
+					),
+				),
 				array(
-					'args'   => array(
-						'name' => array(
-							'description' => __( 'Unique registered name for the block.' ),
-							'type'        => 'string',
+					'methods'             => array( WP_REST_Server::READABLE, WP_REST_Server::CREATABLE ),
+					'callback'            => array( $this, 'get_item' ),
+					'permission_callback' => array( $this, 'get_item_permissions_check' ),
+					'args'                => array(
+						'context'    => $this->get_context_param( array( 'default' => 'view' ) ),
+						'attributes' => array(
+							'description'       => __( 'Attributes for the block' ),
+							'type'              => 'object',
+							'default'           => array(),
+							'validate_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_validate_value_from_schema( $value, $schema );
+							},
+						),
+						'post_id'    => array(
+							'description' => __( 'ID of the post context.' ),
+							'type'        => 'integer',
 						),
 					),
-					array(
-						'methods'             => array( WP_REST_Server::READABLE, WP_REST_Server::CREATABLE ),
-						'callback'            => array( $this, 'get_item' ),
-						'permission_callback' => array( $this, 'get_item_permissions_check' ),
-						'args'                => array(
-							'context'    => $this->get_context_param( array( 'default' => 'view' ) ),
-							'attributes' => array(
-								/* translators: %s: The name of the block. */
-								'description'          => sprintf( __( 'Attributes for %s block' ), $block_type->name ),
-								'type'                 => 'object',
-								'additionalProperties' => false,
-								'properties'           => $block_type->get_attributes(),
-								'default'              => array(),
-							),
-							'post_id'    => array(
-								'description' => __( 'ID of the post context.' ),
-								'type'        => 'integer',
-							),
-						),
-					),
-					'schema' => array( $this, 'get_public_item_schema' ),
-				)
-			);
-		}
+				),
+				'schema' => array( $this, 'get_public_item_schema' ),
+			)
+		);
 	}
 
 	/**
@@ -136,9 +141,11 @@ class WP_REST_Block_Renderer_Controller extends WP_REST_Controller {
 			// Set up postdata since this will be needed if post_id was set.
 			setup_postdata( $post );
 		}
-		$registry = WP_Block_Type_Registry::get_instance();
 
-		if ( null === $registry->get_registered( $request['name'] ) ) {
+		$registry   = WP_Block_Type_Registry::get_instance();
+		$registered = $registry->get_registered( $request['name'] );
+
+		if ( null === $registered || ! $registered->is_dynamic() ) {
 			return new WP_Error(
 				'block_invalid',
 				__( 'Invalid block.' ),
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 0149a32b92..1773c5d6a7 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.5-alpha-48068';
+$wp_version = '5.5-alpha-48069';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.