Media: Sideload external images on the server via a url REST parameter

src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php

@@ -151,6 +151,12 @@ public function get_endpoint_args_for_item_schema( $method = WP_REST_Server::CRE
 				'default'     => true,
 				'description' => __( 'Whether to convert image formats.' ),
 			);
+			$args['url']                = array(
+				'type'              => 'string',
+				'format'            => 'uri',
+				'description'       => __( 'URL of an external image to sideload into the media library, instead of uploading a file.' ),
+				'sanitize_callback' => 'sanitize_url',
+			);
 		}
 
 		return $args;
@@ -289,7 +295,7 @@ public function create_item_permissions_check( $request ) {
 	 * Creates a single attachment.
 	 *
 	 * @since 4.7.0
-	 * @since 7.1.0 Added `generate_sub_sizes` and `convert_format` parameters.
+	 * @since 7.1.0 Added the `generate_sub_sizes`, `convert_format`, and `url` parameters.
 	 *
 	 * @param WP_REST_Request $request Full details about the request.
 	 * @return WP_REST_Response|WP_Error Response object on success, WP_Error object on failure.
@@ -317,6 +323,18 @@ public function create_item( $request ) {
 			add_filter( 'image_editor_output_format', '__return_empty_array', 100 );
 		}
 
+		/*
+		 * When a URL is supplied instead of an uploaded file, sideload the
+		 * remote image on the server. This avoids a cross-origin browser fetch,
+		 * which fails under cross-origin isolation. The sub-size and scaling
+		 * filters applied above still govern whether derivatives are generated.
+		 */
+		if ( ! empty( $request['url'] ) ) {
+			$response = $this->create_item_from_url( $request );
+			$this->remove_client_side_media_processing_filters();
+			return $response;
+		}
+
 		$insert = $this->insert_attachment( $request );
 
 		if ( is_wp_error( $insert ) ) {
@@ -410,6 +428,95 @@ public function create_item( $request ) {
 		return $response;
 	}
 
+	/**
+	 * Sideloads an external image from a URL into the media library.
+	 *
+	 * Downloads the remote file on the server, avoiding a cross-origin browser
+	 * fetch that fails under cross-origin isolation. Whether sub-sizes are
+	 * generated is governed by the filters applied in create_item().
+	 *
+	 * @since 7.1.0
+	 *
+	 * @param WP_REST_Request $request Full details about the request.
+	 * @return WP_REST_Response|WP_Error Response object on success, WP_Error object on failure.
+	 */
+	protected function create_item_from_url( $request ) {
+		// Sideloading downloads and stores a file, so require the upload capability.
+		if ( ! current_user_can( 'upload_files' ) ) {
+			return new WP_Error(
+				'rest_cannot_create',
+				__( 'Sorry, you are not allowed to upload media on this site.' ),
+				array( 'status' => rest_authorization_required_code() )
+			);
+		}
+
+		require_once ABSPATH . 'wp-admin/includes/file.php';
+		require_once ABSPATH . 'wp-admin/includes/media.php';
+		require_once ABSPATH . 'wp-admin/includes/image.php';
+
+		$url     = $request['url'];
+		$post_id = ! empty( $request['post'] ) ? (int) $request['post'] : 0;
+
+		// Derive the filename from the URL path before downloading anything.
+		$url_path = wp_parse_url( $url, PHP_URL_PATH );
+		$filename = $url_path ? wp_basename( $url_path ) : '';
+		if ( '' === $filename ) {
+			return new WP_Error(
+				'rest_invalid_url',
+				__( 'Could not determine a filename from the provided URL.' ),
+				array( 'status' => 400 )
+			);
+		}
+
+		/*
+		 * Download the remote file with WordPress's HTTP API, which validates
+		 * the host and blocks requests to private or local addresses. This is
+		 * the same primitive core's media_sideload_image() relies on.
+		 */
+		$tmp_file = download_url( $url );
+		if ( is_wp_error( $tmp_file ) ) {
+			return $tmp_file;
+		}
+
+		$file_array = array(
+			'name'     => $filename,
+			'tmp_name' => $tmp_file,
+		);
+
+		$attachment_id = media_handle_sideload( $file_array, $post_id );
+
+		if ( is_wp_error( $attachment_id ) ) {
+			/*
+			 * media_handle_sideload() deletes the temp file on success; remove
+			 * it explicitly when the sideload fails.
+			 */
+			if ( file_exists( $tmp_file ) ) {
+				wp_delete_file( $tmp_file );
+			}
+			return $attachment_id;
+		}
+
+		$attachment = get_post( $attachment_id );
+
+		$request->set_param( 'context', 'edit' );
+
+		/*
+		 * media_handle_sideload() fires the standard insert hooks (including
+		 * wp_after_insert_post), but not the REST-specific action, so fire it
+		 * here for parity with the uploaded-file path in create_item().
+		 *
+		 * This action is documented in
+		 * wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
+		 */
+		do_action( 'rest_after_insert_attachment', $attachment, $request, true );
+
+		$response = $this->prepare_item_for_response( $attachment, $request );
+		$response->set_status( 201 );
+		$response->header( 'Location', rest_url( rest_get_route_for_post( $attachment_id ) ) );
+
+		return $response;
+	}
+
 	/**
 	 * Removes filters added for client-side media processing.
 	 *