amp-form-submission-proxy.php

<?php
/**
 * AMP Form Submission Proxy plugin bootstrap.
 *
 * @package   Google\AMP_Form_Submission_Proxy
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2021 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Form Submission Proxy
 * Plugin URI: https://gist.github.com/westonruter/c08134bf8b33a49ef780a70b337d456f
 * Description: Prototype to implement <a href="https://github.com/ampproject/amp-wp/issues/4191">amp-wp#4191</a> specifically to proxy external form submissions, such as to Mailchimp or Genesis eNews Extended.
 * Version: 0.2.1
 * Author: Weston Ruter, Google
 * Author URI: https://weston.ruter.net/
 * License: GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * Gist Plugin URI: https://gist.github.com/westonruter/c08134bf8b33a49ef780a70b337d456f
 * Update URI: https://gist.github.com/westonruter/c08134bf8b33a49ef780a70b337d456f
 */

namespace Google\AMP_Form_Submission_Proxy;

use function register_rest_route;
use WP_REST_Server;
use WP_REST_Request;

const URL_SAFE_BASE64_CHAR_REPLACEMENTS = [
	'+/=',
	'._-',
];

const REST_API_NAMESPACE = 'amp-form-submission-proxy/v1';
const REST_API_ROUTE     = '/proxy';

add_filter(
	'amp_content_sanitizers',
	static function ( $sanitizers ) {
		require_once __DIR__ . '/FormActionRewriteSanitizer.php';
		$sanitizers[ FormActionRewriteSanitizer::class ] = [
			'parsed_home_url' => wp_parse_url( home_url( '/' ) ),
		];
		return $sanitizers;
	}
);

/**
 * URL-safe base64-encode.
 *
 * @see https://stackoverflow.com/a/5835352/93579
 * @param string $input Input.
 * @return string
 */
function url_safe_base64_encode( $input ) {
	return strtr( base64_encode( $input ), URL_SAFE_BASE64_CHAR_REPLACEMENTS[0], URL_SAFE_BASE64_CHAR_REPLACEMENTS[1] );
}

/**
 * URL-safe base64-decode.
 *
 * @see https://stackoverflow.com/a/5835352/93579
 * @param string $input Input.
 * @return string
 */
function url_safe_base64_decode( $input ) {
	return base64_decode( strtr( $input, URL_SAFE_BASE64_CHAR_REPLACEMENTS[1], URL_SAFE_BASE64_CHAR_REPLACEMENTS[0] ) );
}

/**
 * Get REST route.
 *
 * @param string $url URL.
 *
 * @return string
 */
function get_proxy_rest_route( $url ) {
	return rest_url(
		sprintf(
			'/%s%s/%s/%s',
			REST_API_NAMESPACE,
			REST_API_ROUTE,
			urlencode( wp_hash( $url, 'nonce' ) ),
			urlencode( url_safe_base64_encode( $url ) )
		)
	);
}

add_action(
	'rest_api_init',
	static function () {
		register_rest_route(
			REST_API_NAMESPACE,
			REST_API_ROUTE . '/(?P<url_hmac>[0-9a-f]{32})/(?P<base64_url>[a-zA-Z0-9._-]+)',
			[
				'methods'             => WP_REST_Server::CREATABLE,
				'permission_callback' => '__return_true',
				'callback'            => static function ( WP_REST_Request $request ) {
					require_once __DIR__ . '/Proxy.php';
					$proxy = new Proxy( $request );
					return $proxy->submit();
				},
			]
		);
	}
);

FormActionRewriteSanitizer.php

<?php
/**
 * FormActionRewriteSanitizer file.
 *
 * @package Google\AMP_Form_Submission_Proxy
 */

namespace Google\AMP_Form_Submission_Proxy;

use AMP_Base_Sanitizer;
use AmpProject\Dom\Element;

/**
 * Class Sanitizer
 */
class FormActionRewriteSanitizer extends AMP_Base_Sanitizer {

	/** @var array[] */
	protected $DEFAULT_ARGS = [
		'parsed_home_url' => [],
		'rest_route'      => '',
	];

	/**
	 * Sanitize.
	 */
	public function sanitize() {
		if ( empty( $this->args['parsed_home_url']['host'] ) || $this->args['rest_route'] ) {
			return;
		}

		$post_forms = $this->dom->xpath->query( '//form[ @action-xhr and @method = "post" ]' );
		foreach ( $post_forms as $post_form ) {
			$this->convert_form( $post_form );
		}
	}

	/**
	 * Convert form.
	 *
	 * @param Element $post_form Post form.
	 */
	private function convert_form( Element $post_form ) {
		/** @var Element $post_form */
		$action_url = $post_form->getAttribute( 'action-xhr' );

		// Make sure that the form sanitizer converted this non-AMP form.
		$parsed_action = wp_parse_url( $action_url );
		if ( empty( $parsed_action ) || empty( $parsed_action['host'] ) || empty( $parsed_action['query'] ) ) {
			return;
		}
		$query_vars = [];
		parse_str( $parsed_action['query'], $query_vars );
		if ( empty( $query_vars['_wp_amp_action_xhr_converted'] ) ) {
			return;
		}

		// Skip internal form submissions for now.
		if ( $this->args['parsed_home_url']['host'] === $parsed_action['host'] ) {
			return;
		}

		$action_url = remove_query_arg( '_wp_amp_action_xhr_converted', $action_url );

		$post_form->setAttribute( 'action-xhr', get_proxy_rest_route( $action_url ) );

		// Prevent amp-form from failing to redirect with an error:
		// > Redirecting to target=_blank using AMP-Redirect-To is currently not supported, use target=_top instead.
		$post_form->setAttribute( 'target', '_top' );
	}
}

Proxy.php

<?php
/**
 * Proxy file.
 *
 * @package Google\AMP_Form_Submission_Proxy
 */

namespace Google\AMP_Form_Submission_Proxy;

use WP_REST_Response;
use WP_REST_Request;
use Exception;
use AmpProject\Url;
use DOMDocument;
use DOMXPath;
use DOMElement;

// phpcs:disable WordPress.WP.I18n.TextDomainMismatch

/**
 * Class Sanitizer
 */
class Proxy {

	/** @var WP_REST_Request */
	private $request;

	/** @var string|null */
	private $origin;

	/**
	 * Proxy constructor.
	 *
	 * @param WP_REST_Request $request Request.
	 */
	public function __construct( WP_REST_Request $request ) {
		$this->request = $request;
		$this->origin  = $request->get_header( 'origin' );
	}

	/**
	 * Submit.
	 *
	 * @return WP_REST_Response
	 */
	public function submit() {
		if ( empty( $this->origin ) || ! wp_validate_redirect( $this->origin ) ) {
			return new WP_REST_Response(
				[
					'message' => __( 'Missing or invalid <code>Origin</code> request header.', 'amp-form-submission-proxy' ),
				],
				400,
				[ 'Access-Control-Allow-Origin' => '*' ]
			);
		}

		$url        = url_safe_base64_decode( urldecode( $this->request->get_param( 'base64_url' ) ) );
		$parsed_url = wp_parse_url( $url );
		if ( empty( $parsed_url ) || empty( $parsed_url['host'] ) ) {
			return $this->create_error_response( __( 'Form action URL parse error.', 'amp-form-submission-proxy' ), 400 );
		}
		if ( wp_parse_url( home_url( '/' ), PHP_URL_HOST ) === $parsed_url['host'] ) {
			return $this->create_error_response( __( 'Cannot submit to self.', 'amp-form-submission-proxy' ), 400 );
		}
		$expected_hmac = wp_hash( $url, 'nonce' );
		if ( ! hash_equals( $expected_hmac, $this->request->get_param( 'url_hmac' ) ) ) {
			return $this->create_error_response( __( 'HMAC mismatch.', 'amp-form-submission-proxy' ), 400 );
		}

		$headers = $this->request->get_headers();

		// Remove irrelevant headers.
		unset(
			$headers['cookie'],
			$headers['host'],
			$headers['content_length'],
			$headers['content_type'],
			$headers['amp_same_origin']
		);

		$forwarded_headers = [];
		foreach ( $headers as $header_name => $header_value ) {
			// This is the reverse of \WP_REST_Request::canonicalize_header_name().
			$header_name = str_replace( '_', '-', $header_name );

			if ( is_array( $header_value ) ) {
				$header_value = implode( ', ', $header_value );
			}
			$forwarded_headers[ $header_name ] = $header_value;
		}

		$response = wp_remote_post(
			$url,
			[
				'timeout'     => 30,
				'headers'     => $forwarded_headers,
				'body'        => $this->request->get_body_params(),
				'redirection' => 0,
			]
		);
		if ( is_wp_error( $response ) ) {
			return $this->create_error_response(
				__( 'Submission error: ', 'amp-form-submission-proxy' ) . $response->get_error_message(),
				500
			);
		}

		$message     = null;
		$status_code = wp_remote_retrieve_response_code( $response );
		$status_text = wp_remote_retrieve_response_message( $response );

		try {
			// @todo It appears that relative URLs may be made absolute with the wrong base URL.
			$location = wp_remote_retrieve_header( $response, 'location' );
			if ( $status_code >= 300 && $status_code < 400 && $location ) {
				$redirect_url = new Url( $location, new Url( $url ) );

				return new WP_REST_Response(
					[
						'redirecting' => true,
						'message'     => __( 'Redirecting…', 'amp-form-submission-proxy' ),
					],
					200,
					[
						'AMP-Redirect-To'               => (string) $redirect_url,
						'Access-Control-Expose-Headers' => 'AMP-Redirect-To',
						'Access-Control-Allow-Origin'   => $this->origin,
					]
				);
			}
		} catch ( Exception $exception ) {
			$this->create_error_response(
				__( 'An exception occurred while attempting to redirect.', 'amp-form-submission-proxy' ),
				500
			);
		}

		$content_type = wp_remote_retrieve_header( $response, 'content-type' );
		$body         = wp_remote_retrieve_body( $response );
		if ( 0 === strpos( $content_type, 'application/json' ) ) {
			$body = json_decode( $body, true );
			if ( isset( $body['message'] ) && is_string( $body['message'] ) ) {
				$message = $body['message'];
			}
		} elseif ( 0 === strpos( $content_type, 'text/html' ) ) {
			$libxml_previous_state = libxml_use_internal_errors( true );
			$dom                   = new DOMDocument();
			$dom->loadHTML( $body );
			$xpath = new DOMXPath( $dom );
			libxml_clear_errors();
			libxml_use_internal_errors( $libxml_previous_state );

			// Add special handling for Mailchimp success/failure page since redirection is not performed.
			if ( preg_match( '/\.list-manage\.com$/', wp_parse_url( $url, PHP_URL_HOST ) ) ) {
				$error_texts   = $xpath->query( '//div[ @class = "errorText" ]' );
				$template_body = $dom->getElementById( 'templateBody' );
				if ( $error_texts->length > 0 ) {
					$status_code = 400;
					$status_text = 'Bad Request';
					$message     = implode(
						' ',
						array_map(
							static function ( DOMElement $element ) {
								return $element->textContent;
							},
							iterator_to_array( $error_texts )
						)
					);
				} elseif ( $template_body instanceof DOMElement ) {
					$message = $dom->saveHTML( $template_body );
				}
			}
		}

		$data = compact( 'status_code', 'status_text' );
		if ( $message ) {
			$data['message'] = $message;
		}

		return new WP_REST_Response(
			$data,
			$status_code,
			[
				'Access-Control-Allow-Origin' => $this->origin,
			]
		);
	}

	/**
	 * Create error response.
	 *
	 * @param string $message Message.
	 * @param int    $code    Code.
	 * @return WP_REST_Response
	 */
	private function create_error_response( $message, $code = 400 ) {
		return new WP_REST_Response(
			compact( 'message' ),
			$code,
			[ 'Access-Control-Allow-Origin' => $this->origin ]
		);
	}
}

Yes, unless the external endpoint is able to send back JSON responses and/or the AMP-Redirect-To response header. There is one alternative and that is to utilize an iframe. Here you can see an example of that: https://amp-external-subscription-form-in-iframe.glitch.me/