A prototype of native WP multisite single sign-on (SSO)

wp-multisite-sso.php

<?php
/**
 * Plugin Name: Multisite SSO Login
 * Network: true
 * 
 * @see https://x.com/konstruktors/status/1838956536049094659
 */

if ( ! is_multisite() ) {
	return;
}

function is_sso_site() {
	return is_main_site();
}

function get_sso_login_url( $query_args = [] ) {
	$query_args = wp_parse_args( 
		$query_args,
		[
			'multisite_sso_blog_id' => get_current_blog_id(),
		]
	);

	return add_query_arg( $query_args, network_site_url( 'wp-login.php', 'login' ) );
}

function get_sso_callback_url( $blog_id, $query_args = [] ) {
	return add_query_arg( $query_args, get_site_url( $blog_id, 'wp-login.php', 'login' ) );
}

add_filter(
	'login_message',
	function ( $messages ) {
		if ( is_user_logged_in() || is_sso_site() ) {
			return;
		}

		$sso_args = [];

		$redirect_to = filter_input( INPUT_GET, 'redirect_to', FILTER_SANITIZE_URL );
		if ( $redirect_to ) {
			$sso_args['redirect_to'] = rawurlencode( $redirect_to );
		}

		$messages .= '<style type="text/css">
			.login .wp-multisite-sso-login--cta a { font-size:inherit; float:none; display:inline-block; width:100%; clear:both; text-align:center; }
		</style>';

		$messages .= sprintf(
			'<div class="message wp-multisite-sso-login--cta">
				<p><a href="%s" class="button button-primary button-large">%s</a></p>
			</div>',
			esc_url( get_sso_login_url( $sso_args ) ),
			esc_html__( 'Login with Multisite SSO' ),
		);

		return $messages;
	},
	200
);

add_action(
	'login_init',
	function () {
		if ( ! is_sso_site() || is_user_logged_in() ) {
			return;
		}

		$blog_id = isset( $_REQUEST['multisite_sso_blog_id'] ) ? absint( $_REQUEST['multisite_sso_blog_id'] ) : null;

		if ( empty( $blog_id ) ) {
			return;
		}

		$blog = get_blog_details( $blog_id );

		if ( ! $blog ) {
			return;
		}

		add_action(
			'login_form',
			function () use ( $blog_id ) {
				?>
				<input type="hidden" name="multisite_sso_blog_id" value="<?php echo esc_attr( $blog_id ); ?>">
				<?php
			}
		);

		add_filter(
			'wp_login_errors',
			function ( $errors ) use ( $blog ) {
				$message = sprintf( 
					esc_html__( 'You are logging into %s' ), 
					sprintf( 
						'<a href="%s" target="_blank">%s</a>', 
						esc_url( $blog->home ), 
						esc_html( $blog->blogname ) 
					)
				);

				$errors->add( 'multisite_sso_notice', $message, 'message' );

				return $errors;
			}
		);
	}
);

add_filter(
	'login_redirect',
	function ( $redirect_to, $requested_redirect_to, $user ) {
		if ( ! is_sso_site() ) {
			return $redirect_to;	
		}

		$blog_id = isset( $_REQUEST['multisite_sso_blog_id'] ) ? absint( $_REQUEST['multisite_sso_blog_id'] ) : null;

		if ( ! empty( $blog_id ) && $user instanceof WP_User ) {
			// Add the destination blog hostname to the allowed safe-redirect list.
			add_filter(
				'allowed_redirect_hosts',
				function ( $hosts ) use ( $blog_id ) {
					$hosts[] = wp_parse_url( get_site_url( $blog_id ), PHP_URL_HOST );
					$hosts[] = wp_parse_url( get_home_url( $blog_id ), PHP_URL_HOST );

					return $hosts;
				}
			);

			$session_token = wp_generate_password( 42, false, false );

			// Relay the private session token through a back-channel.
			update_user_meta( $user->ID, 'multisite_sso_token', $session_token );

			$args = [
				'multisite_sso_user_id' => $user->ID,
				'multisite_sso_token' => wp_hash( $session_token ), // Transfer hashed for one-time verification.
			];

			if ( ! empty( $requested_redirect_to ) && $redirect_to !== $requested_redirect_to ) {
				$args['redirect_to'] = urlencode( $requested_redirect_to );
			}

			return get_sso_callback_url( $blog_id, $args );
		}

		return $redirect_to;
	},
	100,
	3
);

add_action(
	'login_init',
	function () {
		if ( is_sso_site() || is_user_logged_in() ) {
			return;
		}
		
		$login_user_id = isset( $_GET['multisite_sso_user_id'] ) ? absint( $_GET['multisite_sso_user_id'] ) : null;
		$login_token_hash = isset( $_GET['multisite_sso_token'] ) ? sanitize_text_field( $_GET['multisite_sso_token'] ) : null;

		if ( ! $login_user_id || ! $login_token_hash ) {
			return;
		}

		// Get the session token from the SSO source and delete it to ensure it is used only once.
		$token_source = get_user_meta( $login_user_id, 'multisite_sso_token', true );
		delete_user_meta( $login_user_id, 'multisite_sso_token' );

		if ( $token_source && hash_equals( wp_hash( $token_source ), $login_token_hash ) ) {
			wp_clear_auth_cookie();
			wp_set_auth_cookie( $login_user_id );
			wp_set_current_user( $login_user_id );

			$redirect_to = filter_input( INPUT_GET, 'redirect_to', FILTER_SANITIZE_URL );
			if ( ! $redirect_to ) {
				$redirect_to = get_dashboard_url( $login_user_id );
			}

			wp_redirect( $redirect_to );
			exit;
		}
	}
);