diff --git a/wp-includes/rewrite.php b/wp-includes/rewrite.php
index 705e5f2bc8..943d7d0079 100644
--- a/wp-includes/rewrite.php
+++ b/wp-includes/rewrite.php
@@ -471,6 +471,14 @@ function url_to_postid( $url ) {
 	 */
 	$url = apply_filters( 'url_to_postid', $url );
 
+	$url_host      = str_replace( 'www.', '', parse_url( $url, PHP_URL_HOST ) );
+	$home_url_host = str_replace( 'www.', '', parse_url( home_url(), PHP_URL_HOST ) );
+
+	// Bail early if the URL does not belong to this site.
+	if ( $url_host && $url_host !== $home_url_host ) {
+		return 0;
+	}
+
 	// First, check to see if there is a 'p=N' or 'page_id=N' to match against
 	if ( preg_match('#[?&](p|page_id|attachment_id)=(\d+)#', $url, $values) )	{
 		$id = absint($values[2]);
diff --git a/wp-includes/version.php b/wp-includes/version.php
index af6fcf44cf..0d3a799f3c 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-beta1-41785';
+$wp_version = '4.9-beta1-41786';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.