Ticket #17376: 17376.diff

wp-admin/admin.php

@@ -76 +76 @@
 
 require_once(ABSPATH . 'wp-admin/includes/admin.php');
 
+if( is_multisite() ) {
+        invalid_site_admin_redirect();
+}
+
 auth_redirect();
 
 // Schedule trash collection

wp-admin/includes/ms.php

@@ -811 +811 @@
 </script>
 <?php
 }
+
+/**
+ * Does a canonical redirect for the admin area.
+ *
+ * @since 3.9.0
+ */
+function invalid_site_admin_redirect() {
+        global $current_blog;
+
+        // Redirect to the canonical admin URL if relevant
+        if ( preg_match( '#(.*/wp-admin/)(.*)#', $_SERVER['REQUEST_URI'], $requested_admin_url )
+                && trailingslashit( $requested_admin_url[1] ) !== parse_url( admin_url(), PHP_URL_PATH )
+        ) {
+                wp_redirect(
+                        add_query_arg(
+                                array( 'invalid_site_admin' => '1' ),
+                                get_admin_url( $current_blog->blog_id, $requested_admin_url[2] )
+                        )
+                );
+        }
+
+        if ( isset( $_GET['invalid_site_admin'] ) ) {
+                // Display a notice on the page that was requested
+                add_action( 'all_admin_notices', 'invalid_site_admin_redirect_notice' );
+        }
+
+}
+
+/**
+ * Adds a notice after a canonical redirect for the admin area.
+ *
+ * @since 3.9.0
+ */
+function invalid_site_admin_redirect_notice() {
+        echo '<div class="error">' . __( 'You were redirected because the site whose admin area you were in does not exist.' ) . '</div>';
+}