Issue #2867749 by pfrenssen, claudiu.cristea: Parsing an URL with another URL in the query arguments throws undefined offset notice

core/lib/Drupal/Component/Utility/UrlHelper.php

@@ -142,7 +142,12 @@ class UrlHelper {
 
     // External URLs: not using parse_url() here, so we do not have to rebuild
     // the scheme, host, and path without having any use for it.
-    if (strpos($url, '://') !== FALSE) {
+    // The URL is considered external if it contains the '://' delimiter. Since
+    // a URL can also be passed as a query argument, we check if this delimiter
+    // appears in front of the '?' query argument delimiter.
+    $scheme_delimiter_position = strpos($url, '://');
+    $query_delimiter_position = strpos($url, '?');
+    if ($scheme_delimiter_position !== FALSE && ($query_delimiter_position === FALSE || $scheme_delimiter_position < $query_delimiter_position)) {
       // Split off everything before the query string into 'path'.
       $parts = explode('?', $url);
 

core/tests/Drupal/Tests/Core/UrlTest.php

@@ -736,6 +736,7 @@ class UrlTest extends UnitTestCase {
       ['/?page=1000'],
       ['?page=1000'],
       ['?breed=bengal&page=1000'],
+      ['?referrer=https://kittenfacts'],
       // Paths with various token formats but no leading slash.
       ['/[duckies]'],
       ['/%bunnies'],