Check rootline for feGroups from page with extendToSubpages set

Classes/FrontendEnvironment/Tsfe.php

@@ -144,6 +144,20 @@ protected function initializeTsfe(int $pageId, int $language = 0, ?int $rootPage
                 $userGroups = explode(',', $pageRecord['fe_group']);
             } else {
                 $userGroups = [0, -1];
+
+                // Check rootline for feGroups from page with extendToSubpages set
+                $rootline = BackendUtility::BEgetRootLine($pageId);
+                // remove the current page from the rootline
+                array_shift($rootline);
+                foreach ($rootline as $page) {
+                    // Skip root node, invalid pages and pages which do not define extendToSubpages
+                    if ((int)($page['uid'] ?? 0) <= 0 || !($page['extendToSubpages'] ?? false)) {
+                        continue;
+                    }
+                    $userGroups = explode(',', $page['fe_group']);
+                    // Stop as soon as a page in the rootline has extendToSubpages set
+                    break;
+                }
             }
             $feUser->user = ['uid' => 0, 'username' => '', 'usergroup' => implode(',', $userGroups) ];
             $feUser->fetchGroupData($serverRequest);