diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 79689c0..82989dd 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -689,26 +689,41 @@ function simpletest_clean_database() {
  *   (optional) The relative path to directory of a particular test site. Must
  *   to be of the following pattern: sites/simpletest/12345678. If ommited, all
  *   the test sites' directories found in sites/simpletest will be removed.
+ *
+ * @return bool
+ *   TRUE if directory is removed, FALSE otherwise. Additionally watchdog
+ *   messages will be emitted if any path attempted to remove fails.
+ *
+ * @see file_unmanaged_delete()
  */
 function simpletest_clean_temporary_directories($directory = NULL) {
+  $directories = [];
   $count = 0;
-  $root = \Drupal::root() . '/sites/simpletest/';
-
-  if (is_dir($root)) {
-    // Prevent removing wrong directory accidentally passed as an argument.
-    $directory = $directory ? explode('sites/simpletest/', $directory) : FALSE;
-    $files = [];
-
-    if (isset($directory[1]) && is_dir($root . $directory[1])) {
-      $files[] = $directory[1];
+  $result = FALSE;
+  $simpletest_root = \Drupal::root() . '/sites/simpletest/';
+
+  if (is_dir($simpletest_root)) {
+    // Get the type of $directory argument for debugging in the watchdog error
+    // message if it is not valid string or NULL.
+    $directory = is_string($directory) && !empty($directory) ? $directory : gettype($directory);
+    // Do not recognize any, except expected directory pattern as it may cause
+    // catastrophic consequences on the entire Drupal site.
+    preg_match('/^(sites\/simpletest\/)(.*)/', $directory, $matches);
+
+    if (!empty($matches[2]) && is_dir($simpletest_root . $matches[2])) {
+      $directories[] = $matches[2];
+    }
+    elseif ($directory == 'NULL') {
+      $directories = scandir($simpletest_root);
     }
     else {
-      $files = scandir($root);
+      $logger = \Drupal::logger('Testing');
+      $logger->error('The %path is not eligible for removing in %func().', ['%path' => $directory, '%func' => __FUNCTION__]);
     }
 
-    foreach ($files as $file) {
+    foreach ($directories as $file) {
       if ($file[0] != '.') {
-        $path = $root . $file;
+        $path = $simpletest_root . $file;
         // When the webserver runs with the same system user as the test
         // runner, we can make read-only files writable again. If not, chmod
         // will fail while the file deletion still works if file permissions
@@ -716,11 +731,6 @@ function simpletest_clean_temporary_directories($directory = NULL) {
         $result = file_unmanaged_delete_recursive($path, function ($any_path) {
           @chmod($any_path, 0700);
         });
-        if ($directory) {
-          // We do not need any more if this function was called from within a
-          // running test instead of using "Clean environment" button.
-          return $result;
-        }
         $count++;
       }
     }
@@ -732,6 +742,8 @@ function simpletest_clean_temporary_directories($directory = NULL) {
   else {
     drupal_set_message(t('No temporary directories to remove.'));
   }
+
+  return $result;
 }
 
 /**