diff --git a/core/lib/Drupal/Core/Form/ConfigFormBase.php b/core/lib/Drupal/Core/Form/ConfigFormBase.php
index e7878a7db0..8f24090750 100644
--- a/core/lib/Drupal/Core/Form/ConfigFormBase.php
+++ b/core/lib/Drupal/Core/Form/ConfigFormBase.php
@@ -4,6 +4,7 @@
 
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\Core\Render\Markup;
 
 /**
  * Base class for implementing system configuration forms.
@@ -34,6 +35,24 @@ public static function create(ContainerInterface $container) {
    * {@inheritdoc}
    */
   public function buildForm(array $form, FormStateInterface $form_state) {
+    $config_overrides = $this->getConfigOverrides();
+
+    // If there are config overrides, generate a nice message that informs the
+    // user about this.
+    if ($config_overrides) {
+      $text = '';
+      // Overrides are grouped by config names (there can be multiple
+      // ones per form).
+      foreach ($config_overrides as $config_name => $values) {
+        $text .= "<br /><strong>" . $config_name . "</strong>:";
+        foreach ($values as $key => $value) {
+          $text .= "<br />- " . $key . ": \"" . key($value) . "\" -> \"" . reset($value) . "\"";
+        }
+      }
+      $text = Markup::create($text);
+      drupal_set_message(t("This form has values that are overridden either by a module or by settings files. <br /> You can still change them and they will be saved in the config storage, but the changes will maybe not appear within the site.<br />@overrides", ['@overrides' => $text]), "warning");
+    }
+
     $form['actions']['#type'] = 'actions';
     $form['actions']['submit'] = [
       '#type' => 'submit',
diff --git a/core/lib/Drupal/Core/Form/ConfigFormBaseTrait.php b/core/lib/Drupal/Core/Form/ConfigFormBaseTrait.php
index 4e60d92690..ddffb3ba34 100644
--- a/core/lib/Drupal/Core/Form/ConfigFormBaseTrait.php
+++ b/core/lib/Drupal/Core/Form/ConfigFormBaseTrait.php
@@ -3,6 +3,7 @@
 namespace Drupal\Core\Form;
 
 use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Component\Utility\DiffArray;
 
 /**
  * Provides access to configuration for forms.
@@ -58,6 +59,96 @@ protected function config($name) {
   }
 
   /**
+   * Gets an array of all config keys that are overridden.
+   *
+   * @return array
+   *   An associative array which contains all config values that are
+   *   overridden by either modules or the settings files. The array is first
+   *   keyed by the name of the config, then the key (concatenated with dots if
+   *   the keys are multidimensional) and last the original string of the
+   *   config, the value is the overridden string. Example:
+   *   'system.site' => array('page.403' => array('original' => 'overridden'))
+   */
+  protected function getConfigOverrides() {
+    /** @var \Drupal\Core\Config\ConfigFactoryInterface $config_factory */
+    if (method_exists($this, 'configFactory')) {
+      $config_factory = $this->configFactory();
+    }
+    elseif (property_exists($this, 'configFactory')) {
+      $config_factory = $this->configFactory;
+    }
+    if (!isset($config_factory) || !($config_factory instanceof ConfigFactoryInterface)) {
+      throw new \LogicException('No config factory available for ConfigFormBaseTrait');
+    }
+
+    $diff = [];
+
+    // Forms define which config objects they are using for editing, therefore
+    // these are also the ones which can be overwritten.
+    foreach ($this->getEditableConfigNames() as $name) {
+      // Loading the config with ->get() which means they are loaded overridden.
+      $config = $config_factory->get($name);
+
+      // Diff the overwritten with the original config, returns a diff with only
+      // values that have overrides, others are removed.
+      if (!$config_values = $config->getOriginal('', FALSE)) {
+        continue;
+      }
+      $diff_overwritten = DiffArray::diffAssocRecursive($config->get(), $config_values);
+      // We are also interested in the original strings, so we do the same on
+      // the other side around.
+      $diff_original = DiffArray::diffAssocRecursive($config_values, $config->get());
+
+      // Config objects are multidimensional. Flatten them with the keys
+      // concatenated with dots.
+      $this->flattenArray($diff_overwritten);
+      $this->flattenArray($diff_original);
+
+      // Loop over the arrays and generate the return array.
+      foreach ($diff_overwritten as $key => $value) {
+        $diff[$name][$key] = [$diff_original[$key] => $value];
+      }
+
+    }
+    return $diff;
+  }
+
+  /**
+   * Flattens a nested array.
+   *
+   * The scheme used is:
+   *   'key' => array('key2' => array('key3' => 'value'))
+   * Becomes:
+   *   'key.key2.key3' => 'value'
+   *
+   * This function takes an array by reference and will modify it.
+   *
+   * @param array &$messages
+   *   The array that will be flattened.
+   * @param array $subnode
+   *   Current subnode being parsed, used internally for recursive calls.
+   * @param string $path
+   *   Current path being parsed, used internally for recursive calls.
+   */
+  private function flattenArray(array &$messages, array $subnode = NULL, $path = NULL) {
+    if (NULL === $subnode) {
+      $subnode = &$messages;
+    }
+    foreach ($subnode as $key => $value) {
+      if (is_array($value)) {
+        $node_path = $path ? $path . '.' . $key : $key;
+        $this->flattenArray($messages, $value, $node_path);
+        if (NULL === $path) {
+          unset($messages[$key]);
+        }
+      }
+      elseif (NULL !== $path) {
+        $messages[$path . '.' . $key] = $value;
+      }
+    }
+  }
+
+  /**
    * Gets the configuration names that will be editable.
    *
    * @return array