nico.b
--- a/core/modules/filter/filter.module
+++ b/core/modules/filter/filter.module
+      $private_image_path = 'private://' . $private_image_path;
+      $private_image_path = urldecode($private_image_path);
+      $image_object = \Drupal::service('image.factory')->get($private_image_path);
+      if ($image_object->getFileSize()) {
+        // The image has the right path. Erroneous images are dealt with below.
+        continue;
+      }
+    }
+    elseif (mb_substr($src, 0, $base_path_length) === $base_path) {
      // Remove the $base_path to get the path relative to the Drupal root.
      // Ensure the path refers to an actual image by prefixing the image source
      // with the Drupal root and running getimagesize() on it.
      $local_image_path = $local_dir . mb_substr($src, $base_path_length);
      $local_image_path = rawurldecode($local_image_path);
+
+      // Check for query string and remove it if present.
--- a/core/modules/filter/filter.module
+++ b/core/modules/filter/filter.module
    // Verify that $src starts with $base_path.
    // This also ensures that external images cannot be referenced.
    $src = $image->getAttribute('src');
-    if (mb_substr($src, 0, $base_path_length) === $base_path) {
+    if (mb_substr($src, 0, $private_path_length) === $private_path) {