Update to Drupal 8.2.2. For more information, see https://www.drupal.org/project/drupal/releases/8.2.2

core/modules/editor/editor.module

@@ -467,6 +467,78 @@ function _editor_delete_file_usage(array $uuids, EntityInterface $entity, $count
   }
 }
 
+/**
+ * Implements hook_file_download().
+ *
+ * @see file_file_download()
+ * @see file_get_file_references()
+ */
+function editor_file_download($uri) {
+  // Get the file record based on the URI. If not in the database just return.
+  /** @var \Drupal\file\FileInterface[] $files */
+  $files = \Drupal::entityTypeManager()
+    ->getStorage('file')
+    ->loadByProperties(['uri' => $uri]);
+  if (count($files)) {
+    foreach ($files as $item) {
+      // Since some database servers sometimes use a case-insensitive comparison
+      // by default, double check that the filename is an exact match.
+      if ($item->getFileUri() === $uri) {
+        $file = $item;
+        break;
+      }
+    }
+  }
+  if (!isset($file)) {
+    return;
+  }
+
+  // Temporary files are handled by file_file_download(), so nothing to do here
+  // about them.
+  // @see file_file_download()
+
+  // Find out if any editor-backed field contains the file.
+  $usage_list = \Drupal::service('file.usage')->listUsage($file);
+
+  // Stop processing if there are no references in order to avoid returning
+  // headers for files controlled by other modules. Make an exception for
+  // temporary files where the host entity has not yet been saved (for example,
+  // an image preview on a node creation form) in which case, allow download by
+  // the file's owner.
+  if (empty($usage_list['editor']) && ($file->isPermanent() || $file->getOwnerId() != \Drupal::currentUser()->id())) {
+    return;
+  }
+
+  // Editor.module MUST NOT call $file->access() here (like file_file_download()
+  // does) as checking the 'download' access to a file entity would end up in
+  // FileAccessControlHandler->checkAccess() and ->getFileReferences(), which
+  // calls file_get_file_references(). This latter one would allow downloading
+  // files only handled by the file.module, which is exactly not the case right
+  // here. So instead we must check if the current user is allowed to view any
+  // of the entities that reference the image using the 'editor' module.
+  if ($file->isPermanent()) {
+    $referencing_entity_is_accessible = FALSE;
+    $references = empty($usage_list['editor']) ? [] : $usage_list['editor'];
+    foreach ($references as $entity_type => $entity_ids) {
+      $referencing_entities = entity_load_multiple($entity_type, $entity_ids);
+      /** @var \Drupal\Core\Entity\EntityInterface $referencing_entity */
+      foreach ($referencing_entities as $referencing_entity) {
+        if ($referencing_entity->access('view', NULL, TRUE)->isAllowed()) {
+          $referencing_entity_is_accessible = TRUE;
+          break 2;
+        }
+      }
+    }
+    if (!$referencing_entity_is_accessible) {
+      return -1;
+    }
+  }
+
+  // Access is granted.
+  $headers = file_get_content_headers($file);
+  return $headers;
+}
+
 /**
  * Finds all files referenced (data-entity-uuid) by formatted text fields.
  *

core/modules/editor/src/Tests/EditorPrivateFileReferenceFilterTest.php

@@ -0,0 +1,100 @@
+<?php
+
+namespace Drupal\editor\Tests;
+
+use Drupal\file\Entity\File;
+use Drupal\Tests\BrowserTestBase;
+use Drupal\user\Entity\Role;
+use Drupal\user\RoleInterface;
+
+/**
+ * Tests Editor module's file reference filter with private files.
+ *
+ * @group editor
+ */
+class EditorPrivateFileReferenceFilterTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = [
+    // Needed for the config: this is the only module in core that utilizes the
+    // functionality in editor.module to be tested, and depends on that.
+    'ckeditor',
+    // Depends on filter.module (indirectly).
+    'node',
+    // Pulls in the config we're using during testing which create a text format
+    // - with the filter_html_image_secure filter DISABLED,
+    // - with the editor set to CKEditor,
+    // - with drupalimage.image_upload.scheme set to 'private',
+    // - with drupalimage.image_upload.directory set to ''.
+    'editor_private_test',
+  ];
+
+  /**
+   * Tests the editor file reference filter with private files.
+   */
+  function testEditorPrivateFileReferenceFilter() {
+    $author = $this->drupalCreateUser();
+    $this->drupalLogin($author);
+
+    // Create a content type with a body field.
+    $this->drupalCreateContentType(['type' => 'page', 'name' => 'Basic page']);
+
+    // Create a file in the 'private:// ' stream.
+    $filename = 'test.png';
+    $src = '/system/files/' . $filename;
+    /** @var \Drupal\file\FileInterface $file */
+    $file = File::create([
+      'uri' => 'private://' . $filename,
+    ]);
+    $file->setTemporary();
+    $file->setOwner($author);
+    // Create the file itself.
+    file_put_contents($file->getFileUri(), $this->randomString());
+    $file->save();
+
+    // The image should be visible for its author.
+    $this->drupalGet($src);
+    $this->assertSession()->statusCodeEquals(200);
+    // The not-yet-permanent image should NOT be visible for anonymous.
+    $this->drupalLogout();
+    $this->drupalGet($src);
+    $this->assertSession()->statusCodeEquals(403);
+
+    // Resave the file to be permanent.
+    $file->setPermanent();
+    $file->save();
+
+    // Create a node with its body field properly pointing to the just-created
+    // file.
+    $node = $this->drupalCreateNode([
+      'type' => 'page',
+      'body' => [
+        'value' => '<img alt="alt" data-entity-type="file" data-entity-uuid="' . $file->uuid() . '" src="' . $src . '" />',
+        'format' => 'private_images',
+      ],
+      'uid' => $author->id(),
+    ]);
+
+    // Do the actual test. The image should be visible for anonymous users,
+    // because they can view the referencing entity.
+    $this->drupalGet($node->toUrl());
+    $this->assertSession()->statusCodeEquals(200);
+    $this->drupalGet($src);
+    $this->assertSession()->statusCodeEquals(200);
+
+    // Disallow anonymous users to view the entity, which then should also
+    // disallow them to view the image.
+    Role::load(RoleInterface::ANONYMOUS_ID)
+      ->revokePermission('access content')
+      ->save();
+    $this->drupalGet($node->toUrl());
+    $this->assertSession()->statusCodeEquals(403);
+    $this->drupalGet($src);
+    $this->assertSession()->statusCodeEquals(403);
+  }
+
+}

core/modules/editor/tests/editor_private_test/config/install/editor.editor.private_images.yml

@@ -0,0 +1,34 @@
+format: private_images
+status: true
+langcode: en
+editor: ckeditor
+settings:
+  toolbar:
+    rows:
+      -
+        -
+          name: Media
+          items:
+            - DrupalImage
+        -
+          name: Tools
+          items:
+            - Source
+  plugins:
+    language:
+      language_list: un
+    stylescombo:
+      styles: ''
+image_upload:
+  status: true
+  scheme: private
+  directory: ''
+  max_size: ''
+  max_dimensions:
+    width: null
+    height: null
+dependencies:
+  config:
+    - filter.format.private_images
+  module:
+    - ckeditor

core/modules/editor/tests/editor_private_test/config/install/filter.format.private_images.yml

@@ -0,0 +1,23 @@
+format: private_images
+name: 'Private images'
+status: true
+langcode: en
+filters:
+  editor_file_reference:
+    id: editor_file_reference
+    provider: editor
+    status: true
+    weight: 0
+    settings: {  }
+  filter_html:
+    id: filter_html
+    provider: filter
+    status: false
+    weight: -10
+    settings:
+      allowed_html: '<img src alt data-entity-type data-entity-uuid>'
+      filter_html_help: true
+      filter_html_nofollow: false
+dependencies:
+  module:
+    - editor

core/modules/editor/tests/editor_private_test/editor_private_test.info.yml

@@ -0,0 +1,9 @@
+name: 'Text Editor Private test'
+type: module
+description: 'Support module for the Text Editor Private module tests.'
+core: 8.x
+package: Testing
+version: VERSION
+dependencies:
+  - filter
+  - ckeditor