When adding a private file via the editor in Drupal and Drupal Core 8.2.x before 8.2.7 and between 8.3.0-alpha1 up to 8.3.0-rc1, the editor will not correctly check access for the file being attached, resulting in an access bypass.
CWE-863 - Incorrect Authorization
Authorization is a security mechanism performed by an application to grant or deny access to the requested resources by verifying the privileges of the user. When an application lacks effective authorization mechanisms, it enables unauthorized users to gain unintended privileges and illegitimate access to resources. Such a vulnerability may result in exposure of sensitive information, denial of service, arbitrary code execution, and complete system takeover.