Drupal8: Can't upload images via CKEditor/kcfinder

To authenticate, CKEditor calls CRM_Utils_System::loadBootStrap() followed by CRM_Core_Permission::check().

No Drupal8-specific method exists, but when I created one, CRM_Core_Permission_Drupal8::check() couldn't load the current user. I assume something is failing in the loadBootstrap() call but I can't figure out what it is.

This SE post seems to contain details, but I'm not Drupal-savvy enough to interpret it.