Craft CMS versions >= 5.0.0-RC1, <= 5.9.13 and >= 4.0.0-RC1, <= 4.17.7 contain an authorization bypass in the assets/preview-file endpoint. The action does not enforce per-asset view authorization before returning preview content, allowing an authenticated low-privileged user to supply a controlled assetId for an asset they are not permitted to view and still receive preview response data (previewHtml), including a private preview image route containing the target private assetId. Fixed in 5.9.14 and 4.17.8.

Craft CMS: Authorized asset "preview file" requests bypass allows users without asset access to retrieve private preview metadata