BookStack is a platform for storing and organising information and documentation. In BookStack before version 0.30.5, a user with permissions to edit a page could set certain image URL's to manipulate functionality in the exporting system, which would allow them to make server side requests and/or have access to a wider scope of files within the BookStack file storage locations. The issue was addressed in BookStack v0.30.5. As a workaround, page edit permissions could be limited to only those that are trusted until you can upgrade.