Executive brief
FluentCMS is an ASP.NET Core content management system used to manage multi-tenant websites. A site administrator assigned to one site can bypass authorization controls to view, upload, and modify files and folders belonging to other sites in the same deployment, compromising data isolation and integrity across organizational boundaries.
Technical details
The vulnerability is a cross-tenant authorization bypass in the PermissionManager.HasAccess() method and related File/Folder services. The root cause is improper caching: authorization data (user roles, roles, permissions) is fetched for one site and cached using null-coalescing operators (??=), then reused without revalidation when evaluating subsequent sites. Additionally, FolderService and FileService access objects by UUID without validating that the requesting user has authorization for the object's owning site. An authenticated administrator restricted to Site A can enumerate Site B, supply a Site B folder UUID to the File Management API, and read, upload, or modify Site B assets. The attack requires valid authentication but no special privileges beyond admin access to a single site.
Affected products
- FluentCMS FluentCMS up to 0.0.5
Timeline
- 2026-07-25: disclosed: Issue #2409 opened by researcher
- 2026-09-07: advisory: CVE-2026-86271 published