Executive brief
Concrete CMS, a popular open-source content management system, contains a flaw in its conversation/messaging file attachment feature that allows users without proper permissions to upload files into the file manager. The vulnerability stems from checking permissions after the file has already been stored, rather than before, enabling unauthorized file import that could lead to storage quota abuse or placement of unauthorized files in shared repositories.
Technical details
This is an authorization bypass vulnerability in the conversation attachment upload endpoint (CVE-2026-68533). The root cause is that the "Add Message Attachments" permission check occurs after files have been imported into the file manager, rather than before. An attacker with a low-privilege user account lacking the attachment permission, or an unauthenticated visitor on a guest-posting configuration, can exploit the endpoint to upload and store files of allowed types. The attack requires network access to the endpoint and either valid user credentials or a guest-posting setup; no special user interaction is needed. An attacker can successfully import approved file types into the file manager despite lacking authorization. The vulnerability was patched in Concrete CMS 9.5.3.
Affected products
- Concrete CMS Concrete CMS below 9.5.3
Timeline
- 2026-09-15: disclosed
- 2026-09-15: patched: Fixed in version 9.5.3