CVE-2026-87028: Cross-Board IDOR in the Board Custom Slot Preview in Concrete CMS 9.0.0 through 9.5.3 Discloses Restricted Page Summary Fields
Concrete CMS 9 through 9.5.3 did not confirm that a board InstanceItem submitted to the custom-slot preview endpoint belonged to the board instance the requesting user was authorized to edit, and did not enforce page-view permission before generating page-backed summary content. As a result, an authenticated user holding edit-board-contents permission on a single board instance could submit the identifier of an item belonging to a different board instance and receive summary fields, including the page title and description, of an underlying page the same user was otherwise forbidden to view. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 5.3 with vector CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N. Thanks Pakung for reporting.
Affected Software
Event History
Frequently Asked Questions
Who can exploit this issue?
An authenticated user needs edit-board-contents permission for at least one board instance. They can then submit an InstanceItem identifier associated with a different board instance.
What information can be exposed?
The preview endpoint can return page-backed summary fields, including the title and description, for an underlying page that the user is not permitted to view.
Does exploitation require user interaction or elevated administrative access?
No user interaction is required. The supplied CVSS vector identifies the attack as network-accessible with low privileges required; the required privilege is edit-board-contents permission on a board instance.