Concrete CMS up to 9.5.2 Frontend Calendar Lightbox Endpoint /ccm/calendar/view_event bID/occurrence_id authorization

Summaryinfo

A vulnerability was found in Concrete CMS up to 9.5.2 and classified as problematic. Impacted is an unknown function of the file /ccm/calendar/view_event of the component Frontend Calendar Lightbox Endpoint. Such manipulation of the argument bID/occurrence_id leads to authorization. This vulnerability is listed as CVE-2026-18121. The attack may be performed from remote. There is no available exploit.

Detailsinfo

A vulnerability was found in Concrete CMS up to 9.5.2. It has been rated as problematic. Affected by this issue is an unknown code of the file /ccm/calendar/view_event of the component Frontend Calendar Lightbox Endpoint. The manipulation of the argument bID/occurrence_id with an unknown input leads to a authorization vulnerability. Using CWE to declare the problem leads to CWE-639. The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data. Impacted is confidentiality. CVE summarizes:

Concrete CMS 9.5.2 and below is vulnerable to an authorization bypass (IDOR) because the frontend calendar lightbox endpoint (/ccm/calendar/view_event/{bID}/{occurrence_id}) does not verify that the caller is permitted to view the calendar that owns the requested event occurrence. The controller loads the occurrence directly from an attacker‑supplied, sequential identifier without confirming that it belongs to the calendar configured on the referenced block. An unauthenticated visitor who can render any public calendar block with lightbox properties enabled could therefore supply an arbitrary occurrence identifier and disclose event metadata — title, date, description, page link, and configured event attributes — from calendars they are not permitted to view. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 6.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N. Thanks riodrwn for reporting.

The advisory is shared for download at documentation.concretecms.org. This vulnerability is handled as CVE-2026-18121 since 07/28/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 09/11/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Type

• Content Management System

Name

• Concrete CMS

Version

• 9.5.0
• 9.5.1
• 9.5.2

License

• open-source

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