CVE-2026-18121 Missing authorization in Concrete CMS
Unauthenticated attackers can view private calendar event details by manipulating IDs in the frontend lightbox endpoint if lightbox properties are enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-18121?
CVE-2026-18121 is a medium-severity missing authorization vulnerability in Concrete CMS, published on 10 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2026-18121?
How do I fix CVE-2026-18121?
No fix has been published for CVE-2026-18121 yet. Limit exposure of the affected software until one is available.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-18121 being exploited?
There is no known exploitation of CVE-2026-18121 in the wild and no public exploit code.
Sources
Also known as GHSA-cwc3-87mf-4ggw.