CVE-2026-18121: Concrete CMS 9.5.2 and below is vulnerable to an authorization bypass (IDOR) in the Calendar block's frontend event dialog (/ccm/calendar/view_event/{bID}/{occu

Versions reported by the vendor

Affected version ranges reported by the assigning authority (ConcreteCMS). Independent of NVD's CPE analysis and usually ahead of it.

• Concrete CMS Concrete CMS

  • 5.0.0 and later · up to and including 9.5.2affected · git

Other highest-scoring records for the same primary product.

• CVE-2026-10721Concrete CMS below 9.5.2 is vulnerable to PHP Object Injection via unserialize() calls in the in Permission, Cache, and Search components
  33Monitor
• CVE-2026-7888Concrete CMS below 9.5.3 is vulnerable to PHP Object Injection via unserialize() calls in the Workflow, Form block, and File/Set components that lack the allowe
  33Monitor
• CVE-2026-81898Concrete CMS below version 9.5.3 is vulnerable to Stored XSS via country-less Address attribute in Express association views
  30Monitor

Published before and after this one in the same product

• ← EarlierCVE-2026-68527Sep 10, 2026 · Concrete CMS 8.3.0 through 9.5.2 is vulnerable to an authorization bypass through user-controlled key (cross-calendar IDOR) in the Calendar event edit dialog
  23Monitor
• ← EarlierCVE-2026-84432Sep 10, 2026 · Concrete CMS 9 through 9.5.2 is vulnerable to CSRFin the Boards custom slot dialog controller
  21Monitor
• Later →CVE-2026-81905Sep 10, 2026 · Concrete CMS below 9.5.3 does not enforce validation-hash type on redemption, allowing a hash issued for one purpose to be redeemed for another.
  25Monitor

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Root cause

The endpoint fetches the record by the ID sent in the request, but never checks whether the record belongs to the requesting user.

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

1const invoice = await Invoice.findById(req.params.id);2res.json(invoice);

1const invoice = await Invoice.findOne({2  id: req.params.id,3  ownerId: req.user.id,4});5if (!invoice) return res.sendStatus(404);6res.json(invoice);

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.