# 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.

- **Severity:** Medium 6.3 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 10 Sep 2026
- **Updated:** 11 Sep 2026
- **Also known as:** GHSA-cwc3-87mf-4ggw

## 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.

Weakness: CWE-862 (Missing Authorization), CWE-639 (Authorization Bypass Through User-Controlled Key)

## 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.

## Is CVE-2026-18121 being exploited?

There is no known exploitation of CVE-2026-18121 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-18121 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-18121 in the wild and no public exploit code.

Affected product: Concrete CMS. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-18121/ (Markdown: https://insights.sbomify.com/CVE-2026-18121/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-18121
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-18121
- OSV: https://osv.dev/vulnerability/CVE-2026-18121

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-18121/
Summaries are generated and may contain mistakes; the official description above is the record.