# CVE-2026-87031: Concrete CMS REST API user creation permission bypass

Source: https://junglewise.ai/threats/cve-2026-87031-concrete-cms-rest-api-user-creation-permission-bypass  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-16. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-87031 · Severity: low · CVSS 2.7 · Published 2026-09-16

Concrete CMS, a web content management system, contains a vulnerability in its REST API user creation endpoint that fails to properly verify user permissions. This allows an attacker with a valid OAuth token to create new user accounts without administrative approval or email verification, potentially enabling them to modify website content and inject malicious code.

Technologies: [Concrete CMS](https://junglewise.ai/threats/technologies/concrete-cms). Vendors: [Concrete CMS](https://junglewise.ai/threats/vendors/concrete-cms).

## Quick answers

**Is CVE-2026-87031 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-87031 being exploited in the wild.

**How severe is CVE-2026-87031?** Low severity, with a CVSS score of 2.7, published 16 September 2026.

**Which products and versions are affected?** Concrete CMS 9.2.0 through 9.5.3.

**Is there a fix for CVE-2026-87031?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-87031 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-87031?** Check whether you run Concrete CMS at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability exists in the POST /ccm/api/1.0/users endpoint (add() method in concrete/src/Api/Controller/Users.php) in Concrete CMS versions 9.2.0 through 9.5.3. The endpoint does not perform permission checks before creating user accounts, allowing any request carrying a valid OAuth token with the users:add scope—including client_credentials tokens with no user context—to create active, validated accounts. This bypasses email verification and administrator approval workflows. Under default registration settings, these accounts can edit page content, creating a stored cross-site scripting (XSS) attack vector and enabling further system compromise. Patches are available in versions after 9.5.3.

## Affected products

- Concrete CMS Concrete CMS 9.2.0 through 9.5.3

## Timeline

- 2026-09-16: disclosed

## References

- [https://github.com/concretecms/concretecms](https://github.com/concretecms/concretecms)
- [https://documentation.concretecms.org/developers/introduction/version-history/954-release-notes](https://documentation.concretecms.org/developers/introduction/version-history/954-release-notes)

## Related threats

- [Concrete CMS OAuth REST API authorization bypass](https://junglewise.ai/threats/cve-2026-85387-concrete-cms-oauth-rest-api-authorization-bypass)
- [Concrete CMS unauthenticated Express entry search disclosure](https://junglewise.ai/threats/cve-2026-18120-concrete-cms-unauthenticated-express-entry-search-disclosure)
- [Concrete CMS access control bypass in board custom-slot preview](https://junglewise.ai/threats/cve-2026-87028-concrete-cms-access-control-bypass-in-board-custom-slot-preview)
- [Concrete CMS stored XSS via XML file upload in Form Block](https://junglewise.ai/threats/cve-2026-85386-concrete-cms-stored-xss-via-xml-file-upload-in-form-block)
- [Concrete CMS stored XSS in user timezone field](https://junglewise.ai/threats/cve-2026-85385-concrete-cms-stored-xss-in-user-timezone-field)