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CVE-2026-18110 – Unauthenticated User Enumeration – Concrete CMS 9.0.0 through 9.5.2

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Summary

CVE-2026-18110 is a missing authorization vulnerability (CWE-862) in Concrete CMS affecting the user selector autocomplete endpoint. An unauthenticated attacker can query the endpoint to enumerate every backend user account, exposing internal user IDs, usernames, and email addresses of administrators. The issue carries a CVSS score of 8.7 (High) and requires no authentication or user interaction to exploit.

Technical details

  • Root cause: The /ccm/system/user/autocomplete endpoint, used to power the "Preview as User" panel and other user-selector components in the Concrete CMS dashboard, does not perform an authorization check before returning results.
  • Trigger conditions: An attacker submits an empty search query to the endpoint and paginates through the returned results; the CSRF token used by the endpoint is bound to display options rather than user identity, so it does not prevent this abuse.
  • Attack vector: Network — the endpoint can be reached directly over HTTP/HTTPS with no prior authentication.
  • Impact: Full enumeration of backend/administrative accounts, disclosing internal user IDs, usernames, and email addresses. No password hashes or session data are exposed, but the leaked account information can be used to facilitate targeted phishing, credential stuffing, or brute-force attacks against identified administrator accounts.

Affected software

  • Concrete CMS versions 9.0.0 through 9.5.2

Severity

  • CVSS v4.0 Base Score: 8.7 (High)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade to Concrete CMS 9.5.3 or later, which addresses this issue as part of a broader security release.
  • If immediate patching is not possible: Restrict or monitor access to /ccm/system/user/autocomplete at the network or web-server layer (e.g., via WAF rules or reverse-proxy access controls) to limit exposure to authenticated dashboard sessions, and monitor logs for repeated/paginated unauthenticated requests to this endpoint as a possible indicator of enumeration attempts.

References

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