CVE-2026-81921: Concretecms Concrete CMS

Medium severity, CVSS 5.4. EPSS: 0.2% chance of exploitation in the next 30 days.

Concrete CMS 8.5.3 through 9.5.2 enabled the OAuth 2.0 refresh-token grant using the unmodified upstream League grant, which issued new access tokens from a valid refresh token without re-checking the associated account's active status. A user who obtained a refresh token while active could therefore continue to mint valid access tokens after being deactivated or suspended (uIsActive=0), so deactivating an account did not revoke its API access. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 2.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:L/VA:L/SC:L/SI:L/SA:L. Thanks sh4d0byss for reporting.

Affected products

• Concretecms Concrete CMS: from 8.5.3, before 9.5.3 (fixed in 9.5.3)

Published 2026-09-15. Last modified 2026-09-18.