CVE-2026-18426: Concretecms Concrete CMS

Medium severity, CVSS 6.5. EPSS: 0.2% chance of exploitation in the next 30 days.

Concrete CMS 9.0.0 through 9.5.2 did not enforce a block-level edit-permission check on the Express Form block's control-management actions, which relied solely on CSRF token validation. Because the token is bound to the user and action rather than to a specific block, page, or form, an authenticated user with edit access to one Express Form could reuse a validly obtained token to add, modify, or delete controls on Express Forms they were not authorized to edit, including injecting a control whose value is later rendered as HTML to achieve stored XSS. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 2.0 with vector CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:P/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N. Thanks Yat Wu for reporting.

Affected products

• Concretecms Concrete CMS: from 9.0.0, before 9.5.3 (fixed in 9.5.3)

Published 2026-09-15. Last modified 2026-09-21.