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CVE-2026-18121— Concrete CMS 9.5.2 and below is vulnerable to an authorization bypass (IDOR) in the Calendar block's frontend event dialog (/ccm/calendar/view_event/{bID}/{occurrence_id}).

Quick assessment

Affected
Concrete CMS Concrete CMS
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Concrete CMS 9.5.2 及更早版本存在一个授权绕过（IDOR，Insecure Direct Object Reference，不安全直接对象引用）漏洞。该漏洞源于前端日历轻弹层（lightbox）端点（ ）未验证调用者是否有权限查看包含所请求事件实例的日历。 控制器会直接从攻击者提供的、按顺序递增的标识符加载事件实例，而未确认该实例确实属于所引用区块中配置的日历。因此，任何能够渲染启用了轻弹层属性的公开日历区块的未认证访问者，都可以提供一个任意的事件实例标识符，从而泄露他们原本无权查看的日历中的事件

CVSS 6.3 · Medium

Possible ATT&CK Techniques 1 AI

T1039 · Data from Network Shared Drive
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I. Basic Information for CVE-2026-18121

Vulnerability Information
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Vulnerability Title
Concrete CMS 9.5.2 and below is vulnerable to an authorization bypass (IDOR) in the Calendar block's frontend event dialog (/ccm/calendar/view_event/{bID}/{occurrence_id}).
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
授权机制缺失
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Concrete CMS Concrete CMS 5.0.0 ~ 9.5.2 -

II. Public POCs for CVE-2026-18121

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III. Intelligence Information for CVE-2026-18121

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Vendor Pages for CVE-2026-18121 (1)

Same Patch Batch · Concrete CMS · 2026-09-10 · 5 CVEs total

CVE-2026-81905 6.3 MEDIUM Concrete CMS below 9.5.3 does not enforce validation-hash type on redemption, allowing a h
CVE-2026-81906 6.3 MEDIUM [UNREVIEWED] OAuth Callback Login Bypasses Deactivated-Account Checks
CVE-2026-68527 5.9 MEDIUM Concrete CMS 8.3.0 through 9.5.2 is vulnerable to an authorization bypass through user-con
CVE-2026-84432 5.3 MEDIUM Concrete CMS 9 through 9.5.2 is vulnerable to CSRFin the Boards custom slot dialog contro

IV. Related Vulnerabilities

Same product: Concrete CMS
Same vendor: Concrete CMS
Same weakness: CWE-862

V. Comments for CVE-2026-18121

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