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CVE-2026-8204— Concrete CMS 9.5.0 and below is vulnerable to Authorization Bypass in the Calendar Event Frontend Dialog

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是Concrete CMS开源的一个面向团队的开源内容管理系统。 Concrete CMS 9.5.0及之前版本存在安全漏洞，该漏洞源于日历事件前端对话框中的授权绕过，可能导致跨日历数据泄露。

AI Predicted 6.5 Difficulty: Easy EPSS 0.21% · P12

Possible ATT&CK Techniques 1 AI

T1530 · Data from Cloud Storage

Affected Version Matrix 1

VendorProduct Version RangeStatus
Concrete CMS Concrete CMS 5≤ 9.5.0 affected
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I. Basic Information for CVE-2026-8204

Vulnerability Information
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Vulnerability Title
Concrete CMS 9.5.0 and below is vulnerable to Authorization Bypass in the Calendar Event Frontend Dialog
Source: CVE Program / CVE List V5
Vulnerability Description
Concrete CMS 9.5.0 and below is vulnerable to authorization Bypass in the Calendar Event Frontend Dialog which can allow cross-calendar data disclosure. A public calendar block can be used as a pivot point to access private calendar data. The Concrete CMS security team gave this vulnerability a CVSS v.4.0 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 Winston Crooker 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
通过用户控制密钥绕过授权机制
Source: CVE Program / CVE List V5
Vulnerability Title
Concrete CMS 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Concrete CMS是Concrete CMS开源的一个面向团队的开源内容管理系统。 Concrete CMS 9.5.0及之前版本存在安全漏洞，该漏洞源于日历事件前端对话框中的授权绕过，可能导致跨日历数据泄露。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Concrete CMS Concrete CMS 5 ~ 9.5.0 -

II. Public POCs for CVE-2026-8204

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III. Intelligence Information for CVE-2026-8204

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Vendor Pages for CVE-2026-8204 (1)

Same Patch Batch · Concrete CMS · 2026-05-21 · 41 CVEs total

CVE-2026-8240 Concrete CMS 9.5.0 and below is vulnerable to unauthenticated page metadata disclosure in
CVE-2026-8416 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8412 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8237 Concrete CMS 9.5.0 and below is vulnerable to IDOR in the`/ccm/frontend/conversations/mess
CVE-2026-8434 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8435 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8414 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8433 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8239 Concrete CMS 9.5.0 and below is vulnerable to IDOR in '/ccm/frontend/conversations/get_rat
CVE-2026-8411 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8432 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8236 Concrete CMS 9.5.0 and below is vulnerable to IDOR combined with a missing authentication
CVE-2026-8413 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8415 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8427 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete
CVE-2026-8238 Concrete CMS 9.5.0 and below is vulnerable to IDOR in '/ccm/frontend/conversations/message
CVE-2026-8245 Concrete CMS 9.5.0 and below is vulnerable to Reflected XSS in Legacy Pagination via HTML
CVE-2026-7890 Concrete CMS 9.5.0 is vulnerable to SSRF via RSS Displayer Block
CVE-2026-8139 Concrete CMS 9.5.0 and below is vulnerable to Stored XSS via external-link page cvName
CVE-2026-8409 Concrete CMS 9 before 9.5.0 is vulnerable to Cross Site Request Forgery (CSRF) at concrete

Showing top 20 of 41 CVEs. View all on vendor page → →

IV. Related Vulnerabilities

Same product: Concrete CMS
Same vendor: Concrete CMS
Same weakness: CWE-639

V. Comments for CVE-2026-8204

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