CVE-2026-8204: Concrete5/concrete5 Concrete CMS Is Vulnerable To Vulnerability

MODERATE
Advisory Identifier: GHSA-x2fp-hj8c-mmxh
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CVSS 5.3
CWE-639
Published
Advisory Summary

CVE-2026-8204 is a MODERATE (CVSS 5.3) concrete cms is vulnerable to vulnerability (CWE-639: Authorization Bypass Through User-Controlled Key) affecting Concrete5/concrete5 libraries in the Packagist ecosystem. The issue involves concrete cms is vulnerable to authorization bypass in the calendar event frontend dialog. Remediation is available by upgrading affected packages to version 9.5.1.

CVE-2026-8204 at a Glance

CVE Identifier CVE-2026-8204
GHSA Identifier GHSA-x2fp-hj8c-mmxh
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-639: Authorization Bypass Through User-Controlled Key
Affected Product Concrete5/concrete5
Package Ecosystem Packagist
Published Date
Last Updated

Security Impact

CVSS: 5.3

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality Low (C:L)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2026-8204

To remediate this vulnerability, upgrade all affected Concrete5/concrete5 dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

concrete5/concrete5 fixed: 9.5.1 
Upgrade concrete5/concrete5 to 9.5.1
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for concrete5/concrete5

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-8204, inspect your project dependencies for Concrete5/concrete5:

inspect dependencies for concrete5/concrete5

Analyze the terminal output to locate any version of Concrete5/concrete5 in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 9.5.1), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-639: Authorization Bypass Through User-Controlled Key, where the system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data. In Concrete5/concrete5, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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.

CVE-2026-8204 Affected Concrete5/concrete5 Versions

1 component(s)
Fixed Versions:
9.5.1
Product / Package Ecosystem Affected Versions Fixed Version
concrete5/concrete5
 Packagist
< 9.5.1
9.5.1

References

NVD — CVE-2026-8204 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-8204
documentation.concretecms.org/9-x/developers/intr... WEB
https://documentation.concretecms.org/9-x/developers/introduction/version-history/951-release-notes
Concrete5/concrete5 Source Repository REPOSITORY
https://github.com/concretecms/concretecms

Frequently Asked Questions

What is CVE-2026-8204?
CVE-2026-8204 is a security vulnerability in Concrete5/concrete5 involving Concrete CMS is vulnerable to authorization bypass in the Calendar Event Frontend Dialog under CWE-639: Authorization Bypass Through User-Controlled Key.
Which Concrete5/concrete5 versions are affected?
Affected versions include releases prior to 9.5.1 across affected packages: concrete5/concrete5.
What version fixes CVE-2026-8204?
Upgrading to 9.5.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is concrete5/concrete5 affected?
Yes, concrete5/concrete5 is affected in releases prior to 9.5.1. Upgrade your dependency to 9.5.1 or later.
What is the CVSS score of CVE-2026-8204?
CVE-2026-8204 has a CVSS v3.1 base score of 5.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N.
What CWE is associated with CVE-2026-8204?
CVE-2026-8204 is classified under CWE-639: Authorization Bypass Through User-Controlled Key (The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data.).
How can I check Packagist dependencies for CVE-2026-8204?
Run the dependency tree command: 'inspect dependencies for concrete5/concrete5'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-x2fp-hj8c-mmxh",
  "modified" : "2026-06-24T20:57:23Z",
  "published" : "2026-05-21T21:30:38Z",
  "aliases" : [ "CVE-2026-8204" ],
  "summary" : "Concrete CMS is vulnerable to authorization bypass in the Calendar Event Frontend Dialog",
  "details" : "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "concrete5/concrete5"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "9.5.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-8204"
  }, {
    "type" : "WEB",
    "url" : "https://documentation.concretecms.org/9-x/developers/introduction/version-history/951-release-notes"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/concretecms/concretecms"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-639" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-24T18:12:55Z",
    "nvd_published_at" : "2026-05-21T21:16:33Z"
  }
}