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Concrete CmsApplication · Concretecms

CVE-2026-8204

MEDIUM · 5.3 CVSS v3.1 Published 2026-05-21
Fix available
A fix is available. Upgrade to 9.5.1 or later.
See remediation →
62/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · moderate confidence

Concrete CMS versions 9.5.0 and below contain an authorization bypass vulnerability in the Calendar Event Frontend Dialog. The issue allows a public calendar block to be used as a pivot point to access private calendar data, enabling cross-calendar data disclosure without proper authentication or authorization checks.

MitigationUpgrade Concrete CMS to a version newer than 9.5.0 once the vendor releases a patch. In the interim, review and restrict calendar block permissions to limit exposure of private calendar data.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Concrete CmsApplication
Affected:< 9.5.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm Concrete CMS installation and version
    Locate the version file (typically version.php in the root directory or check the dashboard under Dashboard > System Information > Version). Compare the installed version number to the affected range: versions 9.5.0 and below are vulnerable; version 9.5.1 and above are not.
    Affected if The installed version is 9.5.0 or lower, or the version cannot be determined and falls within the 9.x branch at or below 9.5.0.
  2. Verify Calendar package is installed and enabled
    Navigate to Dashboard > Extend Concrete > Installed Packages. Look for 'Calendar' in the list of installed packages. Alternatively, check the filesystem for the presence of the 'calendar' package directory under packages/.
    Affected if The Calendar package is installed and active in the Concrete CMS instance.
  3. Identify public calendar blocks on the site
    Search the database or inspect pages with frontend calendar blocks. In the database, query the Pages table for blocks of type 'calendar' or check page templates that render calendar views. Alternatively, browse the site and manually identify any public-facing pages containing calendar functionality.
    Affected if One or more public calendar blocks exist on publicly accessible pages.
  4. Check for existence of private calendars
    Log into the dashboard and navigate to the Calendar management area (typically under Content > Calendar or Pages > Calendar). Examine whether private calendars exist that should not be accessible to anonymous users. Alternatively, query the Calendar database tables for calendar entries marked as private or restricted.
    Affected if Private or restricted calendars exist in the system alongside public calendar blocks, creating the potential for cross-calendar data disclosure.

The environment is affected if running Concrete CMS version 9.5.0 or below with the Calendar package enabled and public calendar blocks present alongside private calendars that should remain inaccessible to anonymous users.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 9.5.1 or later
Fixed in 9.5.1
Interim mitigation

Upgrade Concrete CMS to a version newer than 9.5.0 once the vendor releases a patch. In the interim, review and restrict calendar block permissions to limit exposure of private calendar data.

Recommended fix High confidence

9.5.1

  1. Upgrade Concrete CMS to version 9.5.1 or later to resolve the authorization bypass vulnerability in the Calendar Event Frontend Dialog
  2. After upgrading, verify that public calendar blocks can no longer access private calendar data through the frontend dialog
  3. Test that cross-calendar data disclosure is prevented by attempting to access private calendar events through a public calendar block

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Concrete Cms Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,130
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An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,408.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-8204 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data