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Concrete CmsApplication · Concretecms

CVE-2026-8421

HIGH · 8.8 CVSS v3.1 Published 2026-05-21
Fix available
A fix is available. Upgrade to 9.5.1 or later.
See remediation →
95/100
Remediation priority · Urgent
Remotely reachable No privileges

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 contains a CSRF vulnerability in the install_package() method of concrete/controllers/single_page/dashboard/extend/install.php.  An attacker who can cause an authenticated administrator to visit a crafted page,  and who has placed or caused a package to be present under DIR_PACKAGES/<handle>/, can force the installation of that package without any CSRF protection. Package installation executes the package controller's install() method as the web server user, enabling remote code execution.  In order to be vulnerable, the victim must be passing canInstallPackages. The Concrete CMS security team gave this vulnerability a CVSS v.4.0 score of 7.5 with vector CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N. Thanks  https://github.com/maru1009  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 · high confidence

Concrete CMS 9.5.0 and below lacks CSRF protection in the install_package() method of the dashboard package installer controller. An attacker who tricks an authenticated administrator with canInstallPackages permission to visit a crafted page can silently force installation of a pre-placed malicious package, executing arbitrary code via the package controller's install() method running as the web server user.

MitigationImplement CSRF token validation in the install_package() form and server-side handler, and ensure all package installation actions require valid tokens. Alternatively, restrict the canInstallPackages permission to only highly trusted administrators and monitor for unauthorized package installations.

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
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

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. Identify Concrete CMS version
    Check the version file (typically at /concrete/version.php or /application/config/version.php) or access the dashboard About page
    Affected if Installed version is below 9.5.1 (any version 9.5.0 or lower)
  2. Locate the vulnerable controller file
    Examine the file at concrete/controllers/single_page/dashboard/extend/install.php and search for the install_package() method
    Affected if File exists and the install_package() method lacks concrete.token validation (no $this->validateToken() or $token->validate() call)
  3. Check package directory permissions
    Inspect the DIR_PACKAGES directory (typically /packages) and verify if it is writable by the web server user
    Affected if The packages directory is writable by the web server process, allowing file placement by an attacker
  4. Verify administrator session exposure risk
    Review if administrators with canInstallPackages permission log in from browsers that could visit untrusted sites while authenticated
    Affected if Administrators with package installation privileges browse other sites while logged into Concrete CMS dashboard

You are affected if running Concrete CMS version 9.5.0 or below AND the install_package() method lacks CSRF token validation AND the packages directory is writable.

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

Implement CSRF token validation in the install_package() form and server-side handler, and ensure all package installation actions require valid tokens. Alternatively, restrict the canInstallPackages permission to only highly trusted administrators and monitor for unauthorized package installations.

Recommended fix High confidence

Concrete CMS 9.5.1

  1. Backup your Concrete CMS database and files before upgrading
  2. Download Concrete CMS version 9.5.1 from the official source at documentation.concretecms.org
  3. Upload and extract the 9.5.1 files to your web server, overwriting existing files
  4. Run the database upgrade script if required (typically via the dashboard or command line)
  5. Verify the installation by logging in as an administrator
  6. Navigate to Dashboard > Extend > Install to confirm CSRF protection is now in place
  7. Test that package installation works normally
  8. Review package directory (DIR_PACKAGES) to ensure no unauthorized packages were added during the vulnerability window
Caveat Check the Concrete CMS 9.5.1 release notes for any breaking changes or configuration requirements before upgrading

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

Fix this in Concrete Cms Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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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-2026-8421 in production — separate from our analysis above.

No notes yet

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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