dbcve.org
Concrete CmsApplication · Concretecms

CVE-2026-8426

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 does not validate a CSRF token before processing requests to /dashboard/extend/update/prepare_remote_upgrade/<remoteMPID>. An attacker who controls the remote package returned for a known marketplace item ID can overwrite the package PHP on disk and force its upgrade() method to execute in a single browser navigation. This results in remote code execution as the web server user.   In order to be vulnerable, the victim must be passing canInstallPackages, victim site must be connected to the Concrete marketplace; and the attacker controls the package returned for a marketplace item ID already installed on the victim site. 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

CSRF protection bypass in Concrete CMS allows remote code execution via the /dashboard/extend/update/prepare_remote_upgrade/<remoteMPID> endpoint. An attacker controlling a marketplace package can overwrite PHP files on disk and trigger execution of the upgrade() method as the web server user. Requires victim site to have marketplace connectivity and canInstallPackages permission.

MitigationAdd CSRF token validation to the prepare_remote_upgrade endpoint and validate the integrity of returned marketplace packages before writing to disk; consider disabling marketplace connectivity if not required.

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. Check installed Concrete CMS version
    Locate the version file (typically /application/config/version.php or check the dashboard footer for version number). Compare against 9.5.1
    Affected if Installed version is below 9.5.1 (9.5.0 or lower)
  2. Verify marketplace update endpoint exists
    Inspect the routing configuration files under /application/config/routes.php or /concrete/routes for the endpoint /dashboard/extend/update/prepare_remote_upgrade/
    Affected if The route is registered and the prepare_remote_upgrade controller exists in the codebase
  3. Check CSRF token validation on the upgrade action
    Examine the controller file handling the prepare_remote_upgrade action (typically in /concrete/controllers/backend/dashboard/extend/update.php or similar). Look for missing CSRF token check in the method handling this route
    Affected if The prepare_remote_upgrade method lacks $this->validateCSRFToken() or equivalent validation call
  4. Confirm marketplace connection is possible
    Check /dashboard/extend/connect settings or marketplace configuration at /application/config/concrete.php for marketplace credentials and connection settings
    Affected if The site has valid marketplace credentials configured or can connect to marketplace.concretecms.com

A user is affected if their Concrete CMS installation is version 9.5.0 or below AND the marketplace update feature is accessible with the vulnerable endpoint lacking CSRF protection.

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

Add CSRF token validation to the prepare_remote_upgrade endpoint and validate the integrity of returned marketplace packages before writing to disk; consider disabling marketplace connectivity if not required.

Recommended fix High confidence

Concrete CMS 9.5.1

  1. Back up your database and all website files before proceeding
  2. Download Concrete CMS version 9.5.1 from the official Concrete CMS website (concretecms.com)
  3. Extract the downloaded package files
  4. Replace the existing concrete/ directory files with the new version, or use the built-in update mechanism
  5. If using the web updater, navigate to /dashboard/system/updates or run the CLI updater
  6. Log in as an administrator and complete any post-upgrade tasks
  7. Verify the installation by checking the dashboard shows version 9.5.1
  8. Confirm CSRF protection is now enforced on the /dashboard/extend/update/prepare_remote_upgrade/ endpoint

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
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,930
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,088.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2026-8426 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2026-8426 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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