# CVE-2026-8421: Concrete CMS CSRF in install_package method leads to RCE

Source: https://junglewise.ai/threats/cve-2026-8421-concrete-cms-csrf-in-install-package-method-leads-to-rce  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-21. Updated: 2026-06-24  
License: CC BY 4.0 with a link back

CVE-2026-8421 · Severity: high · CVSS 4 · Published 2026-05-21

Concrete CMS, a popular content management system, is vulnerable to a security flaw that could allow an attacker to take over a website. By tricking a logged-in administrator into visiting a malicious link, an attacker can force the system to install a malicious software package. If successful, this allows the attacker to execute arbitrary code on the server, potentially leading to full site compromise and data theft.

Technologies: [Concrete CMS](https://junglewise.ai/threats/technologies/concrete-cms), [concrete5/concrete5 (Packagist)](https://junglewise.ai/threats/technologies/concrete5-concrete5). Vendors: [Concrete CMS](https://junglewise.ai/threats/vendors/concrete-cms), [Packagist](https://junglewise.ai/threats/vendors/packagist).

## Quick answers

**Is CVE-2026-8421 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-8421 being exploited in the wild.

**How severe is CVE-2026-8421?** High severity, with a CVSS score of 4, published 21 May 2026.

**Which products and versions are affected?** Concrete CMS <= 9.5.0, 9.5.0 and below; concrete5/concrete5 (Packagist) < 9.5.1, ECOSYSTEM:introduced=0,fixed=9.5.1.

**Is there a fix for CVE-2026-8421?** Yes. concrete5/concrete5 (Packagist): upgrade to 9.5.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-8421 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-8421?** Check whether you run Concrete CMS or concrete5/concrete5 (Packagist) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- concrete5/concrete5 (Packagist): upgrade to 9.5.1 or later

  ```sh
  composer require concrete5/concrete5:^9.5.1
  ```


## Technical details

A Cross-Site Request Forgery (CSRF) vulnerability exists in the install_package() method within concrete/controllers/single_page/dashboard/extend/install.php. The vulnerability allows an attacker to bypass CSRF protections and force the installation of a package if they can place a malicious package directory under DIR_PACKAGES/<handle>/ and trick an authenticated administrator with 'canInstallPackages' permissions into visiting a crafted URL. Upon installation, the package controller's install() method is executed as the web server user, resulting in remote code execution (RCE). This issue is addressed in version 9.5.1.

## Affected products

- Concrete CMS Concrete CMS <= 9.5.0

## Identifiers

- CVE-2026-8421
- GHSA-4c8m-6fwx-m7xq

## Timeline

- 2026-05-21: disclosed
- 2026-05-21: advisory
- 2026-05-21: patched: Fixed in version 9.5.1

## References

- [https://documentation.concretecms.org/9-x/developers/introduction/version-history/951-release-notes](https://documentation.concretecms.org/9-x/developers/introduction/version-history/951-release-notes)

## Related threats

- [Concrete CMS OAuth REST API authorization bypass](https://junglewise.ai/threats/cve-2026-85387-concrete-cms-oauth-rest-api-authorization-bypass)
- [Concrete CMS unauthenticated Express entry search disclosure](https://junglewise.ai/threats/cve-2026-18120-concrete-cms-unauthenticated-express-entry-search-disclosure)
- [Concrete CMS REST API user creation permission bypass](https://junglewise.ai/threats/cve-2026-87031-concrete-cms-rest-api-user-creation-permission-bypass)
- [Concrete CMS access control bypass in board custom-slot preview](https://junglewise.ai/threats/cve-2026-87028-concrete-cms-access-control-bypass-in-board-custom-slot-preview)
- [Concrete CMS stored XSS via XML file upload in Form Block](https://junglewise.ai/threats/cve-2026-85386-concrete-cms-stored-xss-via-xml-file-upload-in-form-block)