# CVE-2026-8426: Concrete CMS CSRF to Remote Code Execution in package upgrades

Source: https://junglewise.ai/threats/cve-2026-8426-concrete-cms-csrf-to-remote-code-execution-in-package-upgrades  
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-2026-8426 · 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 full control of a website. By tricking an administrator into visiting a malicious link, an attacker can force the site to download and execute malicious code from a compromised marketplace account. This could lead to a total system takeover, data theft, or service disruption.

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-2026-8426 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-8426 being exploited in the wild.

**How severe is CVE-2026-8426?** 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-2026-8426?** 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-2026-8426 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-8426?** 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

Concrete CMS 9.5.0 and below fails to validate a CSRF token on the `/dashboard/extend/update/prepare_remote_upgrade/<remoteMPID>` endpoint. An attacker can exploit this by inducing an administrator to visit a malicious URL. If the victim's site is connected to the Concrete marketplace and the attacker has compromised or controls a package already installed on the victim's site, they can overwrite PHP files on the disk. This allows the attacker to trigger the package's upgrade() method, resulting in remote code execution (RCE) as the web server user. The vulnerability is patched in version 9.5.1.

## Affected products

- Concrete CMS Concrete CMS <= 9.5.0

## Identifiers

- CVE-2026-8426
- GHSA-5rj5-gfmr-hrc3

## Timeline

- 2026-05-21: disclosed
- 2026-05-21: advisory
- 2026-05-21: patched: Version 9.5.1 released

## 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)