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.

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

Q: 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.

Q: 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.

Q: How is CVE-2026-8421 attacked?

It is attacked over the network, in a network appliance.

Q: 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.

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, concrete5/concrete5 (Packagist). Vendors: Concrete CMS, 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
```
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

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