CVE-2026-8135: Concrete CMS insecure deserialization in ExpressEntryList block

CVE-2026-8135 · Severity: high · CVSS 4 · Published 2026-05-21

Concrete CMS, a popular content management system, is vulnerable to a security flaw that could allow a malicious administrator to take full control of the web server. By exploiting a weakness in how the system handles data through its programming interface (API), an attacker can run unauthorized commands on the server. This could lead to the theft of sensitive customer data, complete service outages, or the use of the server for further attacks.

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

Quick answers

Is CVE-2026-8135 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-8135 being exploited in the wild.
How severe is CVE-2026-8135?
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-2026-8135?
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-2026-8135 attacked?
It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-8135?
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

Concrete CMS 9.5.0 and below contains an insecure deserialization vulnerability within the ExpressEntryList block controller. The root cause is a type-juggling bypass of the '_fromCIF === true' protection mechanism; while standard POST requests treat the input as a string, the REST API uses json_decode(), which evaluates the input as a strict PHP Boolean(true). An authenticated attacker with administrative privileges to add blocks can leverage this bypass to inject a malicious serialized payload into the 'filterFields' database column. Remote code execution is triggered when another administrator views or edits the affected block. The issue is resolved in version 9.5.1.

Affected products

  • Concrete CMS Concrete CMS <= 9.5.0

Identifiers

  • CVE-2026-8135
  • GHSA-pv2v-6w2v-97x6

Timeline

  • 2026-05-21: advisory: Initial disclosure by Concrete CMS and GitHub Advisory Database
  • 2026-05-21: patched: Version 9.5.1 released to address the vulnerability

References

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