External risk intelligence

Concrete CMS Remote Code Execution Vulnerability.

CVE advisorySeverity: HIGH (CVSS 8.9)

CVE-2026-8135

A vulnerability in Concrete CMS versions prior to 9.5.1 could allow unauthorized code execution. An authenticated administrator could exploit this flaw via the REST API, potentially leading to complete server takeover. This poses a significant business risk to organizations using affected versions.

3Halo Surface Signal

Deserialization

Concretecms Concrete Cms

9.5.0 and earlier

External exposure likelihood

Halo Surface Signal score for CVE-2026-8135

Concrete CMS is a web application often deployed as a public-facing site. However, the vulnerability requires administrative privileges to add blocks and specifically targets internal administrative functions, making the attack surface primarily an internal management interface rather than a directly public-facing endpoint in common deployments.

CVSS vector

CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Horizon Alert

Summary of the vulnerability and why it matters

Concrete CMS versions prior to 9.5.1 contain a flaw related to how it handles serialized data within specific block controllers. This weakness, when exploited by an authenticated administrator, can permit the injection of malicious code. Successful exploitation could lead to unauthorized server access and control.

  • Vulnerable Concrete CMS versions
  • Insecure deserialization in block controller
  • Complete server takeover

Attack Path

How an attacker could exploit the issue

This vulnerability allows an attacker to execute arbitrary code on the server. The attack involves an administrator with privileges to add blocks to an area of Concrete CMS. By leveraging the REST API, the attacker can bypass security measures that normally prevent malicious inputs. This allows for the injection of a serialized payload into the block's database column. When this data is later accessed by an administrator, the payload is executed, potentially leading to a complete server takeover.

  • Exposed to attackers with privileges.
  • Attacker injects serialized payload via REST API.
  • Payload executes when block data is viewed.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability in Concrete CMS could allow for unauthorized remote code execution, enabling attackers to take complete control of the server. The issue arises from insecure deserialization within the Express Entry List block controller, which can be exploited by a privileged administrator through the REST API. Successful exploitation could lead to significant business risk due to the potential for server takeover.

  • Attackers require high skill.
  • Requires administrator access and block addition privileges.
  • High business risk and potential server takeover.

Priority actions

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in Concrete CMS allows for remote code execution through insecure deserialization. An attacker with administrative privileges could exploit this by manipulating block data via the REST API, leading to complete server compromise. This risk primarily affects the integrity and availability of the application and its underlying server.

  • Identify Concrete CMS instances and administrative users.
  • Restrict REST API access and block management privileges.
  • Apply vendor updates and monitor system logs.

Frequently asked questions

What is the nature of the vulnerability affecting Concrete CMS versions prior to 9.5.1?

Concrete CMS versions 9.5.0 and below are affected by a remote code execution vulnerability due to insecure deserialization in the Express Entry List block controller. This weakness allows a privileged administrator to bypass intended security mechanisms and inject a malicious serialized payload, leading to server takeover when the block's data is accessed.

How can an attacker exploit the Concrete CMS vulnerability?

An attacker with administrator privileges to add blocks can exploit this vulnerability by leveraging the REST API. This bypasses protections on form POST requests, allowing the attacker to insert a malicious serialized payload into the block's filterFields database column, which is then executed when an administrator views or edits the block.

What is the primary weakness class associated with this Concrete CMS vulnerability?

The primary weakness class associated with this Concrete CMS vulnerability is CWE-502, which relates to 'Insecure Deserialization'. This means the application processes serialized data in an unsafe manner, allowing attackers to inject malicious code that can be executed.

What is the relevance of the Halo Surface Signal score for this Concrete CMS vulnerability?

The Halo Surface Signal score for this vulnerability is 3, labeled as 'Possible'. While Concrete CMS is often public-facing, the exploit requires administrative privileges and targets internal administrative functions, making the primary attack surface an internal management interface rather than a directly public-facing endpoint in most common deployments.

What are the practical steps to mitigate the Concrete CMS remote code execution vulnerability?

To mitigate this vulnerability, identify all Concrete CMS instances and administrative users, restrict REST API access and block management privileges, and apply vendor updates by upgrading to a patched version. Continuous monitoring of system logs is also recommended to detect any suspicious activity.

References

Sources and related resources

Primary sources: NVD, CVE.org.

NVDPublished Modified