CVE-2026-3452: Concrete CMS RCE Vulnerability

CVE-2026-3452 Overview

CVE-2026-3452 is a Remote Code Execution vulnerability affecting Concrete CMS versions below 9.4.8. The vulnerability exists in the Express Entry List block and allows authenticated administrators to inject malicious PHP objects through the columns parameter. Attacker-controlled serialized data can be stored in block configuration fields that are later passed to unserialize() without proper class restrictions or integrity checks, enabling remote code execution on vulnerable systems.

Critical Impact

Authenticated administrators can achieve full remote code execution on the underlying server through PHP object injection, potentially leading to complete system compromise and lateral movement within the network.

Affected Products

• Concrete CMS versions below 9.4.8
• Concretecms Concrete CMS (all versions prior to patched release)

Discovery Timeline

• 2026-03-04 - CVE CVE-2026-3452 published to NVD
• 2026-03-04 - Last updated in NVD database

Technical Details for CVE-2026-3452

Vulnerability Analysis

This vulnerability falls under CWE-502 (Deserialization of Untrusted Data), a critical class of vulnerabilities that can lead to arbitrary code execution. The Express Entry List block in Concrete CMS allows administrators to configure columns for data display. However, the application fails to properly validate and sanitize the columns parameter before processing it through PHP's unserialize() function.

The vulnerability requires administrator-level authentication to exploit, which limits the attack surface but doesn't diminish the severity given that compromised admin credentials or insider threats could leverage this flaw. Once exploited, an attacker gains the ability to execute arbitrary PHP code within the context of the web server, potentially leading to full server compromise.

Root Cause

The root cause of this vulnerability lies in the improper handling of serialized PHP objects within the Express Entry List block configuration. The application stores user-supplied data in block configuration fields and subsequently deserializes this data without implementing proper safeguards such as:

• Class whitelist restrictions on unserialize()
• Integrity checks on serialized data
• Input validation to prevent malicious object injection

This allows attackers to craft malicious serialized objects (gadget chains) that, when deserialized, trigger arbitrary code execution through PHP's magic methods.

Attack Vector

The attack is network-based and targets the Express Entry List block configuration interface. An authenticated administrator can inject a malicious serialized PHP object into the columns parameter. When the CMS processes this configuration data, the unserialize() function instantiates the attacker-controlled objects, triggering property-oriented programming (POP) chains that lead to code execution.

The exploitation flow typically involves:

1. Authenticating to Concrete CMS with administrator privileges
2. Navigating to an Express Entry List block configuration
3. Injecting a crafted serialized PHP object payload into the columns parameter
4. Triggering the deserialization of the malicious payload
5. Achieving remote code execution through gadget chain activation

Detection Methods for CVE-2026-3452

Indicators of Compromise

• Unusual serialized data patterns in Express Entry List block configurations containing unexpected class names
• Web server logs showing repeated access to block configuration endpoints with large or encoded payloads
• Unexpected PHP process spawning or network connections originating from the web server
• Modified files in the Concrete CMS installation directory or webroot
• Database entries containing suspicious serialized objects with known gadget chain class names

Detection Strategies

• Implement web application firewall (WAF) rules to detect serialized PHP object patterns in HTTP requests
• Monitor application logs for configuration changes to Express Entry List blocks, particularly focusing on the columns parameter
• Deploy file integrity monitoring on Concrete CMS installation directories to detect unauthorized modifications
• Utilize endpoint detection and response (EDR) solutions to identify anomalous process execution from web server contexts
• Audit administrator account activities for unauthorized or suspicious configuration changes

Monitoring Recommendations

• Enable verbose logging for Concrete CMS administrative actions and block configuration changes
• Set up alerts for failed authentication attempts followed by successful logins to administrator accounts
• Monitor outbound network connections from the web server for command and control (C2) communication patterns
• Implement database query logging to track changes to block configuration tables

How to Mitigate CVE-2026-3452

Immediate Actions Required

• Upgrade Concrete CMS to version 9.4.8 or later immediately
• Audit existing Express Entry List block configurations for suspicious serialized data
• Review administrator account access and remove unnecessary privileged accounts
• Implement network segmentation to limit the blast radius of potential compromise
• Enable multi-factor authentication for all administrator accounts

Patch Information

The Concrete CMS security team has released version 9.4.8 which addresses this vulnerability. The patch implements proper validation and restrictions on deserialized data in the Express Entry List block. Detailed release notes are available in the ConcreteCMS 9.4.8 Release Notes. Technical details of the fix can be reviewed in the GitHub Pull Request.

Workarounds

• Restrict administrator access to trusted IP addresses only using web server configuration or firewall rules
• Implement additional authentication layers for CMS administrative functions
• Deploy a web application firewall with rules to block serialized PHP object injection attempts
• Disable or restrict access to the Express Entry List block functionality until patching is possible
• Conduct regular security audits of administrator accounts and their activities

# Example: Restrict admin access by IP in Apache .htaccess
<Location "/index.php/dashboard">
    Require ip 192.168.1.0/24
    Require ip 10.0.0.0/8
</Location>