CVE-2026-28495: GetSimple CMS RCE Vulnerability

CVE-2026-28495 Overview

GetSimple CMS is a content management system. The massiveAdmin plugin (v6.0.3) bundled with GetSimpleCMS-CE v3.3.22 allows an authenticated administrator to overwrite the gsconfig.php configuration file with arbitrary PHP code via the gsconfig editor module. The form lacks CSRF protection, enabling a remote unauthenticated attacker to exploit this via Cross-Site Request Forgery against a logged-in admin, achieving Remote Code Execution (RCE) on the web server.

Critical Impact

Remote unauthenticated attackers can achieve full Remote Code Execution on vulnerable GetSimple CMS installations by chaining CSRF with arbitrary PHP code injection into the configuration file.

Affected Products

• GetSimpleCMS-CE v3.3.22
• massiveAdmin plugin v6.0.3
• getsimple-ce getsimple_cms (Community Edition)

Discovery Timeline

• 2026-03-10 - CVE CVE-2026-28495 published to NVD
• 2026-03-12 - Last updated in NVD database

Technical Details for CVE-2026-28495

Vulnerability Analysis

This vulnerability represents a dangerous combination of two security weaknesses that, when chained together, allow complete server compromise. The massiveAdmin plugin bundled with GetSimpleCMS-CE provides administrators with a gsconfig editor module that permits modification of the gsconfig.php configuration file directly through the web interface.

The core issue is the absence of Cross-Site Request Forgery (CWE-352) protection on the form that handles configuration file modifications. Without proper CSRF tokens or other anti-forgery mechanisms, the application cannot distinguish between legitimate administrator requests and malicious requests forged by an attacker.

An attacker can craft a malicious webpage containing a hidden form that, when visited by an authenticated administrator, automatically submits a request to the vulnerable endpoint. This request can inject arbitrary PHP code into the gsconfig.php file, which is subsequently executed by the web server during normal CMS operations.

Root Cause

The root cause is the missing CSRF token validation in the gsconfig editor module of the massiveAdmin plugin. Modern web applications must implement anti-CSRF mechanisms such as synchronizer tokens, same-site cookies, or origin header validation to prevent unauthorized state-changing requests. The absence of these protections in a sensitive functionality like configuration file editing creates a direct path to code execution.

Attack Vector

The attack is network-based and requires user interaction—specifically, a logged-in administrator must be tricked into visiting a malicious page controlled by the attacker. The attacker does not need prior authentication to the CMS; they only need to craft a convincing phishing lure or embed the malicious form on a compromised website that the target administrator might visit.

The exploitation flow involves: (1) attacker creates a malicious webpage with a hidden auto-submitting form targeting the gsconfig editor endpoint, (2) attacker tricks an authenticated GetSimple CMS administrator into visiting this page, (3) the administrator's browser automatically sends the forged request with their valid session credentials, (4) the CMS accepts the request and writes attacker-controlled PHP code to gsconfig.php, and (5) the malicious PHP code executes on subsequent page loads, giving the attacker remote code execution capabilities on the web server.

Detection Methods for CVE-2026-28495

Indicators of Compromise

• Unexpected modifications to the gsconfig.php file, particularly additions of PHP functions like eval(), system(), exec(), passthru(), or shell_exec()
• Web server logs showing POST requests to the massiveAdmin gsconfig editor endpoint originating from external referrer URLs
• Newly created PHP files or shell scripts in the CMS directory structure
• Unusual outbound network connections from the web server process

Detection Strategies

• Implement file integrity monitoring (FIM) on critical configuration files including gsconfig.php to detect unauthorized modifications
• Configure web application firewall (WAF) rules to detect and block requests to sensitive administrative endpoints lacking valid CSRF tokens
• Monitor web server access logs for POST requests to admin endpoints with suspicious or external referrer headers
• Deploy endpoint detection and response (EDR) solutions to identify PHP processes spawning unexpected child processes

Monitoring Recommendations

• Enable detailed access logging on the GetSimple CMS admin directory and alert on requests from unexpected IP addresses or with anomalous referrer headers
• Implement real-time alerting for changes to PHP configuration files within the CMS installation directory
• Monitor for web shell indicators such as newly created PHP files with obfuscated content or common web shell signatures

How to Mitigate CVE-2026-28495

Immediate Actions Required

• Restrict access to the GetSimple CMS administrative interface by IP address using web server configuration or network-level firewall rules
• Consider disabling or removing the massiveAdmin plugin until a patched version is available
• Ensure administrators are educated about phishing risks and avoid clicking untrusted links while logged into the CMS
• Implement Content Security Policy (CSP) headers to reduce the risk of form-based CSRF attacks

Patch Information

Review the GitHub Security Advisory for official patch information and updated versions. Organizations should upgrade to a patched version of GetSimpleCMS-CE and the massiveAdmin plugin as soon as one becomes available.

Workarounds

• Disable the gsconfig editor functionality in the massiveAdmin plugin by removing or renaming the relevant PHP files until a patch is released
• Implement additional authentication layers such as HTTP Basic Authentication on the admin directory
• Use web server configuration to make the gsconfig.php file read-only at the file system level, preventing web-based modifications
• Deploy a reverse proxy with CSRF protection capabilities in front of the CMS administrative interface

# Configuration example - Make gsconfig.php read-only
chmod 444 /path/to/getsimple/gsconfig.php
chown root:root /path/to/getsimple/gsconfig.php

# Apache configuration to restrict admin access by IP
<Directory "/path/to/getsimple/admin">
    Require ip 192.168.1.0/24
</Directory>