CVE-2021-47860: GetSimple CMS Custom JS CSRF Vulnerability

CVE-2021-47860 Overview

CVE-2021-47860 is a Cross-Site Request Forgery (CSRF) vulnerability in the GetSimple CMS Custom JS 0.1 plugin that enables unauthenticated attackers to inject arbitrary client-side code into administrator browsers. This vulnerability creates a dangerous attack chain where attackers can craft malicious websites that, when visited by an authenticated administrator, trigger Cross-Site Scripting (XSS) payloads capable of executing remote code on the hosting server.

Critical Impact

This CSRF vulnerability chains with XSS to achieve Remote Code Execution (RCE), allowing unauthenticated attackers to completely compromise servers hosting GetSimple CMS when administrators are socially engineered to visit malicious pages.

Affected Products

• GetSimple CMS Custom JS Plugin version 0.1
• GetSimple CMS installations with Custom JS plugin enabled
• Web servers hosting vulnerable GetSimple CMS configurations

Discovery Timeline

• 2026-01-21 - CVE CVE-2021-47860 published to NVD
• 2026-01-21 - Last updated in NVD database

Technical Details for CVE-2021-47860

Vulnerability Analysis

This vulnerability represents a chained attack scenario where an initial CSRF weakness enables subsequent XSS and ultimately Remote Code Execution. The GetSimple CMS Custom JS plugin version 0.1 fails to implement proper anti-CSRF protections on endpoints that accept JavaScript code submissions. When an authenticated administrator is tricked into visiting an attacker-controlled webpage, the malicious site can silently submit requests to the CMS on behalf of the administrator, injecting arbitrary JavaScript into the Custom JS plugin's storage.

The injected JavaScript then executes within the context of the administrator's authenticated session when they access the CMS dashboard, providing the attacker with the ability to perform any action the administrator can perform—including uploading malicious PHP files or modifying server configurations to achieve full Remote Code Execution.

Root Cause

The root cause of this vulnerability is the absence of CSRF token validation in the Custom JS plugin's form submission handlers. The plugin accepts POST requests to save JavaScript code without verifying that the request originated from a legitimate user session through the proper CMS interface. This allows any external website to forge requests that appear to come from authenticated administrators, bypassing the intended authorization controls (CWE-352).

Attack Vector

The attack vector is network-based and requires user interaction. An attacker must first create a malicious webpage containing a hidden form or JavaScript that automatically submits a crafted POST request to the vulnerable GetSimple CMS endpoint. The attacker then needs to socially engineer an authenticated administrator into visiting this malicious page. Once the administrator's browser loads the attacker's page, the forged request is sent to the CMS using the administrator's valid session cookies, successfully injecting malicious JavaScript code.

The attack progression follows this pattern:

1. Attacker identifies target GetSimple CMS installation with Custom JS plugin
2. Attacker crafts malicious webpage with auto-submitting form targeting the plugin's save endpoint
3. Administrator is lured to visit the malicious page while authenticated to the CMS
4. Malicious JavaScript is injected into the CMS via CSRF
5. When administrator accesses CMS pages, injected XSS payload executes
6. XSS payload leverages admin privileges to upload webshell or execute server commands

For technical exploitation details, refer to the GitHub CustomJS CSRF Exploit repository and Exploit-DB #49712.

Detection Methods for CVE-2021-47860

Indicators of Compromise

• Unexpected or obfuscated JavaScript code appearing in the Custom JS plugin configuration files
• Suspicious HTTP POST requests to Custom JS plugin endpoints with Referer headers from external domains
• Newly created PHP files in the GetSimple CMS upload directories or theme folders
• Web server access logs showing requests to unknown PHP files following admin panel access
• Administrator session activity from unexpected IP addresses or unusual geographic locations

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block POST requests to CMS admin endpoints lacking valid CSRF tokens or originating from external referrers
• Monitor CMS configuration files for unauthorized modifications, particularly files storing custom JavaScript content
• Deploy file integrity monitoring on GetSimple CMS installation directories to detect unauthorized file uploads
• Analyze web server logs for suspicious patterns such as rapid admin endpoint access followed by new file creation

Monitoring Recommendations

• Enable detailed logging for all GetSimple CMS administrative actions and plugin configuration changes
• Configure alerts for any modification to Custom JS plugin settings or stored JavaScript code
• Monitor outbound network connections from the web server that may indicate post-exploitation activity
• Implement real-time monitoring of PHP file creation events in CMS directories

How to Mitigate CVE-2021-47860

Immediate Actions Required

• Remove or disable the Custom JS 0.1 plugin immediately if not essential for site functionality
• Review Custom JS plugin configuration for any unauthorized or suspicious JavaScript code and remove malicious entries
• Audit GetSimple CMS installation for any signs of compromise including unexpected PHP files or modified core files
• Force re-authentication for all administrator accounts and rotate admin credentials
• Consider migrating to an alternative CMS with active security maintenance if GetSimple CMS updates are unavailable

Patch Information

No official patch has been identified for the Custom JS 0.1 plugin at the time of this advisory. GetSimple CMS and the Custom JS plugin appear to have limited active maintenance. Administrators should consult the GetSimple CMS Homepage and GitHub GetSimpleCMS Repository for any security updates. The VulnCheck Advisory provides additional guidance on this vulnerability.

Workarounds

• Implement additional authentication layers such as HTTP Basic Authentication or IP-based access restrictions on the CMS admin panel
• Deploy a Web Application Firewall configured to validate CSRF tokens and block suspicious cross-origin requests to admin endpoints
• Configure the web server to set restrictive Content-Security-Policy headers that limit JavaScript execution sources
• Use browser extensions or policies that prevent automatic form submission when administrators browse external sites
• Isolate the CMS admin panel to a separate subdomain or path with stricter security controls

# Apache configuration to restrict admin panel access by IP
<Directory "/var/www/html/getsimple/admin">
    Require ip 192.168.1.0/24
    Require ip 10.0.0.0/8
</Directory>

# Nginx configuration to add basic authentication layer
location /admin {
    auth_basic "Administrator Access";
    auth_basic_user_file /etc/nginx/.htpasswd;
    # Additional proxy or fastcgi configuration
}