GetSimple CMS Custom JS 0.1 - CSRF to XSS to RCE

The primary impact of CVE-2021-47860 is the potential for an attacker to execute malicious JavaScript code within the context of an administrator's session. This could allow an attacker to steal sensitive information, such as login credentials or administrative settings. Furthermore, the attacker could modify website content, redirect users to malicious sites, or even gain control of the server itself. The lack of authentication required for exploitation significantly broadens the attack surface, making it easier for attackers to target vulnerable installations. Successful exploitation could have a significant impact on the confidentiality, integrity, and availability of the GetSimple CMS website and its associated data.

Administrators of GetSimple CMS websites using the Custom JS plugin version 0.1 are at significant risk. Shared hosting environments where multiple websites share the same server are particularly vulnerable, as a compromise of one website could potentially lead to the compromise of others. Websites relying on the Custom JS plugin for critical functionality are also at higher risk.

• php: Examine the Custom JS plugin files for suspicious JavaScript code, particularly functions that handle user input. Use grep to search for potentially malicious patterns like eval() or document.write.

grep -r 'eval\(' /path/to/customjs/plugin/

• generic web: Monitor access logs for unusual requests containing JavaScript payloads. Look for POST requests to plugin endpoints with suspicious data. Use curl to test endpoints for XSS vulnerabilities.

curl -X POST -d "<script>alert('XSS')</script>" http://example.com/plugins/customjs/endpoint.php

1. 2026-01-21Disclosure

   disclosure

1. Reserviert2026-01-18
2. Veröffentlicht2026-01-21
3. Geändert2026-04-07
4. EPSS aktualisiert2026-03-23

The primary mitigation for CVE-2021-47860 is to upgrade the Custom JS plugin to a patched version. Unfortunately, no patched version is currently available. As a temporary workaround, administrators should carefully review all plugin code for suspicious activity and implement strict input validation on any user-supplied data. Consider implementing a Web Application Firewall (WAF) with XSS filtering rules to block malicious requests. Regularly scan the website for XSS vulnerabilities using automated tools. Disable the Custom JS plugin entirely if it is not essential for website functionality.