CVE-2021-47917: Simple CMS 2.1 XSS Vulnerability

CVE-2021-47917 Overview

Simple CMS 2.1 contains a persistent cross-site scripting (XSS) vulnerability in user input parameters that allows remote attackers to inject malicious script code. Attackers can exploit the newUser and editUser modules to inject persistent scripts that execute on user list preview, potentially leading to session hijacking and application manipulation.

Critical Impact

Authenticated attackers can inject persistent malicious scripts through user management modules, enabling session hijacking, credential theft, and unauthorized application manipulation when administrators view the user list.

Affected Products

• Simple CMS 2.1

Discovery Timeline

• 2026-02-01 - CVE CVE-2021-47917 published to NVD
• 2026-02-03 - Last updated in NVD database

Technical Details for CVE-2021-47917

Vulnerability Analysis

This persistent cross-site scripting vulnerability affects the user management functionality within Simple CMS 2.1. The application fails to properly sanitize user-supplied input in the newUser and editUser modules before storing it in the database and rendering it back to administrators. When an administrator views the user list preview, the injected malicious JavaScript executes within the context of their authenticated session.

The stored nature of this XSS vulnerability makes it particularly dangerous as the malicious payload persists in the application database and executes each time the affected page is loaded. This allows attackers to target multiple victims without requiring them to click a specially crafted link.

Root Cause

The root cause of this vulnerability is improper input validation and output encoding (CWE-79). The Simple CMS application does not implement adequate sanitization of user-controlled input fields within the user management modules. Specifically, the newUser and editUser functionality accepts and stores raw user input without HTML entity encoding or script filtering, and subsequently renders this content directly into the page without proper output escaping.

Attack Vector

The attack vector is network-based, requiring an attacker to have low-level privileges (authenticated access) to the CMS. The attacker submits malicious JavaScript code through the user input parameters in the newUser or editUser modules. Once stored, the payload executes whenever an administrator accesses the user list preview functionality, enabling potential session hijacking, administrative credential theft, or manipulation of application settings.

The attack requires passive user interaction—an administrator must view the compromised user list for the payload to execute. The attacker can craft payloads to steal session cookies, redirect users to phishing pages, or perform actions on behalf of the authenticated administrator.

Detection Methods for CVE-2021-47917

Indicators of Compromise

• Unexpected JavaScript code or HTML tags present in user profile fields within the CMS database
• Unusual <script> tags, event handlers (e.g., onerror, onload), or encoded payloads in user management entries
• Reports of unexpected browser behavior or redirects when administrators access user management pages

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block XSS payloads in HTTP POST requests to user management endpoints
• Enable Content Security Policy (CSP) headers and monitor for policy violation reports indicating script injection attempts
• Review application logs for suspicious input patterns containing script tags or JavaScript event handlers in user creation/modification requests

Monitoring Recommendations

• Monitor database tables storing user information for entries containing potentially malicious HTML or JavaScript content
• Implement real-time alerting for CSP violations that may indicate attempted XSS exploitation
• Regularly audit user management module access logs for patterns indicating reconnaissance or exploitation attempts

How to Mitigate CVE-2021-47917

Immediate Actions Required

• Review existing user records in the Simple CMS database for injected script content and sanitize any malicious entries
• Restrict access to user management modules to only essential administrative personnel
• Consider temporarily disabling the newUser and editUser functionality until a proper fix is implemented

Patch Information

No official vendor patch has been identified in the available CVE data. Organizations using Simple CMS 2.1 should check the Simple PHP Scripts website for updates and security advisories. Additional technical details are available in the VulnCheck Advisory and Vulnerability Lab #2302.

Workarounds

• Implement server-side input validation to strip or encode HTML special characters (<, >, ", ', &) from all user input fields before database storage
• Apply output encoding when rendering user-supplied data in HTML contexts to prevent script execution
• Deploy a Content Security Policy (CSP) header with strict script-src directives to mitigate the impact of any injected scripts
• Consider using a web application firewall (WAF) with XSS detection rules as an additional defense layer

# Example Apache configuration to add CSP headers
# Add to .htaccess or virtual host configuration
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'self';"
Header set X-Content-Type-Options "nosniff"
Header set X-XSS-Protection "1; mode=block"