CVE-2021-47919: Simple CMS 2.1 XSS Vulnerability

CVE-2021-47919 Overview

Simple CMS 2.1 contains a reflected cross-site scripting (XSS) vulnerability in the preview.php file. The vulnerability exists due to improper sanitization of the id parameter, allowing attackers to inject malicious script code through crafted GET requests. When a victim visits a malicious link, the injected script executes in the context of their browser session, potentially enabling session hijacking, credential theft, or phishing attacks.

Critical Impact

Attackers can inject arbitrary JavaScript code through the id parameter in preview.php, potentially hijacking user sessions, stealing credentials, or redirecting victims to malicious sites.

Affected Products

• Simple CMS version 2.1

Discovery Timeline

• 2026-02-01 - CVE CVE-2021-47919 published to NVD
• 2026-02-03 - Last updated in NVD database

Technical Details for CVE-2021-47919

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The flaw resides in the preview.php file of Simple CMS 2.1, where the application fails to properly validate, sanitize, or encode user-supplied input in the id parameter before reflecting it back to the user in the rendered HTML response.

The network-based attack vector requires user interaction, as victims must click a malicious link or visit a compromised page containing the exploit. Once triggered, the malicious script executes within the victim's browser context, inheriting the security context and session credentials of the authenticated user.

Root Cause

The root cause of this vulnerability is the lack of input validation and output encoding in the preview.php file. When processing the id parameter from GET requests, the application directly incorporates user-supplied data into the HTML output without proper sanitization. This allows attackers to break out of the intended data context and inject executable JavaScript code.

Attack Vector

The attack is executed over the network and requires an authenticated user to interact with a malicious link. An attacker crafts a URL containing malicious JavaScript in the id parameter. When a victim clicks the link, the payload is reflected in the server's response and executed by the victim's browser. This can lead to:

• Session cookie theft and session hijacking
• Credential harvesting through fake login forms
• Redirection to phishing pages
• Modification of page content to deceive users
• Execution of actions on behalf of the authenticated user

The vulnerability exploits the trust relationship between the user's browser and the legitimate Simple CMS application. For technical details on the exploitation mechanism, see the Vulnerability Lab ID #2301 advisory.

Detection Methods for CVE-2021-47919

Indicators of Compromise

• Unusual URL patterns containing script tags or JavaScript event handlers in the id parameter of preview.php
• Web server access logs showing GET requests to preview.php with encoded or obfuscated payloads
• Reports from users of unexpected browser behavior or redirects when accessing Simple CMS

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block common XSS patterns in URL parameters
• Deploy endpoint detection solutions to identify suspicious browser activity indicative of XSS exploitation
• Configure Content Security Policy (CSP) headers to prevent inline script execution and report violations
• Monitor for anomalous HTTP requests containing typical XSS payload signatures

Monitoring Recommendations

• Enable detailed access logging on web servers hosting Simple CMS installations
• Configure real-time alerting for requests containing suspicious characters or encoding patterns in URL parameters
• Review web server logs regularly for patterns matching XSS exploitation attempts targeting preview.php

How to Mitigate CVE-2021-47919

Immediate Actions Required

• Restrict access to the preview.php file if it is not required for business operations
• Implement a Web Application Firewall (WAF) with XSS protection rules in front of Simple CMS installations
• Deploy Content Security Policy (CSP) headers to mitigate the impact of successful XSS attacks
• Educate users about the risks of clicking unknown or suspicious links

Patch Information

No official patch information is available from the vendor at this time. Organizations should consult the Simple PHP Scripts Overview page for updates. Review the VulnCheck Security Advisory for additional guidance.

Workarounds

• Add server-side input validation to sanitize the id parameter, removing or encoding special characters such as <, >, ", ', and &
• Implement output encoding using functions like htmlspecialchars() in PHP when rendering user-supplied data
• Disable or remove the preview.php file if preview functionality is not essential
• Deploy HTTP security headers including X-XSS-Protection, X-Content-Type-Options, and Content-Security-Policy

# Apache .htaccess configuration to add security headers
<IfModule mod_headers.c>
    Header set X-XSS-Protection "1; mode=block"
    Header set X-Content-Type-Options "nosniff"
    Header set Content-Security-Policy "default-src 'self'; script-src 'self'"
</IfModule>