CVE-2021-47870: GetSimple CMS My SMTP XSS Vulnerability

CVE-2021-47870 Overview

CVE-2021-47870 is a Stored Cross-Site Scripting (XSS) vulnerability affecting GetSimple CMS My SMTP Contact Plugin version 1.1.2. The plugin implements input sanitization using the htmlspecialchars() function, but this protection can be bypassed by encoding dangerous characters as escaped hex bytes. This sanitization bypass allows attackers to inject arbitrary client-side JavaScript code that persists in the application and executes in the context of an administrator's browser session when visiting a page containing the malicious payload.

Critical Impact

Attackers can inject persistent malicious scripts that execute in administrator browsers, potentially leading to session hijacking, administrative account takeover, or further exploitation of the CMS environment.

Affected Products

• GetSimple CMS My SMTP Contact Plugin version 1.1.2
• GetSimple CMS installations with the vulnerable My SMTP Contact Plugin

Discovery Timeline

• 2026-01-21 - CVE-2021-47870 published to NVD
• 2026-01-21 - Last updated in NVD database

Technical Details for CVE-2021-47870

Vulnerability Analysis

This vulnerability falls under CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The core issue lies in the insufficient input validation mechanism employed by the My SMTP Contact Plugin. While the developers implemented htmlspecialchars() to encode special characters and prevent XSS attacks, the function fails to handle hex-encoded byte sequences properly.

When user input containing malicious JavaScript is submitted through the contact form with dangerous characters encoded as hex bytes (e.g., \\x3c for < and \\x3e for >), the sanitization function does not recognize these as dangerous characters. The hex-encoded payload passes through the filter and is stored in the application. When an administrator later views the submitted contact form data, the browser decodes and executes the malicious script.

Root Cause

The root cause stems from an incomplete sanitization implementation. The htmlspecialchars() function in PHP is designed to convert special characters like <, >, &, and quotes into their HTML entity equivalents. However, this approach only addresses direct character input and does not account for alternative encoding schemes such as hex byte sequences, Unicode representations, or other obfuscation techniques.

The plugin stores user-submitted form data without adequate output encoding or Content Security Policy enforcement, allowing the stored XSS payload to execute when rendered in the administrator's browser context.

Attack Vector

The attack is network-based and requires an attacker to submit a specially crafted contact form entry containing hex-encoded malicious JavaScript. The attack flow is as follows:

1. The attacker submits a contact form with JavaScript payload encoded using hex byte notation
2. The htmlspecialchars() sanitization fails to detect the encoded malicious characters
3. The payload is stored in the application database
4. When an administrator views the contact submissions, the browser interprets and executes the malicious script
5. The script runs with the administrator's session privileges, enabling session theft, CSRF token harvesting, or administrative action execution

For detailed technical analysis and proof-of-concept code, see the GitHub PoC for gSMTP Exploit and Exploit-DB #49798.

Detection Methods for CVE-2021-47870

Indicators of Compromise

• Unusual contact form submissions containing hex-encoded sequences (e.g., \\x3c, \\x3e, \\x22)
• Administrator session cookies appearing in external server logs
• Unexpected administrative actions or configuration changes following contact form review
• JavaScript execution errors in browser console when viewing contact submissions

Detection Strategies

• Implement web application firewall (WAF) rules to detect hex-encoded XSS payloads in form submissions
• Monitor server logs for contact form entries containing suspicious encoded character patterns
• Deploy endpoint detection to identify unauthorized script execution in administrator browser sessions
• Review GetSimple CMS plugin versions and flag installations running My SMTP Contact Plugin 1.1.2

Monitoring Recommendations

• Enable verbose logging for all administrative interface access and form data review events
• Configure browser-side Content Security Policy violation reporting
• Implement real-time alerting for new contact form submissions containing encoded sequences
• Monitor for outbound connections from administrator workstations following CMS access

How to Mitigate CVE-2021-47870

Immediate Actions Required

• Disable or remove the My SMTP Contact Plugin version 1.1.2 until a patched version is available
• Review existing contact form submissions for potentially malicious payloads
• Rotate administrator session tokens and credentials as a precautionary measure
• Implement Content Security Policy headers to restrict inline script execution

Patch Information

No official vendor patch has been identified in the available CVE data. Users should monitor the GetSimple Official Site and GitHub GetSimpleCMS Repository for security updates. Consider contacting the plugin maintainer directly or reviewing the VulnCheck Advisory for GetSimple XSS for the latest remediation guidance.

Workarounds

• Disable the My SMTP Contact Plugin entirely if contact form functionality is not critical
• Implement a reverse proxy or WAF with XSS filtering capabilities in front of the GetSimple CMS installation
• Add server-side input validation that normalizes and decodes all input before sanitization
• Restrict administrative interface access to trusted IP addresses only
• Consider migrating to an alternative contact form solution with more robust input handling

# Example .htaccess configuration to restrict admin access
<Files "admin.php">
    Order Deny,Allow
    Deny from all
    Allow from 192.168.1.0/24
    Allow from 10.0.0.0/8
</Files>

# Add Content-Security-Policy header
<IfModule mod_headers.c>
    Header set Content-Security-Policy "script-src 'self'; object-src 'none'"
</IfModule>