CVE-2026-7016: MaxSite CMS Ushki Plugin XSS Vulnerability

CVE-2026-7016 Overview

A stored cross-site scripting (XSS) vulnerability has been identified in MaxSite CMS versions up to 109.3. The vulnerability exists in the ushki Plugin component, where improper input validation allows attackers to inject malicious scripts through the f_ushka_new and f_ushk parameters. While the vendor classifies this as a "Self-XSS" requiring user interaction, it still represents a violation of secure coding standards and could potentially be leveraged in targeted attacks.

Critical Impact

Attackers with high privileges can inject malicious scripts that persist in the application. The vulnerability requires user interaction but can lead to integrity violations within the affected CMS instance.

Affected Products

• MaxSite CMS versions up to 109.3
• MaxSite CMS ushki Plugin (affected component)

Discovery Timeline

• 2026-04-26 - CVE-2026-7016 published to NVD
• 2026-04-29 - Last updated in NVD database

Technical Details for CVE-2026-7016

Vulnerability Analysis

This stored XSS vulnerability arises from the absence of proper output encoding in the ushki Plugin component of MaxSite CMS. When user-supplied input is processed through the f_ushka_new and f_ushk arguments, the application fails to sanitize the data using proper encoding functions such as htmlspecialchars(). This allows malicious JavaScript code to be stored in the application and subsequently executed when the affected page is rendered.

The vendor acknowledged that while they consider this a Self-XSS scenario (requiring the attacker to have elevated privileges and victim interaction), it nonetheless violates secure coding standards. The lack of filtering creates conditions where incorrect or malicious data could be displayed to users, potentially leading to session hijacking, credential theft, or other client-side attacks in targeted scenarios.

Root Cause

The root cause of this vulnerability is the improper handling of user input in PHP output contexts. Specifically, the application directly outputs user-controlled data without applying the htmlspecialchars() function to properly encode HTML special characters. This is a classic input validation failure that enables stored XSS attacks.

Attack Vector

The attack is exploitable remotely over the network but requires the attacker to have high privileges within the CMS and relies on user interaction for successful exploitation. An attacker with administrative access to the ushki Plugin settings can inject malicious JavaScript through the vulnerable parameters. When another user views the affected content, the injected script executes within their browser context.

The security patch demonstrates the fix by adding htmlspecialchars() encoding to user-controlled output:

// Before fix - vulnerable code
$form .= '<p><strong>' . t('Файл для логов:') . '</strong> ' . getinfo('uploads_dir') . ' <input name="f_logging_file" type="text" value="' . $options['logging_file'] . '">';

// After fix - properly encoded
$form .= '<p><strong>' . t('Файл для логов:') . '</strong> ' . getinfo('uploads_dir') . ' <input name="f_logging_file" type="text" value="' . htmlspecialchars($options['logging_file']) . '">';

Source: GitHub Security Commit

Detection Methods for CVE-2026-7016

Indicators of Compromise

• Unusual JavaScript code or HTML tags stored in ushki Plugin configuration fields
• Unexpected <script> tags or event handlers (e.g., onerror, onload) in plugin parameters
• User reports of unexpected browser behavior when accessing CMS admin pages
• Web Application Firewall (WAF) logs showing XSS payload patterns in POST requests to plugin endpoints

Detection Strategies

• Review MaxSite CMS version numbers; installations running 109.3 or earlier are vulnerable
• Implement WAF rules to detect common XSS payload patterns in requests targeting the ushki Plugin
• Audit plugin configuration data for stored malicious content or suspicious script injections
• Monitor administrative user activity for unusual configuration changes in plugin settings

Monitoring Recommendations

• Enable verbose logging for CMS administrative actions, particularly plugin configuration changes
• Deploy client-side security monitoring to detect anomalous JavaScript execution
• Configure alerting for modifications to ushki Plugin settings from unexpected IP addresses
• Implement Content Security Policy (CSP) headers to mitigate impact of successful XSS attacks

How to Mitigate CVE-2026-7016

Immediate Actions Required

• Upgrade MaxSite CMS to version 109.4 or later immediately
• Review ushki Plugin configuration for any suspicious or unexpected content
• Audit administrative user accounts for unauthorized access or privilege abuse
• Implement Content Security Policy (CSP) headers as a defense-in-depth measure

Patch Information

The vulnerability has been addressed in MaxSite CMS version 109.4. The fix applies proper output encoding using the htmlspecialchars() function to prevent XSS injection. The security patch is identified by commit hash 8a3946bd0a54bfb72a4d57179fcd253f2c550cd7. Organizations should upgrade to the patched version as recommended by the vendor.

For reference, consult the following resources:

• MaxSite CMS GitHub Repository
• Security Patch Commit
• VulDB Advisory

Workarounds

• If immediate patching is not possible, restrict administrative access to the ushki Plugin to trusted users only
• Implement strict Content Security Policy headers to prevent inline script execution
• Deploy a Web Application Firewall (WAF) with XSS detection rules for additional protection
• Regularly audit plugin configuration fields for suspicious content until the patch can be applied

# Example CSP header configuration for Apache
# Add to .htaccess or httpd.conf to mitigate XSS impact
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none'"