CVE-2026-0559: MasterStudy LMS WordPress XSS Vulnerability

CVE-2026-0559 Overview

The MasterStudy LMS WordPress Plugin – for Online Courses and Education plugin for WordPress is vulnerable to Stored Cross-Site Scripting (XSS) via the plugin's stm_lms_courses_grid_display shortcode in all versions up to, and including, 3.7.11 due to insufficient input sanitization and output escaping on user supplied attributes. This makes it possible for authenticated attackers, with contributor level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.

Critical Impact

Authenticated attackers with contributor-level access can inject persistent malicious scripts that execute in the context of other users' browsers, potentially leading to session hijacking, credential theft, or administrative account compromise on WordPress sites using this popular LMS plugin.

Affected Products

• MasterStudy LMS WordPress Plugin versions up to and including 3.7.11
• WordPress installations utilizing the MasterStudy LMS plugin for online courses
• Educational websites using the stm_lms_courses_grid_display shortcode functionality

Discovery Timeline

• 2026-02-14 - CVE-2026-0559 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2026-0559

Vulnerability Analysis

This Stored Cross-Site Scripting (XSS) vulnerability exists in the MasterStudy LMS WordPress plugin's shortcode handling mechanism. The vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation), which represents a common but dangerous class of web application security flaws.

The attack requires network access and low-privilege authentication (contributor-level), making it exploitable by any authenticated user who can create or edit posts containing shortcodes. The cross-site scope indicates that the injected scripts can impact users beyond the vulnerable component itself, potentially affecting the entire WordPress session context.

The root cause lies in the plugin's failure to properly sanitize and escape user-supplied attributes within the stm_lms_courses_grid_display shortcode before rendering them in the page output. This allows attackers to craft malicious attribute values containing JavaScript code that persists in the database and executes whenever the affected page is viewed.

Root Cause

The vulnerability stems from insufficient input sanitization and output escaping on user-supplied attributes within the stm_lms_courses_grid_display shortcode handler. When the plugin processes shortcode attributes, it fails to adequately validate or encode special characters that could be interpreted as executable script content. This architectural flaw allows raw user input to flow directly into the HTML output without proper sanitization, creating a persistent XSS condition.

Attack Vector

The attack is executed over the network and requires the attacker to have at least contributor-level authentication on the target WordPress installation. The attacker crafts a malicious shortcode with specially crafted attribute values containing JavaScript payload. When this shortcode is saved (e.g., in a post or page), the malicious script is stored in the database. Subsequently, when any user—including administrators—views the page containing the injected shortcode, the malicious script executes in their browser context.

The vulnerability allows attackers to embed persistent malicious scripts within WordPress content. Since no user interaction beyond viewing the page is required for the script to execute, the attack surface is broad. The injected scripts can perform actions such as stealing session cookies, redirecting users to phishing sites, defacing website content, or escalating privileges by exploiting administrative sessions.

For detailed technical information about this vulnerability, refer to the Wordfence Vulnerability Report and the WordPress Plugin Change Log.

Detection Methods for CVE-2026-0559

Indicators of Compromise

• Suspicious shortcode attributes containing JavaScript event handlers (e.g., onload, onerror, onmouseover) in post content
• Presence of encoded or obfuscated script content within stm_lms_courses_grid_display shortcode parameters
• Unexpected outbound requests to external domains originating from page views
• User reports of unusual behavior or redirects when viewing LMS course pages

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block XSS patterns in shortcode attributes
• Monitor WordPress database for suspicious content patterns in posts and pages using the affected shortcode
• Review audit logs for contributor-level users creating or modifying content with LMS shortcodes
• Deploy Content Security Policy (CSP) headers to restrict inline script execution and report violations

Monitoring Recommendations

• Enable detailed logging for all shortcode rendering operations within the MasterStudy LMS plugin
• Configure SentinelOne Singularity to monitor for suspicious JavaScript execution patterns on WordPress hosts
• Set up alerts for unusual post modification activity from contributor-level accounts
• Monitor for CSP violation reports that may indicate attempted XSS exploitation

How to Mitigate CVE-2026-0559

Immediate Actions Required

• Update the MasterStudy LMS WordPress plugin to a version newer than 3.7.11 immediately
• Audit existing posts and pages for malicious content within stm_lms_courses_grid_display shortcodes
• Review contributor-level user accounts and their recent content modifications for suspicious activity
• Implement Content Security Policy headers to mitigate the impact of any successful XSS exploitation

Patch Information

The security fix for this vulnerability is available in the plugin update. The patch addresses the insufficient input sanitization by implementing proper escaping of user-supplied shortcode attributes before they are rendered in the page output. Technical details of the changes can be reviewed in the WordPress Plugin Change Log.

Organizations should prioritize updating to the patched version through the WordPress admin dashboard or via WP-CLI. The Wordfence Vulnerability Report provides additional context and remediation guidance.

Workarounds

• Temporarily restrict contributor-level users from creating or editing posts containing LMS shortcodes
• Implement server-side input validation to strip potentially malicious attributes from shortcode parameters
• Deploy a WAF rule to filter XSS patterns in requests containing the stm_lms_courses_grid_display shortcode
• Consider temporarily disabling the affected shortcode functionality until the patch can be applied

# WordPress CLI command to update the MasterStudy LMS plugin
wp plugin update masterstudy-lms-learning-management-system

# Verify the installed version after update
wp plugin list --name=masterstudy-lms-learning-management-system --fields=name,version,status

# Search for potentially malicious shortcode content in the database
wp db query "SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%stm_lms_courses_grid_display%' AND (post_content LIKE '%<script%' OR post_content LIKE '%javascript:%' OR post_content LIKE '%onerror%' OR post_content LIKE '%onload%');"