CVE-2021-47913: PHP Melody 3.0 XSS Vulnerability

CVE-2021-47913 Overview

CVE-2021-47913 is a persistent Cross-Site Scripting (XSS) vulnerability affecting PHP Melody 3.0, a popular video content management system. The vulnerability exists within the video editor component, specifically in the WYSIWYG editor functionality, allowing privileged users to inject malicious scripts that persist in the application database.

When exploited, attackers with authenticated access can inject JavaScript payloads that execute in the browsers of other users viewing the affected content. This stored XSS vulnerability poses significant risks including session hijacking, credential theft, and manipulation of the application interface for subsequent users.

Critical Impact

Privileged users can inject persistent malicious scripts through the video editor's WYSIWYG component, enabling session hijacking and application manipulation for any user viewing the compromised content.

Affected Products

• PHP Melody 3.0

Discovery Timeline

• 2026-02-01 - CVE CVE-2021-47913 published to NVD
• 2026-02-03 - Last updated in NVD database

Technical Details for CVE-2021-47913

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 specifically manifests as a stored/persistent XSS vulnerability within the PHP Melody video editor interface.

The root issue lies in insufficient input validation and output encoding within the WYSIWYG editor component. When privileged users create or edit video content, the editor allows HTML and JavaScript content to be submitted and stored without proper sanitization. This malicious content is then rendered without encoding when other users view the affected pages, causing the injected scripts to execute in their browser context.

The network-based attack vector requires user interaction—a victim must view the page containing the injected content. The vulnerability requires low-privilege authentication to exploit, meaning an attacker needs a valid account with access to the video editor functionality.

Root Cause

The vulnerability stems from improper neutralization of user-supplied input in the video editor's WYSIWYG component. PHP Melody 3.0 fails to adequately sanitize HTML content submitted through the rich text editor before storing it in the database. Additionally, when this content is retrieved and displayed to users, the application does not perform proper output encoding, allowing script tags and event handlers to execute in the victim's browser.

Attack Vector

The attack follows a stored XSS pattern where a privileged attacker with access to the video editor submits malicious JavaScript code embedded within video descriptions, titles, or other editable fields processed by the WYSIWYG editor. The malicious payload is stored in the application database and subsequently served to any user who views the compromised content.

Typical exploitation scenarios include injecting scripts that steal session cookies, perform actions on behalf of the victim, redirect users to phishing sites, or modify the displayed page content. Since this is a persistent XSS vulnerability, the malicious script executes every time a user loads the affected page, potentially impacting a large number of victims over time.

The vulnerability is exploited through the WYSIWYG editor interface by crafting input that bypasses client-side filters (if any) and exploits the lack of server-side sanitization. Technical details and proof-of-concept information can be found in the Vulnerability Lab #2291 advisory.

Detection Methods for CVE-2021-47913

Indicators of Compromise

• Unexpected JavaScript code present in video descriptions, titles, or other WYSIWYG-editable fields in the PHP Melody database
• User reports of unexpected browser behavior, redirects, or pop-ups when viewing video content
• Session tokens being transmitted to unknown external domains

Detection Strategies

• Implement Content Security Policy (CSP) headers to detect and block unauthorized script execution
• Monitor database content for suspicious HTML tags and JavaScript patterns in user-submitted fields
• Review web application firewall (WAF) logs for XSS payload patterns targeting the video editor endpoints

Monitoring Recommendations

• Enable detailed logging for the PHP Melody admin panel and video editor activities
• Set up alerts for database modifications to video content fields that contain script-related HTML elements
• Monitor network traffic for unusual outbound connections from user browsers when accessing the video platform

How to Mitigate CVE-2021-47913

Immediate Actions Required

• Update PHP Melody to the latest patched version as indicated in the PHP Melody Vulnerability Report
• Review and audit existing video content in the database for potentially malicious JavaScript injections
• Implement strict Content Security Policy headers to limit script execution sources
• Consider temporarily restricting access to the video editor for non-essential users until patching is complete

Patch Information

PHPSugar has released a security update addressing this vulnerability. Administrators should consult the official PHP Melody Vulnerability Report for specific patch details and update instructions. Apply the vendor-provided patch immediately and verify that the update has been successfully installed.

Workarounds

• Implement server-side input validation to strip or encode HTML tags and JavaScript from user input before storage
• Deploy a Web Application Firewall (WAF) configured with XSS protection rules to filter malicious input
• Restrict video editor access to only trusted administrators until the official patch can be applied
• Add Content Security Policy headers to prevent inline script execution as a defense-in-depth measure

# Example Apache configuration for Content Security Policy header
<IfModule mod_headers.c>
    Header set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'self';"
</IfModule>