CVE-2018-25116: MyBB Thread Redirect Plugin XSS Flaw

CVE-2018-25116 Overview

CVE-2018-25116 is a Cross-Site Scripting (XSS) vulnerability affecting the MyBB Thread Redirect Plugin version 0.2.1. The vulnerability exists in the custom text input field for thread redirects, where insufficient input validation allows attackers to inject malicious SVG scripts. When other users view the affected thread, the injected scripts execute in their browser context, enabling arbitrary script execution.

Critical Impact

Attackers can inject malicious SVG scripts through the thread redirect custom text field, enabling session hijacking, credential theft, and malicious actions performed on behalf of authenticated users.

Affected Products

• MyBB Thread Redirect Plugin version 0.2.1
• MyBB forums utilizing the vulnerable Thread Redirect Plugin

Discovery Timeline

• 2026-01-23 - CVE CVE-2018-25116 published to NVD
• 2026-01-26 - Last updated in NVD database

Technical Details for CVE-2018-25116

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting (XSS). The flaw occurs because the Thread Redirect Plugin fails to properly sanitize user-supplied input in the custom text field used for thread redirects. Attackers with sufficient privileges to create or modify thread redirects can embed malicious SVG elements containing JavaScript code.

When a victim navigates to a page containing the malicious redirect entry, the browser interprets the SVG payload and executes the embedded script. This stored XSS attack persists in the database, affecting all users who subsequently view the compromised content. The network-based attack vector requires low privileges but does require user interaction, as victims must view the affected page for the payload to execute.

Root Cause

The root cause of this vulnerability is insufficient input validation and output encoding in the Thread Redirect Plugin's handling of custom text fields. The plugin accepts user-provided content and renders it directly in the HTML output without proper sanitization, allowing SVG-based script injection. Specifically, the plugin fails to strip or encode potentially dangerous HTML elements and event handlers within SVG tags, which browsers interpret as executable content.

Attack Vector

The attack vector involves an authenticated attacker with permissions to create thread redirects submitting a malicious payload containing SVG elements with embedded JavaScript. The payload is stored in the forum database and rendered whenever users view the affected content. The malicious script can then perform actions such as stealing session cookies, redirecting users to phishing pages, modifying page content, or performing actions on behalf of the victim user.

The vulnerability is triggered through SVG onload events or similar mechanisms that execute JavaScript when the browser parses the SVG element. For detailed technical analysis and proof-of-concept information, refer to the Exploit-DB entry #49505 and the VulnCheck Advisory.

Detection Methods for CVE-2018-25116

Indicators of Compromise

• Presence of <svg> tags with embedded onload, onerror, or similar event handlers in thread redirect custom text fields
• Unusual JavaScript execution or browser console errors when viewing thread redirects
• User reports of unexpected browser behavior or redirects when viewing forum threads
• Database entries containing SVG elements with script content in redirect-related tables

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block SVG-based XSS payloads in form submissions
• Deploy Content Security Policy (CSP) headers to restrict inline script execution and mitigate XSS impact
• Conduct regular security audits of database content for suspicious SVG or script tags in user-controlled fields
• Monitor browser-side error logs and security events for script injection attempts

Monitoring Recommendations

• Enable detailed logging for forum plugin activities, particularly thread redirect modifications
• Configure real-time alerting for database modifications containing potentially malicious patterns
• Implement user behavior analytics to detect anomalous session activity that may indicate session hijacking
• Review access logs for patterns indicating exploitation attempts against thread redirect functionality

How to Mitigate CVE-2018-25116

Immediate Actions Required

• Disable or remove the Thread Redirect Plugin version 0.2.1 until a patched version is available
• Review existing thread redirects for malicious content and remove any suspicious entries
• Implement strict Content Security Policy headers to prevent inline script execution
• Restrict permissions for creating and modifying thread redirects to trusted administrators only

Patch Information

No official patch information is currently available from the vendor. Administrators should monitor the GitHub repository for updates. Until a patch is released, it is recommended to disable the plugin or implement manual input sanitization. Additional details about the vulnerability can be found in the VulnCheck Advisory.

Workarounds

• Remove or disable the Thread Redirect Plugin until a security update is available
• Manually sanitize the custom text field input by stripping HTML tags and encoding special characters at the application level
• Implement a Content Security Policy that blocks inline scripts and restricts script sources to trusted origins
• Restrict plugin functionality to administrators only and review all redirect entries before publishing

# Example: Add CSP header in MyBB configuration or .htaccess
# Apache configuration example
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; object-src 'none'"

# Alternatively in PHP (add to global.php or similar)
# header("Content-Security-Policy: default-src 'self'; script-src 'self'; object-src 'none'");