CVE-2018-25309: MyBB Recent Threads XSS Vulnerability

CVE-2018-25309 Overview

CVE-2018-25309 is a persistent cross-site scripting (XSS) vulnerability affecting the MyBB Recent Threads plugin version 17.0. This vulnerability allows attackers to inject malicious scripts by creating threads with crafted subject lines, which are then executed in the browsers of all users viewing the index page.

Critical Impact

Attackers can execute arbitrary JavaScript in victim browsers, potentially leading to session hijacking, credential theft, and unauthorized actions on behalf of authenticated users.

Affected Products

• MyBB Recent Threads Plugin 17.0
• MyBB forums utilizing the vulnerable Recent Threads plugin

Discovery Timeline

• 2026-04-29 - CVE CVE-2018-25309 published to NVD
• 2026-04-29 - Last updated in NVD database

Technical Details for CVE-2018-25309

Vulnerability Analysis

This persistent XSS vulnerability exists due to insufficient input sanitization in the MyBB Recent Threads plugin. When users create forum threads, the plugin fails to properly sanitize or escape the thread subject parameter before rendering it on the index page. This allows malicious actors to embed script tags or other JavaScript constructs within thread titles.

Because the malicious payload is stored in the database and rendered every time the index page loads, this constitutes a stored (persistent) XSS attack. Every user who visits the forum's index page will have the attacker's JavaScript executed in their browser context, making this a particularly dangerous vulnerability in high-traffic forums.

The vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation), which encompasses cross-site scripting flaws where user-controllable input is embedded into web pages without proper encoding or validation.

Root Cause

The root cause of this vulnerability is the absence of proper output encoding when displaying thread subjects on the forum index page. The Recent Threads plugin directly renders user-supplied thread subject data without applying HTML entity encoding or other sanitization techniques that would neutralize potentially malicious script content.

Attack Vector

The attack is network-based and requires a low-privileged user account capable of creating forum threads. The attacker crafts a malicious thread with script tags embedded in the subject line parameter. Once the thread is created, the malicious JavaScript is stored in the database and subsequently executed in the browsers of all users viewing the index page where recent threads are displayed.

This stored XSS attack can be leveraged to steal session cookies, redirect users to phishing sites, perform actions on behalf of victims, or spread malware through drive-by downloads.

Technical details and proof-of-concept information are available through the Exploit-DB #44420 entry and the VulnCheck MyBB XSS Advisory.

Detection Methods for CVE-2018-25309

Indicators of Compromise

• Thread subjects containing script tags such as <script>, <img onerror=, or other HTML event handlers
• Database entries in the threads table with unexpected JavaScript code in subject fields
• Unusual JavaScript execution errors in browser developer consoles when viewing forum pages
• Reports from users about unexpected redirects or pop-ups when viewing the forum index

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block XSS payloads in form submissions
• Monitor server logs for POST requests to thread creation endpoints containing suspicious script patterns
• Deploy Content Security Policy (CSP) headers to detect and report inline script execution violations
• Conduct regular security scans of the forum database for stored XSS payloads in thread subjects

Monitoring Recommendations

• Enable detailed logging on the MyBB application to capture thread creation events
• Configure browser-based XSS Auditor or CSP violation reporting to detect exploitation attempts
• Set up alerts for unusual patterns in thread subject content during moderation review
• Implement automated database scanning to detect HTML/JavaScript injection in user-generated content

How to Mitigate CVE-2018-25309

Immediate Actions Required

• Disable or remove the Recent Threads plugin version 17.0 until a patched version is available
• Review existing threads in the database for malicious content in subject fields and sanitize as needed
• Implement Content Security Policy headers to mitigate the impact of any stored XSS payloads
• Consider temporarily restricting thread creation privileges to trusted users

Patch Information

Administrators should check the MyBB Plugin Details page for updated versions of the Recent Threads plugin that address this vulnerability. If no patched version is available, consider alternative plugins with proper input sanitization or implement custom fixes.

Workarounds

• Apply server-side input validation to sanitize thread subjects before storage
• Implement HTML entity encoding for all user-supplied content displayed on the forum
• Use a Web Application Firewall to filter malicious payloads in form submissions
• Deploy strict Content Security Policy headers to prevent inline script execution

Administrators can implement basic output encoding by modifying the plugin to escape HTML entities in thread subjects before rendering. However, this should be considered a temporary measure until an official patch is available.