CVE-2025-48940

Executive Summary

CVE-2025-48940 is a Local File Inclusion (LFI) vulnerability in MyBB, a widely used open-source forum software. The flaw resides in the upgrade component, which fails to properly validate user-supplied input. An attacker can exploit this by sending a specially crafted parameter to include arbitrary local files from the server. The vulnerability is rated with a CVSS score of 7.2 (High), indicating significant confidentiality impact. Exploitation requires the installer to be unlocked (no install/lock file present) and the upgrade script to be accessible. This can occur during a fresh installation, a re-installation, or by an authenticated administrator. The issue is resolved in MyBB version 1.8.39.

Supply Chain & Package Security

MyBB is a PHP-based application distributed as a source package. The vulnerability does not directly affect package managers like NPM, PyPI, Go, or Ruby. However, organizations using MyBB should verify the integrity of their installation and ensure they are running version 1.8.39 or later. The patch is available through the official MyBB website and GitHub repository.

CVSS Breakdown & Threat Level

• CVSS Score: 7.2 (High)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N) – but requires installer unlocked or admin access
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: High (C:H)
• Integrity Impact: None (I:N)
• Availability Impact: None (A:N)

The threat level is High due to the potential for reading sensitive files (e.g., configuration files containing database credentials) without authentication under certain conditions.

Deep Dive & Mechanics

The vulnerability exists in the upgrade component of MyBB. During the upgrade process, the application includes files based on user input without proper sanitization. Specifically, the install/index.php script processes parameters that determine which upgrade steps to execute. An attacker can manipulate these parameters to traverse directories and include arbitrary files from the server.

Root Cause: The application uses user-supplied input to construct file paths without validating or sanitizing path traversal sequences (e.g., ../). This allows an attacker to read files outside the intended directory.

Exploitation Prerequisites:

1. The installer must be unlocked (no install/lock file present).
2. The upgrade script must be accessible via install/index.php.
3. The attacker must either:
   • Have access to a fresh installation (forum not yet installed).
   • Be able to re-install the forum (requires admin privileges).
   • Be an authenticated administrator.

Impact: Successful exploitation allows an attacker to read sensitive files such as:

• inc/config.php (database credentials)
• /etc/passwd (user enumeration)
• Application source code (for further vulnerability discovery)

Vulnerable vs. Patched Code

Vulnerable Code (MyBB < 1.8.39): The upgrade script likely uses a parameter like step to include files:

$step = $_GET['step'];
include("upgrade_".$step.".php");

This allows path traversal: ?step=../../../../etc/passwd%00 (null byte injection may be needed depending on PHP version).

Patched Code (MyBB 1.8.39): The fix validates the step parameter against a whitelist of allowed values:

$allowed_steps = array('1', '2', '3', '4', '5');
$step = $_GET['step'];
if (in_array($step, $allowed_steps)) {
    include("upgrade_".$step.".php");
} else {
    // Handle error
}

This prevents arbitrary file inclusion by restricting input to predefined values.

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Download MyBB 1.8.38 from the official archive.
2. Install on a local server (e.g., XAMPP, LAMP) with PHP 7.x.
3. Ensure the install/lock file is absent.
4. Access http://localhost/mybb/install/index.php.

Exploitation Steps:

1. Identify the vulnerable parameter (likely step).
2. Craft a payload to read /etc/passwd:

   http://localhost/mybb/install/index.php?step=../../../../../../../../etc/passwd%00
   

3. If successful, the file content will be displayed or included in the response.

Note: No public exploit code is available at this time. The above is a conceptual demonstration.

Blue Team

Detection:

• Monitor web server logs for requests to install/index.php with suspicious step parameters containing ../ or null bytes.
• Alert on any access to the installer when the forum is already installed and locked.

Hunting:

• Search for files like install/lock that should exist after installation. If missing, investigate.
• Review access logs for unusual patterns in the step parameter.

Indicators of Compromise (IOCs)

• File Paths:
  • install/index.php accessed with step parameter containing path traversal sequences.
• Log Entries:
  • Apache/Nginx access logs showing requests like:

    GET /install/index.php?step=../../../../etc/passwd HTTP/1.1
    

• System Files:
  • Unexpected presence of /etc/passwd or other sensitive files in web server responses.
• Network:
  • Outbound connections from the web server to unknown IPs (if attacker exfiltrates data).

Detection Engineering

YARA Rule

rule CVE_2025_48940_MyBB_LFI {
    meta:
        description = "Detects attempts to exploit CVE-2025-48940 in MyBB"
        author = "Valters IT"
        date = "2025-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-48940"
    strings:
        $traversal = /step=[^&]*\.\.\// nocase
        $nullbyte = /%00/ nocase
    condition:
        $traversal or $nullbyte
}

Sigma Rule

title: MyBB LFI Exploitation Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects path traversal attempts in MyBB install/index.php
author: Valters IT
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'GET'
        cs-uri-query|contains: 
            - 'step=../'
            - 'step=..%2f'
            - '%00'
    condition: selection
falsepositives:
    - Legitimate upgrade steps (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-48940 MyBB LFI Attempt"; flow:to_server,established; content:"GET"; http_method; content:"/install/index.php"; http_uri; content:"step="; http_uri; pcre:"/step=[^&]*\.\.\//i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade to MyBB 1.8.39 immediately.
2. If upgrade is not possible, ensure the install/lock file exists to prevent access to the installer.
3. Restrict access to install/ directory via web server configuration (e.g., .htaccess):

   <Directory /path/to/mybb/install>
       Require ip 127.0.0.1
   </Directory>
   

Long-term Measures:

• Implement a Web Application Firewall (WAF) to block path traversal patterns.
• Regularly update MyBB to the latest version.
• Conduct security audits of custom plugins and themes.
• Monitor logs for suspicious activity.

Valters IT