CVE-2025-29457

Executive Summary

CVE-2025-29457 is a disputed information disclosure vulnerability in MyBB 1.8.38. The issue arises within the "Import a Theme" administrative function, where a remote attacker with administrative privileges can potentially obtain sensitive information. The vendor disputes this finding, citing that the actions are within the allowed scope of Board administrators and that SSRF mitigations are in place. Despite the dispute, the exposure of sensitive data through theme import functionality represents a risk in environments where administrative accounts may be compromised or where least privilege is not strictly enforced. The vulnerability is classified under CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor) and aligns with MITRE ATT&CK tactic T1190 (Exploit Public-Facing Application). No public proof-of-concept or exploit code is available at this time.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, indicating that no official CVSS vector has been assigned by NVD. However, based on the description and the disputed nature, a hypothetical assessment would consider:

• Attack Vector (AV): Network (the function is accessible via the web interface)
• Attack Complexity (AC): Low (the import function is standard)
• Privileges Required (PR): High (requires admin login)
• User Interaction (UI): None
• Scope (S): Unchanged
• Confidentiality (C): High (sensitive information disclosure)
• Integrity (I): None
• Availability (A): None

Hypothetical CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N
Hypothetical Score: 4.9 (Medium)

Threat Level: Medium. The requirement for administrative privileges reduces the likelihood of exploitation by external attackers, but the potential for sensitive data exposure makes it a concern for internal threats or post-compromise scenarios.

Deep Dive & Mechanics

The vulnerability is located in the theme import functionality of MyBB 1.8.38. When an administrator imports a theme, the system processes XML or other archive files containing theme definitions. The issue likely stems from insufficient validation or sanitization of the imported data, allowing an attacker to craft a malicious theme file that, when processed, reveals sensitive server information such as file paths, database credentials, or configuration details.

Technical Flow:

1. An administrator accesses the Admin CP and navigates to the "Import a Theme" section.
2. The administrator uploads a theme archive (e.g., .xml or .zip).
3. MyBB parses the theme file, extracting metadata, templates, and settings.
4. Due to a lack of proper input validation, the parser may include or expose server-side variables, error messages, or file system paths in the response.
5. The attacker, posing as an administrator, can then view this sensitive information in the import result or error messages.

The vendor disputes the finding, arguing that:

• The actions are within the allowed scope of Board administrators.
• SSRF mitigations are in place, preventing server-side request forgery attacks.

However, the core issue remains: an administrator (or an attacker who has compromised an admin account) can extract sensitive information that should not be exposed, even to privileged users.

Vulnerable vs. Patched Code

No official patch has been released by MyBB. The vendor disputes the vulnerability, so no patched code is available. The vulnerable code is presumed to be in the theme import handler, likely within files such as admin/modules/style/themes.php or similar.

Vulnerable Code (Hypothetical):

// Vulnerable: Directly outputs error messages or file contents
$theme_data = file_get_contents($_FILES['theme']['tmp_name']);
$xml = simplexml_load_string($theme_data);
if (!$xml) {
    echo "Error: " . libxml_get_last_error();
}

Patched Code (Hypothetical):

// Patched: Sanitize output and suppress detailed error messages
$theme_data = file_get_contents($_FILES['theme']['tmp_name']);
$xml = simplexml_load_string($theme_data);
if (!$xml) {
    error_log("Theme import failed: " . libxml_get_last_error());
    echo "Error: Invalid theme file.";
}

Without an official patch, administrators must rely on workarounds and monitoring.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Administrative credentials to the MyBB Admin CP.
• Network access to the MyBB instance.

Exploitation Steps:

1. Log in to the MyBB Admin CP as an administrator.
2. Navigate to Styles > Themes > Import a Theme.
3. Craft a malicious theme XML file that includes references to sensitive files (e.g., ../../inc/config.php) or triggers verbose error messages.
4. Upload the file and observe the response for leaked information.

Example Malicious Theme XML:

<?xml version="1.0" encoding="UTF-8"?>
<theme>
    <name>Malicious Theme</name>
    <templates>
        <template name="header">
            <![CDATA[<?php echo file_get_contents('../../inc/config.php'); ?>]]>
        </template>
    </templates>
</theme>

Note: This is a conceptual example. Actual exploitation may vary based on MyBB's parsing logic.

Blue Team

Detection and Prevention:

• Restrict administrative access to trusted users only.
• Implement multi-factor authentication (MFA) for admin accounts.
• Monitor Admin CP access logs for unusual theme import activity.
• Use Web Application Firewall (WAF) rules to block suspicious file uploads.
• Regularly review and audit theme files before import.

Honeypot Strategy:

• Deploy a decoy MyBB instance with fake admin credentials to lure attackers.
• Monitor for theme import attempts and log the source IP.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (None available, but monitor for unknown theme files)
• Network Indicators:
  • POST requests to /admin/index.php?module=style-themes&action=import
  • Unusual file uploads with .xml or .zip extensions
• Log Entries:
  • Admin CP access logs showing theme import actions from unexpected IPs
  • Error logs containing file paths or configuration data
• Behavioral:
  • Sudden appearance of new themes in the system
  • Unauthorized changes to theme settings

Detection Engineering

YARA Rule

rule Detect_Malicious_MyBB_Theme_Import {
    meta:
        description = "Detects malicious theme files targeting MyBB CVE-2025-29457"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-29457"
    strings:
        $s1 = "file_get_contents" nocase
        $s2 = "../../" nocase
        $s3 = "config.php" nocase
        $s4 = "simplexml_load_string" nocase
    condition:
        any of ($s1, $s2, $s3) and $s4
}

Sigma Rule

title: MyBB Theme Import Information Disclosure Attempt
id: 8f4c3b2a-1d5e-4f7c-9a8b-0c1d2e3f4a5b
status: experimental
description: Detects attempts to exploit CVE-2025-29457 via malicious theme import
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'POST'
        c-uri: '/admin/index.php'
        cs-uri-query: 'module=style-themes&action=import'
    condition: selection
falsepositives:
    - Legitimate theme imports by authorized administrators
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"MyBB Theme Import Information Disclosure Attempt (CVE-2025-29457)";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/admin/index.php"; http_uri;
    content:"module=style-themes&action=import"; http_uri;
    classtype:attempted-recon;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Restrict Admin CP access to trusted IPs and enforce strong passwords.
2. Enable multi-factor authentication for all administrative accounts.
3. Monitor and audit all theme import activities.
4. Implement a Web Application Firewall (WAF) to inspect uploaded files.

Long-Term Fixes:

• Await an official patch from MyBB. In the meantime, consider disabling the theme import feature if not required.
• Apply the principle of least privilege: limit the number of administrators who can import themes.
• Regularly review and update MyBB to the latest version.

Workaround:

• Manually review all theme files before import using a sandboxed environment.
• Modify the theme import handler to suppress detailed error messages and validate file contents.

Valters IT
Securing Digital Frontiers