CVE-2025-29458

Executive Summary

CVE-2025-29458 is a disputed information disclosure vulnerability in MyBB 1.8.38. The issue is triggered via the Change Avatar function, which allegedly allows a remote attacker to obtain sensitive information. The vendor disputes this, citing that the actions are within the allowed scope of Board administrators and that SSRF mitigations are in place. No public proof-of-concept, exploit code, or patch exists at this time. The vulnerability is classified under CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor) and is mapped to MITRE ATT&CK tactic T1190 (Exploit Public-Facing Application).

Supply Chain & Package Security

MyBB is a self-hosted PHP forum application. It is not distributed via NPM, PyPI, Go, or Ruby registries. The supply chain risk is limited to the integrity of the official MyBB download source and any third-party plugins or themes. Administrators should verify the authenticity of their MyBB installation and audit any custom modifications.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0 in the NVD data, indicating that no official CVSS vector has been assigned. This is likely due to the dispute between the reporter and the vendor. Based on the description, a hypothetical CVSS v3.1 vector would be:

• Attack Vector (AV): Network (N) – the attacker can trigger the issue remotely.
• Attack Complexity (AC): Low (L) – no special conditions are required.
• Privileges Required (PR): Low (L) – the attacker likely needs a valid user account to access the Change Avatar function.
• User Interaction (UI): None (N) – no user interaction is required beyond the attacker’s own actions.
• Scope (S): Unchanged (U) – the vulnerability does not cross a trust boundary.
• Confidentiality (C): High (H) – sensitive information could be exposed.
• Integrity (I): None (N) – no data modification.
• Availability (A): None (N) – no denial of service.

Hypothetical Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N Hypothetical Score: 6.5 (Medium)

Threat Level: Medium. The lack of a public exploit and the vendor’s dispute reduce the immediate risk, but the potential for information disclosure remains.

Deep Dive & Mechanics

The vulnerability is centered on the Change Avatar functionality in MyBB 1.8.38. The exact mechanism is not publicly documented, but based on the description and CWE-200, it likely involves:

1. File Path Disclosure: The Change Avatar function may improperly handle file paths or error messages, revealing the server’s directory structure.
2. Server-Side Request Forgery (SSRF) Bypass: The vendor claims SSRF mitigations are in place, but the reporter suggests that sensitive information can still be obtained. This could involve bypassing URL validation to make the server request internal resources (e.g., file:///etc/passwd or internal cloud metadata endpoints).
3. Error-Based Information Leakage: The function might return verbose error messages that include stack traces, database credentials, or configuration details.

The dispute arises because the vendor considers the reported behavior to be within the normal administrative capabilities, while the reporter argues that it can be exploited by a lower-privileged user or in a chained attack.

Vulnerable vs. Patched Code

No official patch exists. The vulnerable code is in MyBB 1.8.38. Without a patch, the following hypothetical vulnerable code pattern is provided for illustration:

Vulnerable Code (hypothetical):

// In avatar upload handler
$avatar_url = $_POST['avatar_url'];
$image_data = file_get_contents($avatar_url); // No validation
// Process image...

Patched Code (hypothetical):

// In avatar upload handler
$avatar_url = $_POST['avatar_url'];
if (!filter_var($avatar_url, FILTER_VALIDATE_URL) || 
    !preg_match('/^https?:\/\/[a-zA-Z0-9.-]+\/.*$/', $avatar_url)) {
    die("Invalid URL");
}
$image_data = file_get_contents($avatar_url);
// Process image...

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A valid user account on the target MyBB forum.
• Access to the Change Avatar function.

Exploitation Steps:

1. Log in to the MyBB forum.
2. Navigate to User CP > Change Avatar.
3. In the “Use a URL” field, enter a malicious URL pointing to an internal resource, e.g.:

   file:///etc/passwd
   

   or

   http://169.254.169.254/latest/meta-data/
   

4. Submit the form.
5. If the server returns the contents of the requested resource, sensitive information is disclosed.

Note: No public exploit code is available. The above is a theoretical approach based on the vulnerability description.

Blue Team (Detection & Defense)

Detection:

• Monitor web server logs for requests to /usercp.php?action=avatar with unusual avatar_url parameters.
• Look for URLs containing file://, 169.254.169.254, or internal IP addresses (e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).
• Alert on error messages containing file paths or stack traces.

Defense:

• Apply input validation to the avatar_url parameter: allow only http:// and https:// schemes, and block private IP ranges.
• Disable the allow_url_fopen PHP directive if not required.
• Use a Web Application Firewall (WAF) to block SSRF attempts.
• Restrict outbound traffic from the web server to only necessary external services.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Outbound HTTP requests to internal IP addresses (e.g., 10.0.0.1, 192.168.1.1, 169.254.169.254).
  • Requests with file:// scheme in the avatar_url parameter.
• File IOCs:
  • Access to /etc/passwd, /etc/shadow, or other sensitive files via the web server.
• Log IOCs:
  • Web server logs containing file:// or internal IP addresses in the query string.
  • PHP error logs showing file inclusion attempts.

Detection Engineering

YARA Rule

rule CVE_2025_29458_MyBB_Avatar_SSRF {
    meta:
        description = "Detects SSRF attempts via MyBB Change Avatar function"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-29458"
    strings:
        $s1 = "avatar_url" nocase
        $s2 = "file://" nocase
        $s3 = "169.254.169.254" nocase
        $s4 = "10." ascii
        $s5 = "192.168." ascii
        $s6 = "172.16." ascii
    condition:
        $s1 and any of ($s2, $s3, $s4, $s5, $s6)
}

Sigma Rule

title: MyBB Change Avatar SSRF Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects SSRF attempts via the Change Avatar function in MyBB
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'action=avatar'
        cs-uri-query|contains: 'avatar_url='
    keywords:
        - 'file://'
        - '169.254.169.254'
        - '10.'
        - '192.168.'
        - '172.16.'
    condition: selection and keywords
falsepositives:
    - Legitimate use of external avatar URLs (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-29458 MyBB Change Avatar SSRF Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"action=avatar"; http_uri;
    content:"avatar_url="; http_uri;
    pcre:"/(file:\/\/|169\.254\.169\.254|10\.\d{1,3}\.\d{1,3}\.\d{1,3}|192\.168\.\d{1,3}\.\d{1,3}|172\.1[6-9]\.\d{1,3}\.\d{1,3}|172\.2[0-9]\.\d{1,3}\.\d{1,3}|172\.3[0-1]\.\d{1,3}\.\d{1,3})/Ri";
    classtype:attempted-recon;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade MyBB: If a patched version becomes available, upgrade immediately. Currently, no patch exists.
2. Disable Avatar URL Upload: In the Admin CP, navigate to Configuration > Settings > User and Registration Options and set “Allow remote avatars” to “No”.
3. Restrict Outbound Traffic: Use firewall rules to block outbound traffic from the web server to private IP ranges and sensitive endpoints (e.g., cloud metadata).
4. Input Validation: Implement custom validation for the avatar_url parameter to allow only trusted domains and schemes.

Long-Term Measures:

• Monitor the MyBB official website and GitHub for security updates.
• Conduct regular security audits of the MyBB installation and any plugins.
• Implement a Web Application Firewall (WAF) with SSRF protection rules.

Valters IT