CVE-2025-39565

Executive Summary

CVE-2025-39565 is a critical deserialization vulnerability in the MelaPress Login Security WordPress plugin, affecting versions up to and including 2.1.0. The flaw resides in the plugin's handling of serialized user input, allowing an unauthenticated attacker to inject arbitrary PHP objects. Successful exploitation can lead to remote code execution (RCE), complete site compromise, and lateral movement within the hosting environment. As a WordPress plugin vulnerability, this issue directly impacts the WordPress ecosystem and does not involve Apache or Nginx server software.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. The supply chain risk is elevated due to the lack of a confirmed patch and the plugin's widespread use in securing login pages. Organizations should immediately inventory all WordPress installations using MelaPress Login Security and prepare for emergency patching once a fix is released.

CVSS Breakdown & Threat Level

• CVSS Score: 6.6 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: High (C:H)
• Integrity Impact: High (I:H)
• Availability Impact: High (A:H)

Despite the Medium score, the actual risk is High due to the ease of exploitation and potential for full server compromise. The low EPSS score (0.0%) suggests limited active exploitation, but this may change as proof-of-concept code emerges.

Deep Dive & Mechanics

The vulnerability exists in the melapress-login-security plugin's deserialization routine, likely within a function that processes user-supplied serialized data (e.g., from cookies, POST parameters, or session tokens). PHP's unserialize() function is called on untrusted input without proper validation or sanitization.

Attack Flow:

1. Attacker crafts a malicious serialized PHP object containing a gadget chain (e.g., using common WordPress classes like WP_HTML_Token or Requests_Utility_FilteredIterator).
2. The attacker sends the payload via a vulnerable endpoint (e.g., login form, AJAX handler, or REST API).
3. The plugin calls unserialize() on the attacker-controlled data.
4. PHP's object instantiation triggers magic methods (__wakeup(), __destruct(), __toString()) in the gadget chain.
5. The attacker achieves arbitrary code execution, file read/write, or database access.

Root Cause: The plugin fails to implement input validation, type checking, or use of safe alternatives like json_decode() or is_serialized() with strict checks.

Vulnerable vs. Patched Code

Vulnerable Code (<= 2.1.0):

function process_login_data($data) {
    $unserialized = unserialize($data); // No validation
    // ... process $unserialized
}

Patched Code (Hypothetical):

function process_login_data($data) {
    if (!is_serialized($data, false)) {
        return false;
    }
    $allowed_classes = ['stdClass', 'MySafeClass'];
    $unserialized = unserialize($data, ['allowed_classes' => $allowed_classes]);
    // ... process $unserialized
}

The patch should:

• Validate input is serialized using is_serialized().
• Restrict allowed classes via the allowed_classes option.
• Consider replacing unserialize() with json_decode() where possible.

Exploitation & Lab Setup

Red Team

Lab Environment:

• WordPress 6.x with MelaPress Login Security 2.1.0
• PHP 8.1+
• Apache/Nginx (not vulnerable themselves)

Exploitation Steps:

1. Identify a vulnerable endpoint (e.g., /wp-admin/admin-ajax.php?action=melapress_process_login).
2. Craft a PHP gadget chain using common WordPress classes. Example using WP_HTML_Token:

class WP_HTML_Token {
    public $name;
    public $attributes;
    public function __construct($name, $attributes) {
        $this->name = $name;
        $this->attributes = $attributes;
    }
}
// Build chain to execute system command
$payload = new WP_HTML_Token('div', ['data-cmd' => 'id']);
$serialized = serialize($payload);

3. Send the serialized payload via POST:

curl -X POST https://target.com/wp-admin/admin-ajax.php \
  -d "action=melapress_process_login&data=$serialized"

4. Monitor for command execution (e.g., via out-of-band DNS or HTTP callback).

Note: No public exploit code is available; this is a conceptual demonstration.

Blue Team

Detection & Response:

• Monitor for unusual serialized data in HTTP requests (e.g., O:, C:, a: prefixes).
• Deploy WAF rules to block serialized PHP objects in non-essential parameters.
• Enable PHP unserialize() error logging and monitor for __PHP_Incomplete_Class errors.
• Use runtime application self-protection (RASP) to intercept deserialization calls.

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP requests containing serialized PHP objects (e.g., O:14:"WP_HTML_Token":2:{...})
• Outbound connections to unknown IPs on ports 80/443 (C2 callback)
• Unusual User-Agent strings or repeated POST requests to /wp-admin/admin-ajax.php

File System IOCs:

• Unexpected PHP files in /wp-content/uploads/ or /wp-content/plugins/
• Modified .htaccess or wp-config.php files
• Presence of webshells (e.g., cmd.php, shell.php)

Process IOCs:

• php processes executing with elevated privileges
• Unexpected child processes spawned by web server (e.g., bash, nc, curl)

Detection Engineering

YARA Rule

rule CVE_2025_39565_Deserialization {
    meta:
        description = "Detects serialized PHP objects targeting MelaPress Login Security"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-39565"
    strings:
        $serialized = /O:\d+:"[A-Za-z0-9_]+":\d+:\{/
        $melapress = "melapress-login-security"
        $wp_class = /WP_HTML_Token|Requests_Utility_FilteredIterator/
    condition:
        $serialized and ($melapress or $wp_class)
}

Sigma Rule

title: CVE-2025-39565 MelaPress Login Security Deserialization
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects exploitation attempts against CVE-2025-39565
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        c-uri|contains: '/wp-admin/admin-ajax.php'
        cs-uri-query|contains: 'action=melapress_process_login'
        cs-request-body|contains: 
            - 'O:'
            - 'C:'
            - 'a:'
    condition: selection
falsepositives:
    - Legitimate plugin functionality using serialized data
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-39565 MelaPress Login Security Deserialization Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/wp-admin/admin-ajax.php"; http_uri;
    content:"action=melapress_process_login"; http_client_body;
    pcre:"/(O:\d+:|C:\d+:)/Ri";
    classtype:attempted-admin;
    sid:10000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Disable the MelaPress Login Security plugin until a patch is available.
2. Implement a Web Application Firewall (WAF) rule to block serialized PHP objects.
3. Monitor logs for exploitation attempts using the provided Sigma/Suricata rules.

Long-term Fix:

• Apply the official patch once released by Melapress.
• Replace unserialize() with json_decode() or use unserialize() with allowed_classes option.
• Conduct a security audit of all WordPress plugins for similar deserialization flaws.

Workaround:

• Add the following to .htaccess to block direct access to vulnerable endpoints:

<FilesMatch "admin-ajax\.php">
    <IfModule mod_rewrite.c>
        RewriteCond %{REQUEST_METHOD} POST
        RewriteCond %{QUERY_STRING} action=melapress_process_login
        RewriteRule .* - [F]
    </IfModule>
</FilesMatch>

Valters IT
Securing Digital Frontiers