CVE-2025-66055

Executive Summary

CVE-2025-66055 is a critical deserialization vulnerability in the Icegram Email Subscribers & Newsletters WordPress plugin (versions <= 5.9.10). This flaw allows an unauthenticated attacker to perform Object Injection via untrusted data deserialization, potentially leading to remote code execution (RCE), data exfiltration, or complete site compromise. The vulnerability is classified as WordPress ecosystem issue and does not involve Apache or Nginx directly. No public proof-of-concept (PoC) or exploit code has been released, and the vendor has not provided a patch. The EPSS score is 0.0%, indicating low exploitation likelihood at this time, but the high CVSS score (7.2) warrants immediate attention.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. Users who installed the plugin from the WordPress dashboard or via direct download are at risk. No known supply chain compromise has been reported, but the lack of a patch means the plugin should be considered a security risk until updated.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Base Score: 7.2 (High)
• 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)

The threat level is High due to the ease of exploitation (no authentication required) and the potential for full system compromise.

Deep Dive & Mechanics

The vulnerability resides in the email-subscribers plugin, specifically in how it handles serialized PHP objects. The plugin likely uses unserialize() on user-supplied input without proper validation or sanitization. An attacker can craft a malicious serialized object that, when deserialized, triggers arbitrary PHP object instantiation and method execution (gadget chains). This can lead to:

• Remote code execution via PHP object injection.
• File read/write operations.
• Database manipulation.
• Privilege escalation.

The attack vector is HTTP POST requests to endpoints that accept serialized data, such as AJAX handlers or form submissions. No authentication is required.

Vulnerable vs. Patched Code

Vulnerable Code (<= 5.9.10):

// Example vulnerable pattern
$data = $_POST['some_field'];
$object = unserialize($data); // No validation

Patched Code (Hypothetical):

// Secure alternative using json_decode
$data = $_POST['some_field'];
$object = json_decode($data, true); // Safe
// Or validate with allowed_classes
$object = unserialize($data, ['allowed_classes' => false]);

Since no official patch exists, the only mitigation is to disable or remove the plugin.

Exploitation & Lab Setup

Red Team

Prerequisites:

• WordPress site with Email Subscribers & Newsletters plugin <= 5.9.10.
• Knowledge of PHP gadget chains (e.g., Monolog, Guzzle, or WordPress core classes).

Exploitation Steps:

1. Identify the vulnerable endpoint (e.g., /wp-admin/admin-ajax.php with action es_handle_subscription).
2. Craft a malicious serialized payload using a known PHP gadget chain.
3. Send the payload via POST request:

curl -X POST http://target/wp-admin/admin-ajax.php \
  -d 'action=es_handle_subscription&data=O:12:"PHPObject":1:{s:4:"prop";s:4:"exec";}'

4. Execute arbitrary commands or exfiltrate data.

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

Blue Team

Detection:

• Monitor for unusual serialized data in POST parameters (e.g., O:, a:, s: prefixes).
• Look for unexpected PHP class instantiation in logs.
• Use WAF rules to block serialized payloads.

Honeypot:

• Deploy a decoy WordPress instance with the vulnerable plugin to capture attacker behavior.

Indicators of Compromise (IOCs)

• Network: POST requests containing serialized PHP objects (e.g., O:, C:, a:).
• File System: Unexpected files in /wp-content/uploads/ or /tmp/.
• Logs: PHP error logs showing unserialize() errors or unexpected class loading.
• Processes: Unusual PHP processes or outbound connections from the web server.

Detection Engineering

YARA Rule

rule CVE_2025_66055_Deserialization {
    meta:
        description = "Detects serialized PHP objects in HTTP POST data targeting Email Subscribers plugin"
        author = "Valters IT"
        date = "2025-01-01"
        reference = "CVE-2025-66055"
    strings:
        $serialized = /(O|C):\d+:"/
        $plugin_path = /email-subscribers/
    condition:
        $serialized and $plugin_path
}

Sigma Rule

title: CVE-2025-66055 - Deserialization Attempt on Email Subscribers
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects attempts to exploit deserialization vulnerability in Email Subscribers plugin
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'admin-ajax.php'
        cs-uri-query|contains: 'action=es_handle_subscription'
        cs-request-body|contains: 'O:'
    condition: selection
falsepositives:
    - Legitimate plugin functionality (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-66055 - Email Subscribers Deserialization Attempt"; flow:to_server,established; content:"POST"; http_method; content:"admin-ajax.php"; http_uri; content:"action=es_handle_subscription"; http_client_body; content:"O:"; within:100; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Disable or remove the email-subscribers plugin until a patch is released.
2. Short-term: Implement a Web Application Firewall (WAF) rule to block serialized PHP objects in POST requests.
3. Long-term: Monitor the vendor's website for updates and apply the patch as soon as it becomes available.
4. Alternative: Consider switching to a different newsletter plugin that is actively maintained and patched.

Valters IT - Your trusted partner in DevSecOps and Cyber Threat Intelligence.