CVE-2025-22303

Executive Summary

CVE-2025-22303 is a vulnerability in the WordPress plugin WP Mailster (versions up to and including 1.8.17.0) that allows an attacker to retrieve embedded sensitive data through the insertion of sensitive information into sent data. This is a WordPress ecosystem issue, not related to the underlying web server (Apache or Nginx). The vulnerability has a CVSS score of 5.3 (Medium) and an EPSS score of 0.0%, indicating low exploitation likelihood at this time. No public proof-of-concept (PoC) or exploit code is available, and no patch has been released by the vendor.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. Users should monitor for updates from the vendor and consider removing the plugin if no patch is forthcoming.

CVSS Breakdown & Threat Level

• CVSS Score: 5.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: Low
• Integrity Impact: None
• Availability Impact: None

The threat level is Medium due to the low complexity and no required privileges, but the impact is limited to low confidentiality loss.

Deep Dive & Mechanics

The vulnerability lies in the WP Mailster plugin's handling of email data. The plugin inserts sensitive information (such as email addresses, user data, or internal configuration details) into sent emails without proper sanitization or filtering. An attacker can exploit this by triggering email sending functionality (e.g., via contact forms or subscription forms) and intercepting the email traffic to retrieve embedded sensitive data.

The root cause is a lack of input validation and output encoding when constructing email content. The plugin does not strip or mask sensitive fields before including them in the email body or headers.

Vulnerable vs. Patched Code

Since no patch is available, we can only analyze the vulnerable code pattern. The vulnerable code likely resembles:

// Vulnerable code (wp-mailster <= 1.8.17.0)
$email_body = "User email: " . $_POST['user_email'] . "\n";
$email_body .= "Message: " . $_POST['message'];
wp_mail($to, $subject, $email_body);

A patched version would sanitize the input:

// Patched code (hypothetical)
$email_body = "User email: " . sanitize_email($_POST['user_email']) . "\n";
$email_body .= "Message: " . esc_textarea($_POST['message']);
wp_mail($to, $subject, $email_body);

Exploitation & Lab Setup

Red Team

Prerequisites:

• A WordPress site with WP Mailster plugin version <= 1.8.17.0
• Network access to the site
• Ability to intercept email traffic (e.g., via SMTP logging or man-in-the-middle)

Exploitation Steps:

1. Identify a form or endpoint that sends emails via WP Mailster (e.g., contact form, subscription form).
2. Submit a request with crafted input that triggers the inclusion of sensitive data (e.g., admin email, database credentials).
3. Intercept the outgoing email (e.g., using a local SMTP server or email logging plugin).
4. Extract the sensitive data from the email body or headers.

Example Request:

POST /wp-admin/admin-ajax.php HTTP/1.1
Host: target-site.com
Content-Type: application/x-www-form-urlencoded

action=wp_mailster_send&email=attacker@example.com&message=test

Blue Team

Detection:

• Monitor for unusual email sending patterns from the WordPress site.
• Check email logs for unexpected sensitive data in email bodies.
• Use the detection rules below.

Mitigation:

• Disable or remove the WP Mailster plugin until a patch is available.
• Implement web application firewall (WAF) rules to block suspicious requests.
• Restrict email sending to authenticated users only.

Indicators of Compromise (IOCs)

• Network: Outbound emails containing sensitive data (e.g., admin email addresses, database credentials).
• File: Modified or suspicious log files in /wp-content/plugins/wp-mailster/.
• Process: Unexpected email sending processes on the server.

Detection Engineering

YARA Rule

rule CVE_2025_22303_WP_Mailster {
    meta:
        description = "Detects WP Mailster plugin files with sensitive data insertion vulnerability"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-22303"
    strings:
        $sensitive1 = "user_email" ascii wide nocase
        $sensitive2 = "admin_email" ascii wide nocase
        $sensitive3 = "wp_mail" ascii wide nocase
    condition:
        all of them and filesize < 100KB
}

Sigma Rule

title: CVE-2025-22303 WP Mailster Sensitive Data Leak
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential exploitation of CVE-2025-22303 via email content
author: Valters IT
date: 2025-04-10
logsource:
    category: email
    product: smtp
detection:
    selection:
        body|contains:
            - 'user_email'
            - 'admin_email'
            - 'wp-config'
    condition: selection
falsepositives:
    - Legitimate email sending with user-provided data
level: medium

Suricata Rule

alert tcp any any -> any 25,587,465 (msg:"CVE-2025-22303 WP Mailster Sensitive Data Leak"; flow:to_server; content:"user_email"; nocase; content:"admin_email"; nocase; distance:0; within:100; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Disable or remove the WP Mailster plugin from all WordPress installations.
2. Vendor Contact: Reach out to the vendor (brandtoss) for a patch or update.
3. Alternative Plugin: Consider using a different email plugin that is actively maintained.
4. WAF Rules: Implement WAF rules to block requests containing sensitive keywords in email parameters.
5. Monitoring: Enable email logging and monitor for suspicious outbound emails.

Valters IT
Securing Digital Frontiers