CVE-2025-58218

Executive Summary

CVE-2025-58218 is a critical deserialization of untrusted data vulnerability (CWE-502) in the WordPress plugin Small Package Quotes – USPS Edition by Enituretechnology. This flaw allows an unauthenticated attacker to perform Object Injection, potentially leading to remote code execution, data exfiltration, or complete site compromise. The vulnerability affects all versions up to and including 1.3.9. As of this report, no official patch has been released, and no public proof-of-concept (PoC) code is available. The EPSS score is 0.0%, indicating low exploitation likelihood currently, but the severity (CVSS 7.2) demands immediate attention.

Supply Chain & Package Security

This vulnerability resides in a WordPress plugin ecosystem. The plugin is distributed via the official WordPress plugin repository and third-party marketplaces. Users should verify the integrity of their plugin files and monitor for updates from the vendor. Since no patch exists, the supply chain risk is elevated, and users must consider alternative plugins or implement virtual patching.

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) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – Vulnerability does not affect other components.
• Confidentiality Impact: High (C:H) – Full data access possible.
• Integrity Impact: High (I:H) – Full system modification possible.
• Availability Impact: High (A:H) – Full denial of service possible.

Threat Level: Critical. The combination of network exploitability, no authentication, and high impact makes this a top-priority vulnerability for WordPress site administrators.

Deep Dive & Mechanics

The vulnerability exists in the small-package-quotes-usps-edition plugin, which handles USPS shipping quotes. The plugin likely uses PHP’s unserialize() function on user-supplied input without proper validation or sanitization. When an attacker sends a crafted serialized object, PHP deserializes it, potentially instantiating arbitrary classes and executing magic methods like __wakeup() or __destruct(). This can lead to:

• Remote Code Execution (RCE): By chaining with available gadget chains (e.g., from WordPress core or other plugins).
• File Manipulation: Writing or deleting files on the server.
• Data Theft: Accessing database credentials or sensitive configuration.

The specific entry point is likely a POST parameter or a cookie value processed by the plugin’s AJAX handlers or shortcodes. Since the plugin is unpatched, the exact vulnerable function is not publicly documented, but the pattern is classic PHP object injection.

Vulnerable vs. Patched Code

Since no patch is available, we can only describe the vulnerable pattern and what a fix would look like.

Vulnerable Code (Hypothetical):

// vulnerable.php
$data = $_POST['shipping_data'];
$object = unserialize($data); // No validation

Patched Code (Recommended):

// patched.php
$data = $_POST['shipping_data'];
$allowed_classes = ['stdClass', 'MySafeClass'];
$object = unserialize($data, ['allowed_classes' => $allowed_classes]);
// Or use json_decode() instead
$object = json_decode($data, true);

The fix should either:

1. Use unserialize() with the allowed_classes option to restrict which classes can be deserialized.
2. Replace unserialize() with json_decode() if object serialization is not required.
3. Implement input validation and sanitization before deserialization.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• WordPress site with plugin version <= 1.3.9.
• Network access to the site.

Exploitation Steps:

1. Identify the vulnerable endpoint (e.g., /wp-admin/admin-ajax.php with action usps_quote).
2. Craft a malicious serialized object using PHP gadget chains (e.g., from WordPress core or common plugins).
3. Send the payload via POST request.

Example Payload (Conceptual):

// Generate payload using PHPGGC or manual crafting
$payload = 'O:10:"WP_User_Query":1:{s:5:"query";s:4:"test";}';
// URL-encode and send

Note: No public PoC exists. This is a theoretical exploitation path.

Blue Team (Detection & Hardening)

Detection:

• Monitor for unusual POST parameters containing serialized PHP data (e.g., a:, O:, s: patterns).
• Alert on requests to /wp-admin/admin-ajax.php with action parameters related to USPS.

Hardening:

• Disable the plugin immediately if not critical.
• Implement a Web Application Firewall (WAF) rule to block serialized objects.
• Apply virtual patching via ModSecurity or similar.

ModSecurity Rule (Conceptual):

SecRule REQUEST_BODY "@rx (O:[0-9]+:|\"a:[0-9]+:)" "id:1000001,phase:2,deny,status:403,msg:'PHP Object Injection Attempt'"

Indicators of Compromise (IOCs)

• File Indicators:

  • Unexpected PHP files in /wp-content/uploads/ or /wp-content/plugins/.
  • Modified plugin files (check file integrity via checksums).

• Network Indicators:

  • POST requests to /wp-admin/admin-ajax.php with parameters containing serialized PHP strings.
  • Outbound connections to unknown IPs (potential C2).

• Log Indicators:

  • PHP error logs showing unserialize() errors or class instantiation failures.
  • Web server logs with 403/500 errors on AJAX endpoints.

• YARA Rule:

rule php_object_injection_usps {
    meta:
        description = "Detects PHP serialized objects in POST data targeting USPS plugin"
        author = "Valters IT"
        date = "2025-04-10"
    strings:
        $serialized = /(O:[0-9]+:|\"a:[0-9]+:)/ 
        $usps_action = "usps_quote"
    condition:
        $serialized and $usps_action
}

Detection Engineering

YARA Rule

rule cve_2025_58218_deserialization {
    meta:
        description = "Detects PHP serialized objects in HTTP requests targeting USPS plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-58218"
    strings:
        $serialized_pattern = /(O:[0-9]+:|\"a:[0-9]+:|\"b:[0-9]+:)/
        $usps_endpoint = "admin-ajax.php"
        $usps_action = "usps_quote"
    condition:
        $serialized_pattern and $usps_endpoint and $usps_action
}

Sigma Rule

title: CVE-2025-58218 - PHP Object Injection via USPS Plugin
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP POST requests containing PHP serialized objects targeting the USPS plugin
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'admin-ajax.php'
        cs-uri-query|contains: 'action=usps_quote'
        cs-body|re: '(O:[0-9]+:|"a:[0-9]+:)'
    condition: selection
falsepositives:
    - Legitimate plugin functionality (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-58218 - PHP Object Injection via USPS Plugin"; flow:to_server,established; content:"POST"; http_method; content:"admin-ajax.php"; http_uri; content:"action=usps_quote"; http_client_body; pcre:"/(O:[0-9]+:|"a:[0-9]+:)/"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin if not essential for business operations.
2. Implement virtual patching using WAF rules (see Blue Team section).
3. Monitor logs for exploitation attempts.

Long-term Fix:

• Await an official patch from Enituretechnology. If none is provided, consider migrating to an alternative USPS shipping plugin.
• Apply the principle of least privilege to the WordPress installation.
• Regularly audit plugins for known vulnerabilities.

Workaround:

• If the plugin must remain active, restrict access to the vulnerable endpoint via .htaccess:

<Files "admin-ajax.php">
    Require ip 192.168.1.0/24
</Files>

Valters IT
Securing Digital Frontiers