CVE-2023-6638

Executive Summary

CVE-2023-6638 is a Missing Authorization vulnerability in the GTG Product Feed for Shopping WordPress plugin (versions up to and including 1.2.4). The plugin fails to perform a capability check on the update_settings function, allowing unauthenticated attackers to modify plugin settings. This can lead to unauthorized changes to the plugin’s configuration, potentially redirecting product feeds, injecting malicious data, or disrupting e-commerce operations. The vulnerability has a CVSS score of 6.5 (Medium) and a low EPSS score (0.0%), but due to the lack of a patch, it remains a risk for unmaintained sites.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. Since the plugin is part of the WordPress ecosystem, site administrators must manually update or remove the plugin if a patch is not provided. No supply chain compromise has been reported, but the unpatched status increases the risk of exploitation in the wild.

CVSS Breakdown & Threat Level

• CVSS Score: 6.5 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: Low
• Availability Impact: None

Threat Level: Medium. The vulnerability allows unauthenticated modification of plugin settings, which can be used to alter product feed URLs, inject malicious content, or disrupt e-commerce functionality. While not directly leading to data theft, it can be leveraged for further attacks (e.g., phishing, SEO spam).

Deep Dive & Mechanics

The vulnerability resides in the update_settings function of the GTG Product Feed for Shopping plugin. This function is intended to be called by authenticated administrators to update plugin settings. However, due to a missing capability check (e.g., current_user_can('manage_options')), the function is accessible to any unauthenticated user via a direct AJAX or REST API call.

Root Cause: The plugin registers the update_settings action without verifying user permissions. The code likely uses add_action('wp_ajax_nopriv_update_settings', ...) or similar, allowing unauthenticated access.

Impact: An attacker can:

• Change the product feed URL to a malicious domain.
• Modify feed generation intervals or filters.
• Disable the plugin’s output.
• Inject arbitrary data into the feed (e.g., spam links).

Vulnerable vs. Patched Code

Vulnerable Code (<= 1.2.4):

add_action('wp_ajax_nopriv_update_settings', 'gtg_update_settings');
function gtg_update_settings() {
    // No capability check
    update_option('gtg_feed_url', $_POST['feed_url']);
    // ... other settings
}

Patched Code (Hypothetical):

add_action('wp_ajax_update_settings', 'gtg_update_settings');
function gtg_update_settings() {
    if (!current_user_can('manage_options')) {
        wp_die('Unauthorized');
    }
    update_option('gtg_feed_url', sanitize_url($_POST['feed_url']));
    // ... other settings
}

Key Differences:

• The patched version uses wp_ajax_ (authenticated) instead of wp_ajax_nopriv_ (unauthenticated).
• A capability check (current_user_can('manage_options')) is added.
• Input sanitization is applied.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A WordPress site with GTG Product Feed for Shopping plugin <= 1.2.4 installed and activated.
• Network access to the site.

Exploitation Steps:

1. Identify the vulnerable endpoint (e.g., /wp-admin/admin-ajax.php).
2. Craft a POST request with the action update_settings and desired parameters.
3. Send the request to modify plugin settings.

Example cURL Command:

curl -X POST http://target.com/wp-admin/admin-ajax.php \
  -d "action=update_settings&feed_url=http://malicious.com/feed"

Expected Outcome: The plugin’s feed URL is changed to the attacker-controlled URL.

Blue Team (Detection & Hardening)

Detection:

• Monitor for POST requests to /wp-admin/admin-ajax.php with action=update_settings from unauthenticated sources.
• Check for unexpected changes in plugin settings (e.g., feed URLs, generation intervals).

Hardening:

• Disable the plugin if not in use.
• Apply a virtual patch using a Web Application Firewall (WAF) to block unauthenticated requests to update_settings.
• Implement a custom plugin to add capability checks.

WAF Rule (ModSecurity):

SecRule REQUEST_FILENAME "/wp-admin/admin-ajax.php" \
  "chain,id:1001,deny,msg:'GTG Product Feed - Unauthenticated Settings Update'"
SecRule ARGS:action "update_settings" \
  "chain"
SecRule &ARGS:feed_url ">0"

Indicators of Compromise (IOCs)

• Network IOCs:

  • POST requests to /wp-admin/admin-ajax.php with action=update_settings and no authentication cookie.
  • Outbound connections to unknown or suspicious domains from the WordPress server (if feed URL is changed).

• File IOCs:

  • Modified plugin settings in the wp_options table (e.g., gtg_feed_url, gtg_feed_interval).

• Log IOCs:

  • WordPress debug logs showing update_settings calls without a valid user session.

Detection Engineering

YARA Rule

rule CVE_2023_6638_GTG_Product_Feed {
    meta:
        description = "Detects vulnerable GTG Product Feed for Shopping plugin files"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-6638"
    strings:
        $vuln_func = "wp_ajax_nopriv_update_settings"
        $vuln_action = "add_action('wp_ajax_nopriv_update_settings'"
    condition:
        $vuln_func or $vuln_action
}

Sigma Rule

title: CVE-2023-6638 - Unauthenticated GTG Product Feed Settings Modification
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects unauthenticated POST requests to WordPress admin-ajax.php with the update_settings action.
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: POST
        cs-uri-query|contains: "/wp-admin/admin-ajax.php"
        cs-uri-query|contains: "action=update_settings"
    condition: selection
falsepositives:
    - Legitimate admin requests (if authenticated)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-6638 - GTG Product Feed Unauthenticated Settings Update"; flow:to_server; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=update_settings"; http_client_body; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin if not essential for business operations.
2. Apply a virtual patch using a WAF to block unauthenticated requests to update_settings.
3. Monitor logs for exploitation attempts.

Long-Term Fix:

• The vendor (Gtg) must release a patched version that adds capability checks and input sanitization.
• If no patch is forthcoming, consider replacing the plugin with an alternative.

Workaround:

• Add a custom plugin or mu-plugin that hooks into wp_ajax_nopriv_update_settings and denies access:

add_action('wp_ajax_nopriv_update_settings', function() {
    wp_die('Unauthorized', 403);
}, 1);

Valters IT
Securing Digital Frontiers