CVE-2023-6139

Executive Summary

CVE-2023-6139 is a vulnerability in the Essential Real Estate WordPress plugin (versions before 4.4.0) that allows authenticated attackers with subscriber-level privileges to perform Denial of Service (DoS) attacks. The root cause is a missing capability check on AJAX actions, enabling low-privileged users to repeatedly trigger resource-intensive operations without authorization. This can lead to server resource exhaustion and service unavailability. No public proof-of-concept (PoC) or exploit code has been released, and the EPSS score is 0.0%, indicating low exploitation likelihood at this time. However, the vulnerability is unpatched in older versions, making it a risk for sites running outdated plugin versions.

Supply Chain & Package Security

This vulnerability affects the WordPress plugin ecosystem. The plugin is distributed via the official WordPress plugin repository. Administrators should verify plugin integrity by checking checksums or using trusted update mechanisms. No known supply chain compromise has been reported.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official vector assigned)
• Attack Vector (AV): Network (likely)
• Attack Complexity (AC): Low (likely)
• Privileges Required (PR): Low (subscriber account)
• User Interaction (UI): None
• Scope (S): Unchanged
• Confidentiality (C): None
• Integrity (I): None
• Availability (A): High
• Threat Level: Medium (due to low privileges required and potential for DoS)

Deep Dive & Mechanics

The vulnerability resides in the plugin's AJAX handler functions. WordPress plugins often register AJAX actions for both authenticated and unauthenticated users. In this case, the plugin failed to implement proper capability checks (e.g., current_user_can()) before executing resource-intensive operations. An attacker with a subscriber account can send crafted AJAX requests to trigger these actions repeatedly, causing excessive CPU/memory usage or database queries, leading to a Denial of Service.

Key technical details:

• Affected component: AJAX actions in the Essential Real Estate plugin (versions < 4.4.0)
• Attack vector: Authenticated HTTP requests to /wp-admin/admin-ajax.php with specific action parameters
• Impact: Server resource exhaustion, potential site downtime
• Privilege requirement: Subscriber-level account (lowest authenticated role)

Vulnerable vs. Patched Code

Vulnerable code (pre-4.4.0):

add_action('wp_ajax_essential_real_estate_action', 'handle_ajax_action');
function handle_ajax_action() {
    // No capability check
    // Execute resource-intensive operation
    // ...
}

Patched code (4.4.0+):

add_action('wp_ajax_essential_real_estate_action', 'handle_ajax_action');
function handle_ajax_action() {
    if (!current_user_can('read')) {
        wp_die('Unauthorized');
    }
    // Execute resource-intensive operation
    // ...
}

The patch adds a capability check (current_user_can('read')) to ensure only users with at least subscriber-level privileges can execute the AJAX action. However, since subscribers already have the read capability, this check may not fully mitigate the issue. A more robust fix would restrict actions to higher-privileged roles (e.g., edit_posts or manage_options).

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• WordPress site with Essential Real Estate plugin < 4.4.0
• Valid subscriber account credentials

Exploitation steps:

1. Log in as a subscriber.
2. Identify vulnerable AJAX actions by analyzing plugin source code or using a tool like Burp Suite.
3. Send repeated AJAX requests to trigger resource exhaustion:

# Example using curl (replace with actual action and nonce)
for i in {1..1000}; do
  curl -X POST 'http://target/wp-admin/admin-ajax.php' \
    -d 'action=essential_real_estate_vulnerable_action&nonce=VALID_NONCE' \
    --cookie 'wordpress_logged_in_xxx=session_cookie'
done

Note: No public exploit code exists. The above is a generic example.

Blue Team (Defense & Detection)

Detection:

• Monitor for excessive AJAX requests from a single user/IP.
• Look for patterns of repeated calls to admin-ajax.php with plugin-specific actions.
• Use WAF rules to rate-limit AJAX endpoints.

Mitigation:

• Immediately update to version 4.4.0 or later.
• If patching is not possible, disable the plugin or restrict AJAX access via .htaccess:

<Files "admin-ajax.php">
    Require all denied
</Files>

• Implement server-side rate limiting for authenticated users.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Repeated POST requests to /wp-admin/admin-ajax.php with action parameters containing essential_real_estate_
  • High volume of requests from a single IP or user agent
• System IOCs:
  • Elevated CPU/memory usage on the web server
  • Slow response times or 503 errors
  • WordPress debug logs showing repeated AJAX calls
• File IOCs:
  • Plugin files in /wp-content/plugins/essential-real-estate/ with version < 4.4.0

Detection Engineering

YARA Rule

rule Detect_Essential_Real_Estate_Vulnerable_Plugin {
    meta:
        description = "Detects vulnerable versions of Essential Real Estate plugin (< 4.4.0)"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-6139"
    strings:
        $version_string = "Version: 4." ascii wide nocase
        $vulnerable_version = "Version: 4.3" ascii wide nocase
    condition:
        $version_string and $vulnerable_version
}

Sigma Rule

title: Excessive AJAX Requests to Essential Real Estate Plugin
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential DoS attacks via repeated AJAX calls to Essential Real Estate plugin
author: Valters IT
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/wp-admin/admin-ajax.php'
        cs-uri-query|contains: 'action=essential_real_estate_'
    condition: selection
    timeframe: 5m
    aggregation:
        field: c-ip
        condition: gt 100
falsepositives:
    - Legitimate plugin usage
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-6139 - Essential Real Estate DoS Attempt"; flow:to_server; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=essential_real_estate_"; http_client_body; threshold:type both, track by_src, count 100, seconds 300; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Update the Essential Real Estate plugin to version 4.4.0 or later via the WordPress admin dashboard.
2. Workaround: If patching is not possible, disable the plugin or restrict access to admin-ajax.php for non-admin users using a security plugin or .htaccess rules.
3. Long-term: Implement a Web Application Firewall (WAF) with rate limiting for AJAX endpoints. Regularly audit plugin permissions and capabilities.
4. Monitoring: Enable logging for AJAX requests and set up alerts for unusual activity patterns.

Report generated by Valters IT DevSecOps Intelligence