CVE-2024-53819

Executive Summary

CVE-2024-53819 is a Missing Authorization vulnerability in the Client Invoicing by Sprout Invoices WordPress plugin (slug: sprout-invoices), developed by BoldGrid. This flaw affects all versions up to and including 20.8.0. The vulnerability allows an unauthenticated attacker to access sensitive invoice data or perform unauthorized actions due to missing capability checks on certain AJAX endpoints or REST routes. Given the CVSS score of 5.3 (Medium), the primary impact is confidentiality breach, though the attack complexity is low and requires no privileges. No public exploit code or proof-of-concept has been released, and the vendor has not yet issued a patch.

Supply Chain & Package Security

This vulnerability resides in a WordPress plugin distributed via the official WordPress Plugin Directory. As a WordPress ecosystem issue, it affects any site running the vulnerable plugin version. The plugin is not available on NPM, PyPI, Go, or Ruby registries. Administrators should monitor the plugin’s update channel for a patched version.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – The vulnerable component remains isolated.
• Confidentiality (C): Low (L) – Limited data exposure.
• Integrity (I): None (N) – No data modification.
• Availability (A): None (N) – No denial of service.

Threat Level: Medium. The low EPSS score (0.0%) suggests limited active exploitation, but the ease of exploitation and lack of patch make it a risk for unpatched sites.

Deep Dive & Mechanics

The vulnerability stems from missing authorization checks in the sprout-invoices plugin. The plugin likely registers AJAX actions or REST API endpoints that handle invoice retrieval, creation, or modification. Without proper current_user_can() checks or nonce validation, an unauthenticated attacker can directly call these endpoints to access or manipulate invoice data.

Root Cause: The plugin fails to verify user capabilities before executing sensitive operations. For example, a function hooked to wp_ajax_nopriv_* (allowing unauthenticated access) or a REST route without permission_callback may expose invoice details.

Attack Vector: An attacker sends crafted HTTP requests to the vulnerable endpoints, bypassing authentication. The attack does not require any prior knowledge or session.

Impact: Unauthorized disclosure of invoice metadata (client names, amounts, statuses) or potentially full invoice content. In some configurations, the attacker might also trigger invoice creation or status changes.

Vulnerable vs. Patched Code

Since no patch is available, the following illustrates a typical vulnerable pattern and its secure counterpart.

Vulnerable Code (sprout-invoices <= 20.8.0):

// Example AJAX handler without capability check
add_action('wp_ajax_nopriv_si_get_invoice', 'si_get_invoice_handler');
function si_get_invoice_handler() {
    $invoice_id = $_GET['invoice_id'];
    $invoice = get_post($invoice_id);
    echo json_encode($invoice);
    wp_die();
}

Patched Code (hypothetical fix):

add_action('wp_ajax_si_get_invoice', 'si_get_invoice_handler');
add_action('wp_ajax_nopriv_si_get_invoice', 'si_get_invoice_handler');
function si_get_invoice_handler() {
    // Verify nonce and capability
    if (!wp_verify_nonce($_GET['_wpnonce'], 'si_get_invoice') || !current_user_can('edit_posts')) {
        wp_die('Unauthorized', 403);
    }
    $invoice_id = intval($_GET['invoice_id']);
    $invoice = get_post($invoice_id);
    echo json_encode($invoice);
    wp_die();
}

Exploitation & Lab Setup

Red Team

Prerequisites: A WordPress site with sprout-invoices <= 20.8.0 installed and activated.

Exploitation Steps:

1. Identify vulnerable endpoints by analyzing plugin source or using a fuzzer.
2. Send a direct HTTP GET request to the endpoint without authentication:

   curl -v "http://target/wp-admin/admin-ajax.php?action=si_get_invoice&invoice_id=123"
   

3. If the endpoint returns invoice data, the vulnerability is confirmed.

No public exploit code is available. The above is a generic example based on the vulnerability type.

Blue Team

Detection & Prevention:

• Monitor for unusual AJAX requests to admin-ajax.php with action parameters related to sprout-invoices (e.g., si_get_invoice, si_create_invoice).
• Implement Web Application Firewall (WAF) rules to block unauthenticated access to plugin-specific endpoints.
• Use a WordPress security plugin to enforce capability checks on all AJAX handlers.

Lab Setup:

• Deploy a vulnerable WordPress instance using Docker:

  docker run -d --name wp-vuln -p 8080:80 wordpress:latest
  

• Install the vulnerable plugin manually from the WordPress Plugin Directory (version 20.8.0).
• Simulate exploitation using the curl command above.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests to /wp-admin/admin-ajax.php with action=si_* parameters from unauthenticated sources.
  • Unusual invoice_id values in GET/POST parameters.
• File IOCs:
  • Presence of sprout-invoices plugin version <= 20.8.0 in /wp-content/plugins/sprout-invoices/.
• Log IOCs:
  • WordPress debug log entries showing wp_ajax_nopriv_si_* calls without corresponding user sessions.

Detection Engineering

YARA Rule

rule CVE_2024_53819_SproutInvoices_MissingAuth {
    meta:
        description = "Detects vulnerable sprout-invoices plugin files with missing authorization"
        author = "Valters IT"
        date = "2025-04-17"
        reference = "CVE-2024-53819"
    strings:
        $vuln_func = "wp_ajax_nopriv_si_" nocase
        $cap_check = "current_user_can" nocase
    condition:
        $vuln_func and not $cap_check
}

Sigma Rule

title: CVE-2024-53819 - Missing Authorization in Sprout Invoices
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects unauthenticated AJAX requests to sprout-invoices endpoints
author: Valters IT
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-uri-query|contains: "admin-ajax.php?action=si_"
        sc-status: 200
    condition: selection
falsepositives:
    - Legitimate plugin functionality if misconfigured
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-53819 - Sprout Invoices Missing Authorization"; flow:to_server; content:"/admin-ajax.php"; http_uri; content:"action=si_"; http_uri; nocase; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin until a patch is released.
2. Apply a virtual patch using a WAF to block unauthenticated requests to admin-ajax.php with action parameters starting with si_.
3. Restrict access to admin-ajax.php via .htaccess or server configuration for non-logged-in users (if feasible).

Long-Term Fix:

• Monitor the plugin’s official page for a security update.
• Once patched, update to the latest version immediately.
• Conduct a security audit of all WordPress plugins for similar missing authorization issues.

Workaround:

• Add a custom plugin or mu-plugin that hooks into wp_ajax_nopriv_* and denies all si_* actions unless the user is authenticated.

Valters IT
DevSecOps & Cyber Threat Intelligence