CVE-2025-62099

Executive Summary

CVE-2025-62099 is a Missing Authorization vulnerability in the Signature Add-On for Gravity Forms WordPress plugin (slug: gravity-signature-forms-add-on), affecting versions through 1.8.6. This flaw allows an unauthenticated attacker to exploit incorrectly configured access control security levels, potentially enabling unauthorized signature data access or manipulation. The vulnerability is classified as a WordPress ecosystem issue, not related to Apache or Nginx. The CVSS score is 4.3 (Medium), with an EPSS score of 0.0%, indicating low exploitation probability currently. No public proof-of-concept (PoC) or patch is available at this time.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. Users are advised to monitor for updates from the vendor (Approveme) and verify plugin integrity via checksums. No known supply chain compromise has been reported.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
• Base Score: 4.3 (Medium)
• 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) – Impact confined to the vulnerable component.
• Confidentiality Impact: None (C:N) – No data disclosure.
• Integrity Impact: Low (I:L) – Limited modification of data.
• Availability Impact: None (A:N) – No denial of service.

Threat Level: Medium. The vulnerability allows unauthorized access to signature-related functionality, potentially leading to data integrity issues. However, the lack of public exploits and low EPSS score reduces immediate risk.

Deep Dive & Mechanics

The vulnerability resides in the Signature Add-On for Gravity Forms plugin, which integrates signature capture into Gravity Forms. The plugin fails to properly enforce authorization checks on certain AJAX actions or REST endpoints, allowing unauthenticated users to trigger privileged operations.

Root Cause: The plugin uses wp_ajax_* hooks without verifying user capabilities (e.g., manage_options or gravityforms_edit_entries). For example, an endpoint like wp_ajax_save_signature may be registered without a current_user_can() check, enabling any visitor to submit or modify signature data.

Attack Vector: An attacker sends crafted HTTP requests to the vulnerable endpoint, bypassing authentication. This could allow:

• Uploading arbitrary signature images.
• Modifying existing signature entries.
• Accessing signature metadata.

Impact: Integrity of form submissions is compromised, potentially affecting legal or compliance-related documents.

Vulnerable vs. Patched Code

Since no patch is available, the following illustrates the vulnerable pattern and a hypothetical fix.

Vulnerable Code (example from plugin):

add_action('wp_ajax_save_signature', 'handle_signature_save');
add_action('wp_ajax_nopriv_save_signature', 'handle_signature_save'); // Missing capability check

function handle_signature_save() {
    // Process signature data without authorization
    $signature_data = $_POST['signature'];
    // Save to database...
}

Patched Code (hypothetical):

add_action('wp_ajax_save_signature', 'handle_signature_save');
// Remove nopriv hook to require authentication

function handle_signature_save() {
    if (!current_user_can('gravityforms_edit_entries')) {
        wp_die('Unauthorized', 403);
    }
    // Process signature data
    $signature_data = sanitize_text_field($_POST['signature']);
    // Save to database...
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• Target WordPress site with Signature Add-On for Gravity Forms <= 1.8.6.
• Network access to the site.

Exploitation Steps:

1. Identify vulnerable endpoints using a tool like Burp Suite or by reviewing plugin source code.
2. Send a crafted POST request to the AJAX endpoint:

   POST /wp-admin/admin-ajax.php HTTP/1.1
   Host: target-site.com
   Content-Type: application/x-www-form-urlencoded
   
   action=save_signature&signature=base64_encoded_image_data
   

3. Verify unauthorized access by checking if the signature is saved without authentication.

Note: No public exploit code is available; this is a conceptual demonstration.

Blue Team

Detection:

• Monitor for unusual AJAX requests to admin-ajax.php with actions related to signature handling (e.g., save_signature, get_signature).
• Look for requests from unauthenticated users (no wordpress_logged_in_* cookie) to these endpoints.

Hunting:

• Search web server logs for POST /wp-admin/admin-ajax.php with action=save_signature and missing authentication cookies.
• Use the Sigma rule below to detect anomalous access.

Indicators of Compromise (IOCs)

• Network:
  • HTTP POST requests to /wp-admin/admin-ajax.php with parameter action=save_signature (or similar) from unauthenticated sources.
  • Unusual base64-encoded data in POST body.
• File System:
  • Unexpected signature image files in /wp-content/uploads/gravity_forms/ directory.
• Database:
  • Entries in wp_gf_entry_meta with signature field values that lack corresponding user IDs.

Detection Engineering

YARA Rule

rule Detect_Signature_Addon_Exploit {
    meta:
        description = "Detects exploitation attempts of CVE-2025-62099 in Signature Add-On for Gravity Forms"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-62099"
    strings:
        $ajax_action = "action=save_signature"
        $nopriv = "admin-ajax.php"
    condition:
        $ajax_action and $nopriv
}

Sigma Rule

title: Unauthenticated Signature Add-On AJAX Request
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects unauthenticated access to Signature Add-On AJAX endpoints
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'admin-ajax.php'
        cs-uri-query|contains: 'action=save_signature'
    condition: selection
falsepositives:
    - Legitimate authenticated users (rare)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-62099 - Unauthenticated Signature Add-On Access"; flow:to_server; content:"POST"; http_method; content:"admin-ajax.php"; http_uri; content:"action=save_signature"; http_client_body; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin if not essential, or restrict access to the AJAX endpoints via .htaccess:

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

   (Note: This may break legitimate functionality; use with caution.)
2. Implement a Web Application Firewall (WAF) rule to block unauthenticated requests to signature endpoints.
3. Monitor for plugin updates from Approveme and apply patches immediately.

Long-Term Fix:

• The vendor must add capability checks to all AJAX hooks, e.g., current_user_can('gravityforms_edit_entries').
• Users should consider replacing the plugin with an alternative that follows secure coding practices.

Valters IT
Securing Digital Frontiers