Exploitation Research Plan: CVE-2025-62099 - Missing Authorization in Signature Add-On for Gravity Forms

1. Vulnerability Summary

The Signature Add-On for Gravity Forms (up to version 1.8.6) contains a missing authorization vulnerability within its AJAX handling logic. While the plugin registers AJAX actions for authenticated users, it fails to perform adequate capability checks (e.g., current_user_can( 'gravityforms_edit_entries' )) within the handler functions. This allows any authenticated user, such as a Subscriber, to perform actions intended for administrators or editors, specifically the deletion of signature files associated with form entries.

2. Attack Vector Analysis

• Endpoint: /wp-admin/admin-ajax.php
• Action: gfsignature_delete_signature (inferred from common plugin patterns and the nature of the vulnerability).
• Payload Parameters:
  • action: gfsignature_delete_signature
  • entry_id: The ID of the Gravity Forms entry containing the signature to delete.
  • input_id: The ID of the specific signature field (e.g., 3).
  • nonce: A valid WordPress nonce for the action.
• Authentication: Authenticated user with Subscriber role or higher.
• Preconditions:
  • Gravity Forms and the Signature Add-On must be active.
  • At least one form entry with a signature must exist.

3. Code Flow

1. Registration: The plugin registers the AJAX handler in the main class (likely GFSignature) or an AJAX-specific class:
   add_action( 'wp_ajax_gfsignature_delete_signature', array( $this, 'ajax_delete_signature' ) );
2. Entry Point: An authenticated user sends a POST request to admin-ajax.php with the action parameter set to gfsignature_delete_signature.
3. Handler Execution: The ajax_delete_signature function is called.
4. Insecure Verification: The function typically calls check_ajax_referer( 'gfsignature_delete_signature', 'nonce' ). This verifies the nonce but does not verify if the user has the authority to edit Gravity Forms entries.
5. Sink: The function proceeds to use GFAPI::get_entry( $entry_id ) and deletes the file associated with the signature field, updating the entry meta to remove the signature path.

4. Nonce Acquisition Strategy

The nonce for gfsignature_delete_signature is typically generated using wp_create_nonce( 'gfsignature_delete_signature' ). To obtain this as a Subscriber:

1. Identify Trigger: The plugin likely enqueues the necessary script and nonce on pages where a form with a signature field is present (to allow users to clear their own signature before submission) or in the entry detail view.
2. Create Test Page: Create a public page containing a Gravity Forms shortcode for a form that includes a signature field.
   • wp post create --post_type=page --post_status=publish --post_content='[gravityform id="1"]'
3. Navigate and Extract:
   • Log in as the Subscriber.
   • Navigate to the newly created page.
   • Use browser_eval to find the localized script data. The signature add-on usually localizes data under a global object like gfsignatures_strings or gform_signature_config.
   • Actionable JS: browser_eval("window.gfsignatures_strings?.nonce") or search the page source for gfsignature_delete_signature.

5. Exploitation Strategy

1. Setup Phase:
   • Install Gravity Forms and Signature Add-On.
   • Create a form (ID: 1) with a Signature field (ID: 3).
   • Submit the form as an administrator to create an entry (Entry ID: 1) with a valid signature.
   • Create a Subscriber user.
2. Nonce Collection:
   • Log in as the Subscriber.
   • Access the page where the signature form is rendered.
   • Extract the nonce for gfsignature_delete_signature.
3. Execution Phase:
   • Send a POST request to /wp-admin/admin-ajax.php using the http_request tool.
   • Target Entry: Use the Entry ID created by the admin (e.g., 1).
4. Payload:

   POST /wp-admin/admin-ajax.php HTTP/1.1
   Content-Type: application/x-www-form-urlencoded
   
   action=gfsignature_delete_signature&entry_id=1&input_id=3&nonce=[EXTRACTED_NONCE]
   

6. Test Data Setup

• Gravity Form: A form with at least one "Signature" field type.
• Entry: Submit the form once. Note the entry_id (usually 1 if it's the first submission) and the field ID of the signature (usually a number like 3).
• Subscriber User: wp user create attacker attacker@example.com --role=subscriber --user_pass=password123

7. Expected Results

• HTTP Response: The server should return a 200 OK response, potentially containing a JSON success message like {"success":true}.
• System Impact: The signature file on the server's filesystem associated with Entry ID 1 should be deleted or the reference in the database removed.

8. Verification Steps

1. Check Entry Meta: Use WP-CLI to inspect the entry after the attack.
   • wp db query "SELECT * FROM wp_gf_entry_meta WHERE entry_id = 1 AND meta_key = '3';"
   • Success criteria: The value should be empty or the row should be missing, whereas it previously contained a URL/path to a .png file.
2. Check File Existence: Locate the signature file path from the entry meta before the attack and verify it is missing from the wp-content/uploads/gravity_forms/... directory after the attack.

9. Alternative Approaches

• Direct Action Probe: If the nonce is not easily found on the frontend, check if the admin_init hook is used to handle signature deletions via a GET request (common in older Gravity Forms add-ons). Look for $_GET['action'] == 'delete_signature'.
• Entry ID Brute Force: Since the exploit allows a Subscriber to delete any entry's signature, once the nonce is obtained, the attacker can iterate through entry_id values to delete all signatures on the site.