CVE-2024-11188: Formidable Forms XSS Vulnerability

CVE-2024-11188 Overview

CVE-2024-11188 is a POST-based reflected Cross-Site Scripting (XSS) vulnerability in the Formidable Forms plugin for WordPress. The flaw affects all versions up to and including 6.16.1.2 of the Contact Form Plugin, Survey, Quiz, Payment, Calculator Form & Custom Form Builder. The root cause is insufficient input sanitization and output escaping in Custom HTML Form parameters. Unauthenticated attackers can inject arbitrary web scripts that execute when a user is tricked into clicking a crafted link. This weakness maps to [CWE-79] (Improper Neutralization of Input During Web Page Generation).

Critical Impact
Successful exploitation allows unauthenticated attackers to execute arbitrary JavaScript in a victim's browser session, enabling session theft, credential harvesting, or targeted admin account takeover on WordPress sites running vulnerable Formidable Forms versions.

Affected Products

Strategy11 Formidable Forms (free WordPress plugin) — all versions through 6.16.1.2
Deployments using the Custom HTML Form field feature
WordPress sites exposing form endpoints to unauthenticated visitors

Discovery Timeline

2024-11-23 - CVE-2024-11188 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-11188

Vulnerability Analysis

The vulnerability resides in how Formidable Forms processes Custom HTML Form parameters submitted via HTTP POST requests. The plugin renders user-supplied input back into the response without applying adequate sanitization or context-aware output escaping. Because the injection occurs on reflection, an attacker must induce user interaction, such as a click on a malicious link or form submission. The scope change indicates that injected scripts can affect resources beyond the vulnerable component's security context, including the WordPress administrative interface if the target is an authenticated administrator.

Root Cause

The defect stems from missing sanitization within the plugin's field-handling logic, specifically around functions used to render form fields. The relevant helper code is available in the WordPress FrmFieldsHelper source. User input reaches HTML rendering paths without being passed through WordPress escaping functions such as esc_html(), esc_attr(), or wp_kses(). As a result, POST parameters containing HTML or JavaScript are reflected verbatim in the page markup.

Attack Vector

An attacker crafts a malicious page or link that submits a POST request to a form endpoint on a vulnerable WordPress site, embedding a JavaScript payload in a Custom HTML Form parameter. When a victim triggers the request, the server reflects the payload into the response, and the browser executes it in the origin of the WordPress site. See the Wordfence Vulnerability Analysis for additional technical context. No authentication is required, but user interaction is necessary to trigger the reflection.

Detection Methods for CVE-2024-11188

Indicators of Compromise

POST requests to Formidable Forms endpoints containing HTML tags such as <script>, <img onerror=>, or <svg onload=> in form parameters
Web server access logs showing unusual referrers directing users to form submission URLs
Browser console errors or unexpected outbound requests originating from pages that render Formidable Forms
Administrator sessions exhibiting unexpected AJAX calls to admin-ajax.php shortly after clicking external links

Detection Strategies

Inspect HTTP POST bodies to Formidable Forms handlers for script tags, event handler attributes, and encoded JavaScript payloads
Deploy web application firewall (WAF) rules that flag reflected XSS patterns in frm_action, field IDs, and Custom HTML parameters
Correlate form submissions with subsequent authenticated administrative actions to identify potential session abuse

Monitoring Recommendations

Enable verbose logging on WordPress and reverse proxy layers for all POST requests to /wp-admin/admin-ajax.php and form processing endpoints
Monitor plugin version inventory and alert when Formidable Forms remains at 6.16.1.2 or earlier
Track outbound requests from administrator browsers to detect data exfiltration attempts following XSS execution

How to Mitigate CVE-2024-11188

Immediate Actions Required

Update Formidable Forms to a version later than 6.16.1.2 across all WordPress instances
Audit all custom HTML form fields for unexpected content or recent modifications by non-privileged users
Force password resets and invalidate active sessions for WordPress administrators if exploitation is suspected

Patch Information

Upgrade Strategy11 Formidable Forms to the latest available release that addresses CVE-2024-11188. Review the Wordfence Vulnerability Analysis advisory for the fixed version and reference the FrmFieldsHelper source to confirm the patch applies context-aware escaping.

Workarounds

Disable the Formidable Forms plugin until it can be updated if patching is not immediately feasible
Deploy WAF signatures that block reflected XSS payloads targeting form parameters
Enforce a strict Content Security Policy (CSP) that disallows inline scripts and untrusted script sources
Restrict access to WordPress form pages using authentication or IP allowlisting where practical

bash

# Example WordPress CLI update command
wp plugin update formidable --version=latest
wp plugin list --name=formidable --fields=name,status,version