CVE-2026-19857 Overview
CVE-2026-19857 affects the Formidable Forms WordPress plugin in versions prior to 6.35. The plugin fails to prevent request-derived values from reaching the WordPress shortcode parser during token substitution in a form's custom HTML. Unauthenticated visitors can inject arbitrary shortcodes with attacker-chosen attributes, which are then executed server-side on any page that renders the affected form. The vulnerability is classified under CWE-74 (Improper Neutralization of Special Elements in Output). Exploitation requires meeting non-trivial preconditions, but the resulting shortcode execution runs within the WordPress request context.
Critical Impact
Unauthenticated attackers can trigger arbitrary WordPress shortcodes server-side, potentially exposing site data or invoking sensitive plugin functionality.
Affected Products
- Formidable Forms WordPress plugin versions before 6.35
- WordPress sites publishing forms that substitute request-derived tokens into custom HTML
- Any page on an affected site that renders a vulnerable form
Discovery Timeline
- 2026-09-16 - CVE-2026-19857 published to the National Vulnerability Database (NVD)
- 2026-09-16 - Last updated in the NVD database
Technical Details for CVE-2026-19857
Vulnerability Analysis
Formidable Forms allows administrators to define custom HTML for forms and to embed tokens that the plugin substitutes at render time. Prior to version 6.35, the substitution routine inserts request-derived values into the output before the WordPress shortcode parser processes the resulting HTML. Because the substituted content is not neutralized, an attacker-supplied string containing shortcode syntax such as [shortcode attr="value"] is treated as a legitimate shortcode by WordPress.
The parser then dispatches the shortcode to any handler registered on the site, along with attacker-controlled attributes. The impact depends on which shortcodes are available in the WordPress instance, but the plugin surface commonly includes handlers that read site configuration, execute database queries, or emit user-scoped data. The attacker does not need authentication, though attack complexity is elevated because the target form must expose a token bound to a controllable request parameter.
Root Cause
The root cause is improper output neutralization ([CWE-74]). The plugin trusts token substitution as a safe operation and hands the result to do_shortcode() without stripping or escaping shortcode-relevant characters such as [ and ]. This violates the principle of separating data from executable markup.
Attack Vector
The attack vector is network-based. An unauthenticated visitor submits or references a request whose value is later substituted into the custom HTML of a rendered form. When the containing page loads, WordPress evaluates the injected shortcode server-side. Additional details are available in the WPScan Vulnerability Report.
Detection Methods for CVE-2026-19857
Indicators of Compromise
- Web server access logs containing request parameters with bracket characters [ or ] and shortcode-like tokens targeting form endpoints.
- Unexpected shortcode expansions or plugin output on pages that render Formidable Forms views.
- WordPress debug or PHP error logs referencing unknown shortcode invocations tied to form rendering.
Detection Strategies
- Inspect Formidable Forms configurations for custom HTML that references request-derived tokens and cross-check against the plugin version.
- Deploy web application firewall rules that flag URL and body parameters containing shortcode syntax destined for form pages.
- Correlate anomalous outbound requests or database reads originating from PHP worker processes with form rendering events.
Monitoring Recommendations
- Alert on any Formidable Forms plugin instance running a version below 6.35.
- Monitor WordPress wp-content and plugin update channels for integrity changes and version drift.
- Track HTTP request patterns against form endpoints for repeated attempts containing bracketed payloads.
How to Mitigate CVE-2026-19857
Immediate Actions Required
- Upgrade the Formidable Forms plugin to version 6.35 or later on all WordPress instances.
- Audit all published forms for custom HTML that substitutes request-derived tokens and disable those forms until patched.
- Review WordPress activity logs for signs of shortcode injection attempts against form pages.
Patch Information
The vendor has addressed the vulnerability in Formidable Forms version 6.35. Refer to the WPScan Vulnerability Report for the fixed version reference and additional remediation guidance.
Workarounds
- Remove or rewrite custom HTML in forms so that no token references a value sourced from HTTP request parameters.
- Restrict access to affected form pages using authentication or IP allowlisting until the plugin is upgraded.
- Configure a web application firewall to block bracket characters in query and body parameters targeting form URLs.
# Configuration example: upgrade Formidable Forms via WP-CLI
wp plugin update formidable --version=6.35
wp plugin list --name=formidable --fields=name,status,version
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
