CVE-2026-18331: Formidable Forms <= 6.33.1 - Unauthenticated Stored Cross-Site Scripting via 'frm_user_id' Parameter
The Formidable Forms – WordPress Form Builder for Contact Forms, Calculators, Quizzes & More plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'frmuserid' parameter in all versions up to, and including, 6.33.1 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. By forging frmuserid to match an administrator's user ID — discoverable via the public WordPress REST API — an unauthenticated attacker causes wpksespost() to serve as the only output filter, which preserves the injected payload structurally intact; the plugin's admin JavaScript then decodes and executes it automatically on page load.
Affected Software
Event History
Frequently Asked Questions
Which users are most likely to be impacted if this is exploited?
Administrators are the highest-risk users because an attacker can forge frm_user_id to match an administrator's user ID. When an administrator accesses an injected page, the plugin's admin JavaScript automatically decodes and executes the stored script.
Does an attacker need an account or elevated permissions to exploit this issue?
No. The issue is exploitable by an unauthenticated attacker, and the administrator user ID can be discovered through the public WordPress REST API.
Is victim interaction required for the injected script to run?
The attacker does not need user interaction to inject the payload. The stored script executes automatically when a user accesses the injected page.