CVE-2026-18331 Formidable Forms <= 6.33.1 - Unauthenticated Stored Cross-Site Scripting via 'frm_user_id' Parameter
Unauthenticated attackers can inject arbitrary scripts via the frm_user_id parameter, leading to code execution and potential account takeover when administrators view affected pages.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-18331?
CVE-2026-18331 is a high-severity cross-site scripting vulnerability in strategy11team Formidable Forms – WordPress Form Builder for Contact Forms, Calculators, Quizzes & More, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
The Formidable Forms – WordPress Form Builder for Contact Forms, Calculators, Quizzes & More plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'frm_user_id' 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 frm_user_id to match an administrator's user ID — discoverable via the public WordPress REST API — an unauthenticated attacker causes wp_kses_post() 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.
Weakness: CWE-79
Which versions are affected by CVE-2026-18331?
How do I fix CVE-2026-18331?
No fix has been published for CVE-2026-18331 yet. Limit exposure of the affected software until one is available.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-18331 being exploited?
There is no known exploitation of CVE-2026-18331 in the wild and no public exploit code.
Sources
Also known as GHSA-hjj8-x8wc-9cj2.