# 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.

- **Severity:** High 7.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 26 Aug 2026
- **Updated:** 26 Aug 2026
- **Also known as:** GHSA-hjj8-x8wc-9cj2

## 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 (Improper Neutralization of Input During Web Page Generation)

## 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.

## Is CVE-2026-18331 being exploited?

There is no known exploitation of CVE-2026-18331 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-18331 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-18331 in the wild and no public exploit code.

Affected product: strategy11team Formidable Forms – WordPress Form Builder for Contact Forms, Calculators, Quizzes & More. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-18331/ (Markdown: https://insights.sbomify.com/CVE-2026-18331/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-18331
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-18331
- OSV: https://osv.dev/vulnerability/CVE-2026-18331

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-18331/
Summaries are generated and may contain mistakes; the official description above is the record.