Stored XSS in Formidable Forms WordPress Plugin

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.

Casky was already ahead

This CVE exploits attack patterns that Casky's 323matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

Formidable Forms, a popular WordPress form builder plugin with hundreds of thousands of installations, contains a stored cross-site scripting (XSS) vulnerability in versions up to 6.33.1. The vulnerability exists in the 'frm_user_id' parameter, which lacks proper input sanitization and output escaping. This allows unauthenticated attackers to inject malicious JavaScript that persists in the database and executes whenever any user—including administrators—visits the affected page. The impact is severe: attackers can steal session tokens, redirect users to malicious sites, deface content, or escalate privileges by targeting admin accounts, making this a critical threat to WordPress installations using this plugin.

Casky's platform maps this vulnerability to MITRE ATT&CK techniques TA0001 (Initial Access) and TA0043 (Reconnaissance), with 323 relevant security skills available for detection and response. Practitioners using Casky would identify attack patterns through detection of unsanitized 'frm_user_id' parameters in HTTP requests, unusual JavaScript payloads in form submissions, and script execution in page contexts where forms render. Claude's extended reasoning capabilities help correlate stored XSS indicators—such as database queries retrieving malicious content, anomalous DOM manipulation in page responses, and user-agent patterns associated with reconnaissance activities—allowing security teams to distinguish legitimate form interactions from exploitation attempts and prioritize remediation of this high-severity vulnerability.

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Casky Risk Score

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EPSS Probability

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Shodan Exposure

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GreyNoise Signal

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Composite risk scoring from EPSS, CISA KEV, Shodan, and GreyNoise — 21 security APIs correlated into a single Casky Risk Score. Coming in Casky Pro. Join early access →

323 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-18331.

Abuse of Public-Facing API: Mobile API Abuse

mobile security · low

Run

abusing-dpapi-for-credential-access

red teaming · high

Run

abusing-shadow-credentials-for-privesc

red teaming · high

Run

MITRE ATT&CK Techniques

TA0001 TA0043

Investigate the attack patterns behind CVE-2026-18331

Casky has 323 skills that investigate the attack patterns behind CVE-2026-18331. Run one and get CVSS-scored findings in 3 minutes.

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