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Vulnerability record · CVE-2026-18331 · published 26 August 2026

CVE-2026-18331: Cross-site scripting vulnerability

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.

7.2 CVSS 3.1 High EPSS 0.41% · top 67.5% CWE-79 · Cross-site scripting Deferred
7.2CVSS 3.1 base score
0.41%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
9References
26 Aug 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

References

LinkTags
https://plugins.trac.wordpress.org/browser/formidable/trunk/classes/controllers/FrmEntriesController.php#L752
https://plugins.trac.wordpress.org/browser/formidable/trunk/classes/controllers/FrmHooksController.php#L69
https://plugins.trac.wordpress.org/browser/formidable/trunk/classes/models/FrmEntry.php#L819
https://plugins.trac.wordpress.org/browser/formidable/trunk/classes/models/FrmEntry.php#L846
https://plugins.trac.wordpress.org/browser/formidable/trunk/classes/models/FrmEntry.php#L982
https://plugins.trac.wordpress.org/browser/formidable/trunk/classes/models/fields/FrmFieldType.php#L1830
https://plugins.trac.wordpress.org/browser/formidable/trunk/classes/models/fields/FrmFieldType.php#L2083
https://plugins.trac.wordpress.org/browser/formidable/trunk/js/formidable_admin.js#L1
https://www.wordfence.com/threat-intel/vulnerabilities/id/f03b09ad-257c-48c9-8ecc-36f60f178f80?source=cve

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Source: NIST National Vulnerability Database (record CVE-2026-18331), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.