WordPress Newsletter Plugin Stored XSS via Unvalidated Custom Fields

The Newsletter – Send awesome emails from WordPress plugin for WordPress is vulnerable to Stored Cross-Site Scripting via 'np1' Custom Field Parameter in all versions up to, and including, 9.4.0 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. The subscription endpoint (na=sa) requires no nonce, no capability check, and no CAPTCHA, and the payload can be smuggled past email-address validation by embedding the {profile_1} placeholder in the local part of the submitted address, since WordPress's is_email() permits curly braces there.

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

The Newsletter plugin for WordPress contains a critical stored cross-site scripting (XSS) vulnerability in its custom field parameter handling (np1). Attackers can inject malicious JavaScript through the subscription endpoint without authentication, CAPTCHA protection, or nonce validation. Once injected, the payload persists in the database and executes whenever users access affected pages, potentially compromising visitor sessions, stealing credentials, or redirecting traffic to malicious sites. This vulnerability affects all versions up to 9.4.0 and poses significant risk to WordPress sites relying on this plugin for email marketing, as the lack of authentication checks means any unauthenticated user can launch attacks at scale.

Casky's 323 mapped skills leverage Claude AI with extended reasoning to identify the attack patterns underlying this vulnerability across MITRE ATT&CK techniques TA0001 (Initial Access) and TA0043 (Reconnaissance). Practitioners using Casky would detect this through skills focused on CWE-79 pattern recognition: monitoring for unsanitized input in form parameters, identifying missing output escaping in email subscription handlers, and flagging authentication bypass indicators in endpoints lacking nonce/capability checks. The platform's extended reasoning capabilities would correlate the absence of input validation with successful payload delivery, recognizing the stored XSS execution chain. Security teams would see findings highlighting the unprotected subscription endpoint (na=sa), missing sanitization of the np1 parameter, and insufficient escaping on page rendering—enabling them to prioritize remediation and implement input validation, output encoding, and nonce-based CSRF protections before exploitation occurs.

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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-2026-96566.

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-2026-96566

Casky has 323 skills that investigate the attack patterns behind CVE-2026-96566. Run one and get CVSS-scored findings in 3 minutes.

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