The Newsletter plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'np1' parameter in all versions up to, and including, 8.3.4 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 Newsletter plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'newsletter_form' shortcode in versions up to, and including, 7.8.9 due to insufficient input sanitization and output escaping on user supplied attributes. This makes it possible for authenticated attackers with contributor-level and above permissions to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.

Cross-site scripting vulnerability in Newsletter versions prior to 7.6.9 allows a remote unauthenticated attacker to inject an arbitrary script.

The Newsletter WordPress plugin before 7.4.6 does not escape and sanitise the preheader_text setting, which could allow high privilege users to perform Stored Cross-Site Scripting attacks when the unfilteredhtml is disallowed

The Newsletter WordPress plugin before 7.4.5 does not sanitize and escape the $_SERVER['REQUEST_URI'] before echoing it back in admin pages. Although this uses addslashes, and most modern browsers automatically URLEncode requests, this is still vulnerable to Reflected XSS in older browsers such as Internet Explorer 9 or below.