Simple Membership <= 4.3.5 - Reflected Cross-Site Scripting

The Simple Membership plugin for WordPress is vulnerable to Reflected Cross-Site Scripting via the `list_type` parameter in versions up to, and including, 4.3.5 due to insufficient input sanitization and output escaping. Using this vulnerability, unauthenticated attackers could inject arbitrary web scripts into pages that are being executed if they can successfully trick a user into taking an action, such as clicking a malicious link.