The WP-Members Membership Plugin plugin for WordPress is vulnerable to Stored Cross-Site Scripting via WooCommerce Checkout Multicheckbox/Multiselect Field (scalar bypass of wpmem_sanitize_array) in all versions up to, and including, 3.5.7 due to insufficient input sanitization and output escaping. This makes it possible for authenticated attackers, with subscriber-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. This is only exploitable via the WooCommerce classic checkout path, as the sibling registration, profile, and account-edit handlers implode the value before sanitization, which raises a PHP 8 TypeError on scalar input and fails closed.