The Easy Cart plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'add_to_cart' shortcode in all versions up to and including 1.8. This is due to insufficient input sanitization and output escaping on user-supplied shortcode attributes. Specifically, the ectp_add_to_cart() function uses sanitize_text_field() on shortcode attributes like 'itemid', 'product_name', 'product_desc', 'product_qty', and 'price' before inserting them into double-quoted HTML attributes. While sanitize_text_field() strips HTML tags, it does not escape double quote characters, allowing an attacker to break out of the HTML attribute context and inject arbitrary event handlers. This makes it possible for authenticated attackers, with Contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.