The Order Tip for WooCommerce plugin for WordPress is vulnerable to Unauthenticated Improper Input Validation in all versions up to, and including, 1.5.4. This is due to lack of server-side validation on the `data-tip` attribute, which makes it possible for unauthenticated attackers to apply an excessive or even negative tip amount, resulting in unauthorized discount up to free orders depending on the value submitted.
https://plugins.trac.wordpress.org/browser/order-tip-woo/trunk/frontend/views/tip-form.php#L49
https://plugins.trac.wordpress.org/browser/order-tip-woo/trunk/assets/build/front.bundle.js