WebToffee Gift Cards for WooCommerce < 1.3.1 - Unauthenticated Gift Card Amount Manipulation via wt_credit_amount

The WebToffee Gift Cards for WooCommerce WordPress plugin before 1.3.1 does not validate a user-supplied gift card amount server-side before using it as the cart-item price and store-credit coupon value, allowing unauthenticated users to submit an arbitrary or negative amount, bypassing the configured denominations and manipulating the order total to obtain products without paying.