The WordPress Simple Shopping Cart plugin for WordPress is vulnerable to product price manipulation in all versions up to, and including, 5.1.2. This is due to a logic flaw involving the inconsistent use of parameters during the cart addition process. The plugin uses the parameter 'product_tmp_two' for computing a security hash against price tampering while using 'wspsc_product' to display the product, allowing an unauthenticated attacker to substitute details from a cheaper product and bypass payment for a more expensive item.
The web application does not sufficiently verify inputs that are assumed to be immutable but are actually externally controllable, such as hidden form fields.