Express Payment For Stripe <= 1.28.0 - Authenticated (Contributor+) Stored Cross-Site Scripting via Shortcode Attributes

The Express Payment For Stripe plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'type' attribute of the [stripe-express] shortcode in versions up to, and including, 1.28.0. This is due to insufficient input sanitization and output escaping on the shortcode attribute value, which is concatenated into an HTML attribute in the rendered output of the register_shortcode() function without being passed through esc_attr() or any other escaping function. 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.