Quick PayPal Payments: Payment Button & Form <= 6.0.4 - Authenticated (Contributor+) Stored Cross-Site Scripting via 'form' Shortcode Attribute

The Quick PayPal Payments: Payment Button & Form plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'form' parameter in all versions up to, and including, 6.0.4 due to insufficient input sanitization and output escaping. 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. Because wp_kses_post strips only HTML tags, a quote-breaking payload containing no angle brackets survives post save intact, and the injected script fires for every viewer of the post — including higher-privileged Editors and Administrators who routinely preview a Contributor's pending content.