The WP Express Checkout plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the ‘pec_coupon[code]’ parameter in versions up to, and including, 2.2.8 due to insufficient input sanitization and output escaping. This makes it possible for authenticated attackers with administrator-level access to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. Note: This can potentially be exploited by lower-privileged users if the `Admin Dashboard Access Permission` setting it set for those users to access the dashboard.