Mailgun Subscriptions <= 1.3.1 - Authenticated (Contributor+) Stored Cross-Site Scripting

The Mailgun Subscriptions plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the plugin's 'mailgun_subscription_form' shortcode in all versions up to, and including, 1.3.1 due to insufficient input sanitization and output escaping on user supplied attributes. 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.