This vulnerability exists within the Mailgun Subscriptions plugin due to the lack of proper sanitization and escaping of user inputs within the 'mailgun_subscription_form' shortcode. Authenticated users with contributor-level access or higher can exploit this flaw by injecting malicious scripts. When these scripts are embedded within a page, they execute each time the page is viewed by any user, potentially leading to session hijacking, defacement, or broader website compromise. The risk is particularly concerning for sites with multiple users with content editing privileges, as even lower-level users can leverage this flaw to execute scripts. Proper sanitization should scrub user inputs of potentially harmful characters or code, and output should be escaped before rendering in a browser to ensure safe execution.

To mitigate this vulnerability, restrict access of contributor-level accounts, ensuring only trusted and vetted users have access to edit content containing the 'mailgun_subscription_form' shortcode. Implement a regular review process to audit plugin usage and permissions. Additionally, apply a web application firewall (WAF) to filter out potential malicious script inputs.