MC4WP: Mailchimp for WordPress < 4.15.0 - Unauthenticated Subscription Audience Override

The MC4WP: Mailchimp for WordPress plugin before 4.15.0 does not validate that the Mailchimp audience a sign-up form or bundled integration subscribes a visitor to matches the audience configured by the site owner, allowing unauthenticated visitors to subscribe arbitrary email addresses to any other audience in the site's connected Mailchimp account.