The MC4WP: Mailchimp for WordPress plugin for WordPress is vulnerable to Reflected Cross-Site Scripting via the 'email' parameter when a placeholder such as {email} is used for the field in versions 4.9.9 to 4.9.16 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that execute if they can successfully trick a user into performing an action such as clicking on a link.

Cross-site scripting (XSS) vulnerability in the MailChimp Signup submodule in the MailChimp module 7.x-3.x before 7.x-3.3 for Drupal allows remote authenticated users with the "administer mailchimp" permission to inject arbitrary web script or HTML via unspecified vectors.

Cross-site scripting (XSS) vulnerability in the Easy MailChimp Forms plugin 3.0 through 5.0.6 for WordPress allows remote attackers to inject arbitrary web script or HTML via the updateoptions action to wp-admin/admin-ajax.php.

Multiple cross-site scripting (XSS) vulnerabilities in the MailChimp module 7.x-2.x before 7.x-2.7 for Drupal allow remote attackers to inject arbitrary web script or HTML via vectors related to (1) a predictable "webhook URL key" and (2) improper sanitization of "Webhook variables from POST requests."