Formie is a Craft CMS plugin for creating forms. Prior to 2.1.6, users with access to a form's settings can include malicious Twig code into fields that support Twig. These might be the Submission Title or the Success Message. This code will then be executed upon creating a submission, or rendering the text. This has been fixed in Formie 2.1.6.

verbb/formie Server-Side Template Injection for variable-enabled settings