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.
The product uses a template engine to insert or process externally-influenced input, but it does not neutralize or incorrectly neutralizes special elements or syntax that can be interpreted as template expressions or other code directives when processed by the engine.