HeyForm is an open-source form builder. Prior to version 3.0.0-rc.9, "completeSubmission" accepts a "hiddenFields: [{id, name, value}]" array from the submitter and stores it verbatim in "submission.hiddenFields", without validating the supplied "id"/"name" against the form's declared "form.hiddenFields" schema. An anonymous form submitter can therefore inject arbitrary key/value pairs (including XSS payloads, fake authorization metadata, integration-relevant values) into the stored submission. These fields are subsequently forwarded as-is to every webhook integration registered on the form. Version 3.0.0-rc.9 contains a patch for the issue.