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.
Exploitability
AV:NAC:LPR:NUI:NScope
S:CImpact
C:NI:LA:N5.8/CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:NInput Validation