Formie: Unauthenticated users can overwrite incomplete submissions via submit action

Impact

The anonymous front-end action formie/submissions/submit (SubmissionsController::actionSubmit) trusted a client-supplied submissionId when loading an incomplete submission, with no session binding, ownership check, or edit token validation.

An unauthenticated attacker could enumerate sequential submission IDs and overwrite or hijack another user's in-progress (multi-page / save-for-later) submission. Tampered data could be persisted and forwarded via notifications and integrations when the submission was completed.

Completed submissions were not affected (isIncomplete=true filter). This is an incomplete remediation of GHSA-pgxq-p76c-x9cg, which added edit-token validation to save-submission but not to submit.

Patches

Fixed in 3.1.31 (Craft 5) and 2.2.23 (Craft 4).

When resuming an existing submission on a site request, the submission ID must match the session-bound submission (Form::getCurrentSubmission()) or a valid submissionEditToken when editingSubmission is set. Cross-form submission ID reuse is also rejected.

Workarounds

No reliable workaround without upgrading. Disabling multi-page forms or save-for-later reduces exposure but does not remove the vulnerability.

• Reported by Jorge González (jorge@jmilla.es)