Impact In versions from 1.5.0 up to and including 3.0.0, any authenticated portal user could complete and tamper with another user's open task by submitting it on their behalf. The task submission endpoint accepted a task ID and a payload, but it never checked whether the task actually belonged to the user making the call. An attacker who held a valid login (a normal "burger" OAuth token) and who knew or guessed another user's task ID could: - Mark someone else's task as completed. - Overwrite the data submitted with that task — the "verzonden_data" — with arbitrary input of their choosing. - Receive the full task back in the GraphQL response, including the form data that the legitimate owner had already entered. This leaks personal data belonging to the original user. Functionally this means a malicious authenticated user could submit and alter forms in any other user's name, while at the same time reading what that user had previously filled in. Both the integrity of submitted data and the confidentiality of form contents are affected. The vulnerable code was introduced together with the Taak V2 implementation (commit "bb1c1ecf", 2024-06-04) and first shipped in the "1.5.x" release line. Earlier 1.x releases did not contain this resolver. Patches Upgrade to 3.0.1 or later. Fix commit: "8e699add" — "Add auth check for task submission". Workarounds Until the upgrade is applied, block the "submitTaakV2" GraphQL mutation at the API gateway, or restrict the "/graphql" endpoint to trusted networks. Technical details The resolver "nl.nlportal.zgw.taak.service.TaakService.submitTaakV2(id, submission, authentication)" fetched the task object by UUID and immediately transitioned it to the "AFGEROND" state, writing "record.data.portaalformulier.verzondenData" from caller-supplied input. No check verified that the task's "identificatie" matched the authenticated burger. The fix adds a call to a new "isAuthorizedForTaak(authentication, objectsApiTask)" before the status change. The check compares "identificatie.type" and "identificatie.value" against the authenticated principal and validates the task's "eigenaar" for "bedrijf" "machtigingen". Credits Discovered during the nl-portal-backend-libraries penetration testing engagement (phase 1, May 2026). Vendor attribution to be added before publication.