An upsert on the primary key in Ash ignores attribute multitenancy, so one tenant can read, overwrite and take over another tenant's records

Authorization Bypass Through User-Controlled Key vulnerability in ash-project ash allows a user of one tenant to read, overwrite and take over another tenant's records via an upserting create action.

Under attribute multitenancy, Ash.Actions.Create (lib/ash/actions/create/create.ex) and Ash.Actions.Create.Bulk (lib/ash/actions/create/bulk.ex) upserted on the primary key, or on an all_tenants? identity, without scoping the conflicting record to the request's tenant. A caller of a create action with upsert? true that accepts the primary key, supplying a record id from another tenant, updated that record's upsert fields and received all of its attributes in the result. With the default upsert fields the tenant attribute was written too, moving the record into the caller's tenant. Read and update policies do not run for upserts, and tenant-scoped create policies pass because the tenant is the caller's own.

This issue affects ash: from 1.22.0 before 3.37.2.

Add upsert_condition expr(<tenant attribute> == ^tenant()) to every create action that upserts on the primary key or on an all_tenants? identity, and do not combine it with return_skipped_upsert?. An upsert into another tenant's record then fails with Ash.Error.Changes.StaleRecord (or is skipped in a bulk create) and leaves that record unchanged. This does not help on the Mnesia data layer, which ignores upsert conditions in affected versions.

Reachable when a resource uses attribute multitenancy and exposes to callers of one tenant a create action with upsert? true that either upserts on the primary key (no upsert_identity) and accepts it, or upserts on an all_tenants? identity, and the caller knows or can enumerate a key belonging to another tenant (for example a serial integer id, or an id from a shared link or export).